Sexual selection and alternative reproductive tactics in the nursery-web spider Pisaura mirabilis
Paolo Giovanni Ghislandi
PhD Thesis
Sexual selection and alternative reproductive tactics in the nursery-web spider Pisaura mirabilis
Paolo Giovanni Ghislandi
SpiderLab
Genetics, Ecology and Evolution, Department of Bioscience
Faculty of Science and Technology
Aarhus University
November 2017
SUPERVISORS:
Prof. Trine Bilde
Department of Bioscience, Aarhus University, Denmark
Dr. Cristina Tuni
Department Biologie II, Ludwig-Maximilians University München, Germany
CREDITS:
Photos: Paolo Ghislandi
SpiderLab Logo: Lena Grinsted
CITE AS:
Ghislandi P.G. (2017). Sexual selection and alternative reproductive tactics in the nursery- web spider Pisaura mirabilis. Ph.D. dissertation. Genetics, Ecology and Evolution, Department of Bioscience, Aarhus University, Denmark
NUMBER OF PAGES: 103
PRINTED BY:
Fællestrykkeriet – SUN, Aarhus University
Table of Contents
Acknowledgements……………………………………………………………………………...... 1
Summary…………………………………………………………………………………………….3
Resumé…………………………………………………………………………………………...... 5
State of the ART……………………………………………………………………………………7
Reproductive tactics in the nursery-web spider Pisaura mirabilis……………………...………11
Types of ARTs: fixed or plastic traits?...... 15
Which ecological drivers influence the expression and maintenance of ARTs?...... 20
References………………………………………………………………………………………….23
Chapter I - Behavioural plasticity and population variation in the expression of alternative reproductive tactics in the nuptial gift giving spider Pisaura mirabilis: are guys from the hood doing like we do? Manuscript...... 28
Chapter II - Resource availability, mating opportunity, and sexual selection intensity influence the expression of male alternative reproductive tactics. Re-submitted to The Journal of Evolutionary Biology...... 56
Appendix to Chapter II - The effect of environmental variation on the expression and maintenance of ARTs……………………………………………………………………………...89
Chapter III - Silk wrapping of nuptial gifts aids cheating behaviour in male spiders
Behavioral Ecology (2017), 00(00), 1–6. doi:10.1093/beheco/arx028……………………………93
Chapter IV - Extreme male mating behaviours: anecdotes in a nuptial gift-giving spider Arachnology (2015) 16 (8), 273–275……………………………………………………………..100 Acknowledgements
I‟ve always been the spider guy. I had my first pet spider when I was 13, as a present from my father, a young adult female Chilean rose tarantula (Grammostola rosea, Walckenaer, 1837). I had no clue it was actually a female, so I called it “Ranieri”, like the prince of Monaco, just because to me it looked so regal and majestic that it really deserved that name. I spent a massive amount of hours watching what Ranieri was doing (not much by the way, as most mygalomorphs..), as there was something special over her that I was not really able to decode or easily grasp. Something I never experienced before, in the face of the huge amount of whatever other animal I previously kept in a terrarium, cage or aquarium. To me, Ranieri seemed different. The growing interest and passion for that difference brought me to have raised and bred more than 60 spider species from 20 different families and to be about to finish a PhD on spiders behaviour. Thanks, Ranieri.
Among all the people I want to thank for this great experience, the first one is my partner, Daniela
Boncinelli, who have always supported and encouraged me to pursue my PhD study far before I started it and continued doing that until today. And she does it with whatever project I decide to realize, from the smallest to the ones potentially changing our lives.
A huge, huge thanks goes to my parents and my entire family, for always supporting and feeding my interest and for having allowed me to keep whatever animal at home not to limit my passion, even when it came to black widows, brown recluse spiders or deadly poisonous centipedes. Grazie mamma, grazie papà.
Thanks to Myriam and Luciano Boncinelli, Daniela‟s parents, for all the support and lovely advice helping me figuring out which was the right direction to go.
Thanks to all the friends who were always with me, before, during and (hopefully) after this PhD.
Then thanks to myself, for having never given up on my passion and dedication, even when it seemed it wasn‟t really leading me anywhere, or when I had the feeling I could have had better things to do.
1 To my supervisor Trine Bilde, thanks for all the things I‟ve learnt, for helpful discussions, for
“cooling down” my love for spiders when it had to be more scientific and less emotional, and for all the kindness and human support which had nothing to do with being one‟s supervisor.
Thanks to my co-supervisor, Cristina Tuni, for being a friend more than a co-worker, and for involving me in a lot of supercool side projects taking time away from my real, massive Phd project.
Thanks to Virginia “Vivi” Settepani, Magnus Jacobsen and André Walter for being great friends and helpful colleagues, for all the fun in South Africa doing field work, for Pevestorf and for all the rather unfruitful fishing trips.
Thanks to Jesper Bechsgaard, Bram Vanthournout, Tommaso Manenti, Marie Rosenstand and “the
LMU crew”, Philipp Sprau, Petri Niemelä, Marco La Fortezza and Alex Hutfluss for all the beers, fun and also, sometimes, advice. Thanks to all the students who helped me collecting data.
I would have never forgotten to thank Fabio De Vita, the biggest spider lover and experienced breeder I have ever met, and Carlo Paoletti, the biggest amphibian lover and curiosity-driven- zoology fan I have ever met, for being two of my best friends and for walking me in the world of terraristics and taxonomy when I was just a bit more than a kid. Grazie fratellone, grazie Carletto.
Last but not least, thanks to Emanuele Biggi and Francesco Tomasinelli, for all the hours at the phone talking about spiders when I was a kid, for being great friends and for the always too-few collecting trips together, grazie Ema and grazie Francio!
And obviously, thanks to something like 7.360 Pisaura mirabilis which “spontaneously” collaborated to my PhD project.
2 Summary
The evolution and maintenance of alternative reproductive tactics ARTs represent a topic of broad interest in evolutionary and behavioural biology. In this thesis, I studied the characteristics and factors that influence the expression of ARTs in the nuptial gift-giving spider Pisaura mirabilis, where males offer either genuine or worthless nuptial gifts but can also mate without a gift, and where males also facultatively perform thanatosis, a death feigning behaviour associated with courtship. I addressed the following questions: 1. Are ARTs fixed or plastic? I conducted a study in five different populations to investigate whether nuptial gift traits and thanatosis represent fixed or plastic tactics, and show that ARTs are plastic labile traits. 2. Is there variation in the expression of
ARTs display among populations? Populations may diverge in the expression of ARTs in response to variation in ecological factors. As a first step to gain insight into population differences, I determined variation in the expression of ARTs among populations, and found evidence for population variation in the frequencies of trait expression. 3. Which ecological factors influence the expression and maintenance of ARTs? Ecological factors may influence the expression of ARTs, and variation in ecological factors over the reproductive season may influence the maintenance of
ARTs. I conducted a field study to explore whether the nuptial gift tactics vary with ecological factors. I found patterns of correlation between sexual selection, male body condition and prey availability and the expression of tactic, suggesting that the interplay and variation in ecological factors over time influence the maintenance of ARTs in wild populations. 4. How do seasonal changes in ecological factors shape ARTs? Based on the empirical data collected in my thesis, I aimed to develop and parameterize a model to investigate how variation in ecological factors influence the maintenance of ARTs, this work is recently initiated and ongoing, and represented as a short sketch of the model. 5. What is the role of silk wrapping in alternative nuptial gift tactics?
Silk is used to wrap the nuptial gift, and can potentially be used to enlarge the gift or disguise gift content. I performed a study to test whether males make strategic use of silk when offering nuptial gifts, and found that males wrap worthless gifts with more silk than genuine gifts, possibly to
3 compensate for their lower nutritive value and to increase copulation time. Finally, while collecting data on male reproductive behaviour, I observed that males perform rather extreme behaviours in additions to those reported above, this is reported in a paper presenting extreme ARTs.
4 Resumé
Evolutionen og opretholdelse af alternative reproduktionstaktikker (ARTs) repræsenterer et emne af bred interesse indenfor evolutionær- og adfærdsbiologi. I denne afhandling, har jeg studeret karakteristika og faktorer der influerer udtrykket af ARTs hos edderkoppen, Pisaura mirabilis, hvor hanner oftest tilbyder enten spiselige eller værdiløse bryllupsgaver. Hanner kan dog også opnå parring uden en bryllupsgave. Hanner kan yderligere fakultativt udføre såkaldt thanatosis (spille død), en adfærd associeret med parring. Jeg adresserede følgende spørgsmål: 1) er ARTs fikserede eller plastiske? Jeg udførte et studie i 5 populationer for at undersøge om karakterer omkring bryllupsgaver og thanatosis repræsenterer fikserede eller plastiske taktikker, og finder at ARTs er plastiske labile træk. 2) er der variation i udtrykket af ARTs mellem populationer? Variation i
økologiske faktorer kan forårsage divergens i udtrykket af ARTs mellem populationer. Som et første skridt for at opnå indsigt i populationsforskelle, har jeg bestemt variationen i udtrykket af
ARTs mellem populationer, og fundet at variationen i frekvensen af de enkelte ARTs mellem populationer. 3) hvilke økologiske faktorer influerer udtrykket og opretholdelse af ARTs?
Økologiske faktorer kan influere udtrykket af ARTs, og variationen i økologiske faktorer henover den reproduktive sæson kan influere opretholdelsen af ARTs. Jeg udførte et feltstudie for at udforske om bryllupsgavetaktikker varierer sammen med økologiske faktorer. Jeg fandt korrelationer mellem seksuel selektion, hannernes kropskonditioner og bytte tilgængelighed, og udtrykket af taktik, hvilket tyder på at samspillet og variationen mellem økologiske faktorer over tid influerer opretholdelsen af ARTs i naturlige populationer. 4) hvorledes former ændringer i
økologiske faktorer henover sæsonen ARTs? Baseret på empiriske data indsamlet i min afhandling, har jeg startet udviklingen samt parametriseringen af en model til at undersøge hvorledes variationen i økologiske faktorer influerer opretholdelse af ARTs. Dette arbejde er lige indledende og præsenteret som en simpel skitse. 5) hvilken rolle spiller silkeindpakning for alternative bryllupsgavetaktikker? Hannerne burger silke til at pakke bryllupsgaverne ind, og kan potentielt udnyttes til at forstørre bryllupsgaven eller forklæde indholdet af gaven. Jeg udførte et studie for at
5 teste om hannerne udfører strategisk brug af silke i forbindelse med, at de tilbyder bryllupsgaver, og fandt, at hanner pakker værdiløse bryllupsgaver ind i mere silke end ægte bryllupsgaver, sandsynligvis for at kompensere for deres lavere ernæringsværdi og for at øge kopulationstid.
Endeligt har jeg under indsamlingen af data for hanners reproduktive adfærd observeret, at hanner hellere udfører ekstrem adfærd i tillæg til disse rapporteret herover, hvilket er præsenteret i en artikel om ekstreme ARTs.
6 State of the ART
Competition over limited resources is ubiquitous in nature, promoting selection for traits that facilitate access to and exploitation of the resource itself (Schluter, 1994; Rainey and Travisano,
1998; Dieckmann and Doebeli, 1999). If a single optimal response to such competition is present, then these traits are under directional selection, with the most advantageous trait outcompeting the others and becoming fixed (Darwin, 1859). When more than a single optimal response is present, selection can promote set of traits to evolve, with each trait providing individuals with equal fitness payoffs (Oliveira, Taborsky and Brockmann, 2008). Conspecific individuals might indeed adopt different traits to reach the same end, thus showing phenotypic plasticity (Oliveira, Taborsky and
Brockmann, 2008; Taborsky and Brockmann, 2010). When phenotypic plasticity happens in a sexual context, it may result in alternative reproductive tactics (ARTs) (Gross, 1996; Shuster and
Wade, 2003; Oliveira, Taborsky and Brockmann, 2008). ARTs consist of morphological or behavioural traits which both males and females can adopt, that are different from the prevailing trait that intra-sexual conspecific competitors display to obtain inseminations (Tuttle, 2003;
Oliveira, Taborsky and Brockmann, 2008). ARTs are highly variable from fixed genetically determined phenotypes to plastic and labile behavioural types (Brockmann and Penn, 1992; Gross,
1996; Engqvist, 2007; Locatello et al., 2007; Simmons and Kotiaho, 2007). In the case of fixed
ARTs, traits differ between individuals and are linked to early developmental stages of the individual, which determine into which alternate and exclusive phenotype to develop (Moczek and
Emlen, 2000; Simmons and Kotiaho, 2007; Cardoso, Teles and Oliveira, 2015). Plastic ARTs are expressed within individuals, and might be sequential or labile. Sequential traits are linked to a particular life stage of the individual, while labile plastic traits do not depend on any given life stage of the individual and can be multiply and reversibly displayed (Brockmann and Penn, 1992;
Locatello et al., 2007; Cardoso, Teles and Oliveira, 2015). Plastic traits are driven by the interaction between the internal state of the individual and environmental factors (Schradin, 2013; Cardoso,
Teles and Oliveira, 2015).
7 The study of ARTs represents a hot topic in behavioural and evolutionary biology.
Investigating ARTs provides a particularly interesting opportunity to study how phenotypic variation can evolve, which is a central question in evolutionary biology (Oliveira, Taborsky and
Brockmann, 2008). The great variety of organisms adopting ARTs also allows us to approach the topic from different perspectives, increasing even more the interest related to it and the possibilities to explore it.
Several questions regarding ARTs are yet to be investigated. First, in many animals, it is poorly understood whether and when ARTs are fixed or plastic. This aspect is crucial as it a better understanding on the flexibility of ARTs may provide insights into how behavioural phenotypic plasticity evolves, and on investigating the evolutionary processes maintaining ARTs (Oliveira,
Taborsky and Brockmann, 2008; Hendrickx, Vanthournout and Taborsky, 2015). Secondly, identifying the evolutionary forces and drivers that are involved in maintaining ARTs in wild populations remains a key unsolved issue, as this is only understood in a few species (Oliveira,
Taborsky and Brockmann, 2008). Sexual selection represents that major force driving the evolution of ARTs (Oliveira, Taborsky and Brockmann, 2008; Taborsky and Brockmann, 2010). Phenotypic plasticity may evolve in response to environmental heterogeneity, and variation in ecological factors is likely to shape behavioural flexibility (Oliveira, Taborsky and Brockmann, 2008; Monroe et al., 2016). Studies investigating population differences in the frequencies of ARTs are needed to understand how variation in environmental conditions shape the expression of ARTs.
During my PhD studies, I used the nuptial gift-giving spider Pisaura mirabilis, where males are known to perform alternative tactics to fertilize females (Bilde et al., 2006; Albo et al., 2011;
Ghislandi et al., 2014), to investigate what shapes the expression of ARTs and how they may be maintained. Different tactics in this species include the use of a nuptial gift that is offered to the female during courtship and mating, and which might contain genuine nutritious food or nutritionally worthless items such as food leftovers (Albo et al., 2011; Ghislandi et al., 2014).
Males also display another curios trait, they can perform thanatosis, a “death feigning” behaviour
8 prior to or during mating, which has been proposed to function as male mating effort (Bilde et al.,
2006; Hansen et al., 2008). In a previous study, we developed a theoretical model that propose that in Pisaura mirabilis the genuine and worthless gift traits might be maintained by the interplay of several ecological factors such as prey availability, female mate choice, sperm competition, and male body condition (Ghislandi et al., 2014), with these hypotheses still remaining unexplored.
Also, P. mirabilis males have been proposed to plastically make use of silk-wrapping, with this feature promoting males‟ deception by masking the worthless gift content to females (Albo et al.,
2011). If this hypothesis holds, it might further indicate the importance of behavioural plasticity in the expression of ARTs.
In order to examine the characteristics of ARTs and factors that influence the expression of ARTs in
P. mirabilis, I aimed to answer the following research questions in my PhD thesis:
1. Are ARTs fixed or plastic?
I conducted a study in five different populations to investigate whether nuptial gift traits and
thanatosis represent fixed or plastic tactics, which is presented in Chapter I. I show that
ARTs are plastic labile traits
2. Is there variation in the expression of ARTs among populations?
Populations may diverge in the expression of ARTs in response to variation in ecological
factors. As a first step to gain insight into population differences, I determined variation in
the expression of ARTs among populations, reported in Chapter I. I found evidence for
population variation in the frequencies of trait expression.
3. Which ecological drivers influence the expression and maintenance of ARTs?
Ecological factors may influence the expression of ARTs, and variation in ecological factors
over the reproductive season may influence the maintenance of ARTs. I conducted a field
study to explore whether nuptial gift tactics vary according to the interplay of ecological
factors ,which is presented in Chapter II. We found patterns of correlation between sexual
selection, male body condition and prey availability and the expression of gift (or no gift)
9 tactics, suggesting that the interplay and variation in ecological factors over time influence
the maintenance of ARTs in wild populations (Chapter II).
4. How do seasonal changes in ecological factors shape ARTs?
Based on the empirical data collected in my thesis, we aimed to develop and parameterize a
model to investigate how variation in ecological factors influence the maintenance of ARTs.
This work is recently initiated and ongoing, and the current stage of the study is developing
realistic parameter sets to test. Therefore, at present this model represents an appendix of the
study reported in Chapter II.
5. What is the role of silk wrapping in alternative nuptial gift tactics?
Silk is used to wrap the nuptial gift, and can potentially be used to enlarge the gift or
disguise gift content. I performed a study to test whether males make strategic use of silk
when offering nuptial gifts, which is reported in Chapter III. We found that males wrap
worthless gifts with more silk than genuine gifts, possibly to compensate for their lower
nutritive value and to increase copulation time.
6. Extreme ARTs
While collecting data on male reproductive behaviour, I observed that males perform rather
extreme behaviours in additions to those reported above. This resulted in a paper presenting
extreme ARTs in Chapter IV.
10 Reproductive tactics in the nursery web spider Pisaura mirabilis
During my PhD project, I used the araneomorph spider Pisaura mirabilis (Clerck, 1757) as study organism. Pisaura mirabilis is a paleartic ambush-hunting species belonging to the family
Pisauridae (The World Spider Catalog), which is commonly referred to as nursery-web spider due to maternal care by guarding broods in a dedicated tent-shaped nursery web. Pisaura mirabilis males are known to employ behavioural ARTs to acquire mates (Bilde et al., 2006; Albo et al.,
2011; Ghislandi et al., 2014). One of these tactics include the offer of a nutritious donation to females during courtship (Picture 1,A), a so called nuptial gift, which might occur in the form of a genuine or a worthless gift (Bristowe, 1958; Albo et al., 2011; Ghislandi et al., 2014). Genuine gifts consist of arthropod prey, caught and wrapped in silk by males, while worthless gift are often digested prey leftovers, plant buds, pollen or other inedible token items which are silk-wrapped in a similar way as a genuine gift (Picture 1, C-D) and then offered to the female (Albo et al., 2011;
Ghislandi et al., 2014). Silk wrapping has been proposed to have multiple functions. Males might benefit from enveloping gifts in silk as this could increase their control over the gift by allowing a better grip with their chelicera (Andersen et al., 2008), or make strategic use of silk by wrapping worthless gifts with more silk to better hide the content to females (Albo et al., 2011; Ghislandi et al., 2017). Males are also likely to face metabolic costs associated to gift wrapping, as it was shown that males in poor body condition are impaired in their wrapping abilities (Albo, Toft and Bilde,
2011). On the other hand, females might benefit from silk wrapping, because if it is a condition- dependent trait, it might be an honest signal of male quality, and also because silk represents a nutritious source, which females consume during gift consumption (Lang, 1996; Albo, Toft and
Bilde, 2011; Ghislandi et al., 2017). However, if males use silk to mask gift content, this could be detrimental to females, as they are not able to disclose the content prior to accepting the male and initiate feeding on the gift (Albo et al., 2011; Ghislandi et al., 2014). Females may therefore mate with males they would otherwise have rejected.
11 As a last tactic, males can also mate without offering any gift at all (Stålhandske, 2001; Albo et al.,
2011). Nuptial gifts are under sexual selection imposed by strong female preference for gift-giving males (Stålhandske, 2001; Bilde et al., 2007). Copulation duration is dictated by female gift consumption time, with males transferring sperm as long as the female is occupied with feeding on the gift (Drengsgaard and Toft, 1999) (Picture 1, B). Males offering worthless gifts are equally likely to be accepted as males that offer genuine gifts, i.e. around 90% of mating attempts, while males that offer no gifts have a much lower success rate at 40%, at least with virgin females (Albo et al., 2011). Worthless gifts have been suggested to be less costly to produce than genuine gifts, as males would likely save some of the costs associated to genuine gift production, as for instance missed foraging opportunities (Albo et al., 2011; Ghislandi et al., 2014). However, worthless gift- giving males experience at least one major cost, as the female interrupts copulation sooner, which results in reduced sperm transfer and consequently reduced fertilization opportunity (Albo et al.,
2011). As females are polyandrous (Tuni, Albo and Bilde, 2013), costs associated with offering worthless gifts are even amplified, as males are also likely to loose out in sperm competition by transferring a relatively smaller amount of sperm (Parker, 1970).
When courting, males vibrate their abdomen and repeatedly lift their first pair of legs, twitching around in the female‟s proximity (Bristowe, 1958). Courtship culminates with males touching the ground with the tip of their abdomen, keeping the first two pairs of legs open wide and often lifted, and maintaining this position in front of the female, offering the nuptial gift (Picture 1A) (Bristowe,
1958). Once the female accepts the gift by grabbing it in her chelicera (mouthparts) and starts feeding on it, mating usually begins.
12
Picture 1- A: A male P. mirabilis offering a nuptial gift to a female, adopting the typical courtship position, during an experimental trial at Ludwig-Maximilians University, München, Germany; B: A couple mating in the field during the mating season of 2015 in Trnava, Slovakia. The female consumes the gift while the male transfers sperm; C: The content of a genuine field-collected nuptial gift opened under a stereomicroscope in
München, Germany. Note the intact larva of Coleoptera, Family Coccinellidae; D: The content of a worthless field-collected nuptial gift opened under a stereomicroscope in München, Germany. The donation consists almost entirely of silk, with some arthropod exoskeleton parts.
Sometimes both sexes are holding the gift with their chelicera in a front-to-front position, either before copulation, or when males switch pedipalp (sperm transfer organ), and it might then happen
13 that the female attempts to run away with the gift and avoid copulation. In this situation, Pisaura mirabilis males can also perform another ART called thanatosis, which is a “feigning death” tactic
(Bilde et al., 2006; Hansen et al., 2008). Thanatosis occurs in the form of a very stereotyped posture in which males stretch their legs out, hanging motionless onto the gift when the female attempts to flee (Bilde et al., 2006; Hansen et al., 2008) (Picture 2). This particular position allows males to be dragged around on the substrate, and in this way hanging on to the female via the gift. Once the female stops and starts feeding on the gift, males “revive” and resume copulation (Bilde et al.,
2006). Death feigning has been shown to function as a male mating effort, leading to longer copulations and increasing the probability of a male being accepted by the female (Hansen et al.,
2008). Thanatosis can also occur between mating insertions, i.e. when males return to the front-to- front position prior to switch pedipalp to continue copulation (Hansen et al., 2008; Ghislandi et al.,
2014). Furthermore, thanatosis has also been proposed as a male strategy to keep control of the gift not losing it to females: female spiders are often larger than males, with a struggle over the gift most likely resulting in the male losing it. Indeed, it is extremely unlikely that the female re-accepts the male they were mating with for further copulation if they manage to gain full control over the gift (Andersen et al., 2008; Ghislandi et al., 2014). Thus, males might perform thanatosis in order to increase their probability of mating. Hansen et al. (2008) showed that males are able to facultatively perform thanatosis during copulation, suggesting this trait to represent a plastic labile trait.
Considering the benefits provided by performing thanatosis, males are likely to face costs associated with displaying the trait, as it is not systematically performed (Hansen et al., 2008).
Males might for instance make strategic use of thanatosis when females are resistant to mating and/or to increase their chance when courting a high quality female.
14
Picture 2- A P. mirabilis male performing thanatosis during a laboratory trial at Aarhus University, Denmark.
The female drags the male over the substrate while he remains attached to the gift with his chelicera, and maintaining the typical motionless position.
Types of ARTs: fixed or plastic traits?
ARTs are highly variable, and can be genetically or morphologically fixed, or developmental or behaviourally plastic (Oliveira, Taborsky and Brockmann, 2008; Taborsky and Brockmann, 2010).
In the case of fixed traits, multiple intra-sexual phenotypes are simultaneously present, with each phenotype exclusively associated to a tactic (Oliveira, Taborsky and Brockmann, 2008; Taborsky and Brockmann, 2010). For instance, in the marine isopod Paracerceis sculpta (Holmes, 1904) 3 fixed genetically-determined male phenotypes are present : α males are large and defend territories adopting a fighting tactic, β males mimic females behaviour and morphology, and γ males adopt a sneaking tactic, favoured by their reduced body size (Shuster and Wade, 1991, 2003; Oliveira,
15 Taborsky and Brockmann, 2008). Each of these male phenotypes maintains the same tactic throughout its entire adult life, thus adopting a fixed tactic (Shuster and Wade, 1991; Taborsky and
Brockmann, 2010). When ARTs are performed by a single genotype able to adopt either tactic, then tactics rely on plastic traits. For example males of the scorpion fly Panorpa cognata (Rambur,
1842) might offer females a nuptial gift consisting either of salivary nutritious secretions, or a prey collected in the environment depending on male body condition (Vahed, 1998; Engqvist, 2007).
The same male phenotype can thus plastically perform either tactic, which may for example be condition dependent.
In the case of the nursery-web spider Pisaura mirabilis, previous studies have shown thanatosis to represent a plastic labile trait, that males can reversibly adopt (Hansen et al., 2008).
However, regarding the genuine, worthless and no-gift tactics, the question whether these males‟
ARTs depend on fixed or plastic traits is not well understood (Albo et al., 2011). Part of my PhD study therefore focused on understanding whether, in Pisaura mirabilis, males develop into multiple fixed phenotypes each of them performing a single gift tactic, or into a single phenotype able to plastically display either tactic. To address this question, I tested males from 5 different populations offering them the possibility to wrap a worthless and/or a genuine gift (Chapter I).
Individual males from all populations showed the capability of producing either gift type, confirming these traits to be plastic (Chapter I). Also, males were able to reversibly perform the investigated traits, meaning that they are not linked to any particular life stage of individuals, thus representing plastic labile traits (Chapter I).
Plasticity in male gift traits may not only be revealed by gift contents, but also by other aspects related to gift production. For example, previous studies outlined the possibility that males could plastically adjust silk investment depending on the type of donation they are offering (Albo et al., 2011; Ghislandi et al., 2014). This is due to the fact that males might wrap worthless gifts with additional silk to better mask the non-nutritious content and successfully deceive females into mating (Albo, Toft and Bilde, 2011; Ghislandi et al., 2014). I tested this assumption by combining a
16 field and a laboratory study (Ghislandi et al., 2017) (Chapter III), which indeed demonstrated that males plastically invest more in silk wrapping when offering a worthless donation (Ghislandi et al.,
2017) (Chapter III). However, it is possible that males wrap worthless gifts with more silk layers not necessarily to mask the gift content as previously proposed (Albo et al., 2011; Ghislandi et al.,
2014), but to compensate for the reduced quality of their donation. Since females also feed on the silk around gifts (Lang, 1996), males might deposit more silk in order to prolong copulation by improving the nutritional quality of their donation, keeping the female occupied feeding on the gift for longer to enable longer sperm transfer (Ghislandi et al., 2017) (Chapter III).
Independently from the reasons behind increased silk wrapping of worthless gifts, this finding further underlines male behavioural plasticity in performing gift tactics, with males being able to adjust their behaviour according to conditional contexts in which is expressed (Charmantier et al., 2008; Oliveira, Taborsky and Brockmann, 2008; Dingemanse et al., 2010). Furthermore, there may be no limit to the extreme variation in reproductive traits that behavioural plasticity enables. In fact, we also experienced unexpected male mating behaviours associated to gift wrapping during laboratory experimental trials (Ghislandi, Bilde and Tuni, 2015), where males deviated from the “standard” mode of courtship (Chapter IV). Males detached their own legs to wrap into a nuptial gift (Ghislandi, Bilde and Tuni, 2015) (Chapter IV). Interestingly, we also noticed this behaviour in the field, males carrying nuptial gifts containing their own legs (Ghislandi and Hutfluss, pers. obs.) (Pictures 3A-B), and males also allowed females to feed on their limbs while mating when offering no gifts (Ghislandi, Bilde and Tuni, 2015) (Chapter IV).
This might be a case referred in behavioural biology as “doing the best of a bad job”, meaning that individuals might invest in fitness enhancing traits even when the conditions to adopt this behaviour are not optimal (Dawkins, 1980). The ability of rapidly adopting a tactic and to be capable of suddenly modifying it, further underlines how crucial the role of behavioural plasticity is in performing the gift traits.
17
Picture 3- A: An adult male Pisaura mirabilis collected during the mating season of 2015 in Trnava, Slovakia.
Circled in red is the missing limb which has been self-autotomized and wrapped into a nuptial gift that he was carrying; B: nuptial gift of the male from panel A. The third left leg is visible through the silk.
ARTs have been shown to occur at different frequencies across populations (Emlen, 2008; Oliveira,
Taborsky and Brockmann, 2008; Monroe et al., 2016). This fact has been proposed to be related to differential genetic mechanisms and/or local environmental factors (Emlen, 2008; Monroe et al.,
2016). Tactics also return different fitness pay-offs depending on the environment in which they are displayed (Schluter, 2001), leading to variation in the frequencies of ARTs among populations in response to environmental heterogeneity (Charmantier et al., 2008; Oliveira, Taborsky and
Brockmann, 2008; Chevin, Lande and Mace, 2010).
In Pisaura mirabilis, empirical data suggest possible population differences in the expression of
ARTs. For instance, a study on a Slovakian population showed that all field-collected nuptial gifts consisted of intact fresh prey (Prokop and Maxwell, 2012), with males carrying exclusively genuine donations. By contrast, 41.6% of field-collected nuptial gifts from a German population consisted of worthless donations (Ghislandi et al., 2017). Similarly, studies of a Danish population found
18 thanatosis to be performed in 30-40% of mating encounters (Bilde et al., 2006; Hansen et al., 2008) while empirical data collected from a Slovakian population suggested thanatosis to occur at a lower frequency, approximately 13% (Pavol Prokop, pers. comm.).
In order to evaluate whether and to what extent population differences in the frequencies of performing a particular gift trait and/or thanatosis are present, I tested 5 different populations for the occurrence of these traits in standardized laboratory tests. We chose a British and an Italian population because of their likely isolation from the European mainland due to ecological barriers, consisting respectively of the British Channel and the Alps. The other 3 populations were chosen because of previous knowledge, consisting of a Danish, a German and a Slovakian population. We specifically included the Slovakian population studied in Prokop et al. (2012) in which males were found to only offer genuine gifts. Our results showed that populations differ in the frequency of thanatosis performance and in the frequency of producing a genuine nuptial gift (Chapter I).
However, no differences in the frequency of worthless gifts were found among populations
(Chapter I). Also, behavioural plasticity measured as the proportion of males performing all tactics did not differ among populations. Our study showed that variation in the frequency of P. mirabilis male ARTs is present among populations (Chapter I). Furthermore, we found that worthless gifts are produced also by Slovakian males, confirming this finding in both the mentioned laboratory study of Chapter I, and in a field study I performed in Slovakia, where they occurred at a natural frequency of 26%.
Our findings are consistent with the hypothesis that if individual implementation of a given tactic varies according to environmental heterogeneity, this may result in population differences in the expression of tactics (Emlen, 2008; Oliveira, Taborsky and Brockmann, 2008; Monroe et al., 2016).
19 Which ecological drivers influence the expression and maintenance of ARTs?
Different models have been proposed for the maintenance of ARTs in populations (Oliveira,
Taborsky and Brockmann, 2008; Taborsky and Brockmann, 2010). The mechanisms which is likely the most accredited for ARTs maintenance is frequency-dependent selection (Plaistow et al., 2004;
Oliveira, Taborsky and Brockmann, 2008; Taborsky and Brockmann, 2010). Theory predicts that
ARTs can be maintained when frequency-dependent selection results in each tactic in the population to be equally successful in term of fitness pay-off (Maynard Smith, 1982; Oliveira,
Taborsky and Brockmann, 2008). This results in an Evolutionarily Stable Strategy (ESS) wherein alternative tactics are maintained (Maynard Smith and Price, 1973; Gross, 1996). For instance, in the dwarf spider Oedothorax gibbosus (Blackwall 1841), two males morphs are present, a high- and a low-investing phenotype. The high investing phenotype experiences delayed sexual maturation due to the development of costly structures producing a secretion functioning as a nuptial gift for females (Hendrickx, Vanthournout and Taborsky, 2015). This maturation delay results in a vacant mating niche because females remain unmated in the beginning of the mating season. This vacant niche is occupied by a low-investing male phenotype that matures earlier without the secretion producing structure (Hendrickx, Vanthournout and Taborsky, 2015). The maintenance of these male
ARTs relies on negative frequency-dependent selection, with each morph favoured when rare
(Hendrickx, Vanthournout and Taborsky, 2015).
In some cases, ARTs maintenance have been shown to depend on demographic parameters such as density (Radwan, 2001). Social environment might indeed play an important role on the decision of which alternative tactic to adopt (Radwan, 2001; Oliveira, Taborsky and Brockmann,
2008; Taborsky and Brockmann, 2010). Males of the dragonfly Nannophya pygmaea
(Rambour,1842) for instance, adopt a territorial-defending tactic at lower densities, while they are more likely to switch to a satellite tactic when the number of individuals increases (Tsubaki and
Ono, 1986; Taborsky and Brockmann, 2010).
20 Variation in other ecological factors have also been identified as crucial for maintaining
ARTs in several study organisms (Oliveira, Taborsky and Brockmann, 2008; Taborsky and
Brockmann, 2010), as costs and benefits of tactics might vary in response to environmental changes
(Emlen, 2008; Oliveira, Taborsky and Brockmann, 2008). For instance, if environmental conditions are unpredictable or change to a negligible extent throughout individuals life, fixed ARTs may evolve (Shuster and Wade, 2003; Oliveira, Taborsky and Brockmann, 2008). If environmental changes happen in a more predictable way during an individual development, with the individual exposed to changing environments consecutively, sequential plastic ARTs might be selected
(Alonzo, Taborsky and Wirtz, 2000; Oliveira, Taborsky and Brockmann, 2008). If then unpredictability of changes in environmental factors occurs at a short-lived temporary scale (i.e. for instance number of potential mates or seasonal predator/prey abundance), selection should favour labile plastic ARTs, allowing individuals to respond more rapidly, and reversibly, to changing conditions (Engqvist, 2007; Oliveira, Taborsky and Brockmann, 2008).
During my PhD project, I performed field studies to investigate patterns of correlation between ecological factors and the occurrence of gift and worthless gift traits in P. mirabilis (Chapter II). We based our observations on a model we developed in 2014, where we hypothesized P. mirabilis males to adopt a gift-giving tactic and to offer worthless gifts depending on the interplay of sexual selection, body condition and prey availability (Ghislandi et al., 2014). We found some evidence for female availability influencing the expression of overall gift and worthless gift production, with the number of available females negatively correlating with worthless gift production (Chapter II). This result is best understood in the context of post-copulatory sexual selection, because sperm competition is likely to favour males that produce genuine gifts. OSR, measures as the relative proportion of females in the field positively correlated with males‟ likelihood to produce a nuptial gift and also with males‟ likelihood to switch to genuine gifts production (Chapter II). Possibly, also prey availability and male body condition might play a role. Males overall gift production (genuine and worthless) and worthless gift production positively correlated with prey availability (in one
21 year), and overall gift production also negatively correlated with male body condition (Chapter II).
Resource availability may therefore also influence trait expression.
Our findings underline the importance and also the complexity of ecological conditions in the shaping of Pisaura mirabilis ARTs, confirming what we previously hypothesized in our model, namely that gift and worthless gift production might possibly be effected by the interplay of sexual selection, male body condition and prey availability (Ghislandi et al., 2014). As is the case for several other systems showing plastic labile traits (Oliveira, Taborsky and Brockmann, 2008), nursery-web spider males appear to adjust their tactics depending on environmental conditions
(Chapter II).
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27
Chapter I
Behavioural plasticity and population variation in the expression of alternative reproductive tactics in the nuptial gift giving spider
Pisaura mirabilis: are guys from the hood doing like we do?
Ghislandi P.G., Lauridsen A., Hutfluss A., Tuni C. & T. Bilde
Manuscript
28 1 Behavioural plasticity and population variation in the expression of alternative
2 reproductive tactics in the nuptial gift giving spider Pisaura mirabilis: are guys
3 from the hood doing like we do?
4 Paolo Giovanni Ghislandi* a, Astrid Lauridsen a, Alexander Hutfluss b, Cristina Tuni b✚ & Trine
5 Bilde a✚
6
7 a Department of Bioscience, Aarhus University, Aarhus, Denmark
8 b Department of Biology, Ludwig Maximilian University, Munich, Germany
9 * Corresponding author: [email protected]; Ny Munkegade 116, 8000 Aarhus C, Denmark
10 ✚ These authors contributed equally.
11
12
13
29 14 Abstract
15 Behavioural plasticity allows animals to respond rapidly to variation in environmental
16 conditions. When behavioural plasticity occurs while competing for mates, it may result in
17 alternative reproductive tactics (ARTs). Males of the nursery-web spider Pisaura mirabilis
18 express alternative tactics to acquire mates, these involves the use of genuine or worthless
19 nuptial gifts offered to females during courtship, and thanatosis – a death feigning behaviour
20 that is displayed in a mating context. In this study, we examined i) variation in the display of
21 gift tactics and thanatosis among populations, ii) whether males can plastically adopt either of
22 the gift traits or thanatosis within and between populations. We collected data on the
23 frequency in tactic use and behavioural plasticity in tactic performance in laboratory trials of
24 P. mirabilis males originating from five different populations. We found variation in the
25 percentage of males that adopt the genuine gift tactic, but not the worthless tactic, among
26 populations. Males were much less inclined to construct a worthless than a genuine gift. We
27 also found variation in the percentage of males that perform thanatosis among populations.
28 Behavioural plasticity was pronounced, with some degree of variation among populations. If
29 populations vary in ecological and environmental factors that influence tactic expression, this
30 could explain the population differences observed in this study.
31
30 32 Introduction
33 Genetic isolation caused by ecological barriers is considered a major driver in population
34 divergence, allowing beneficial mutations to become fixed via genetic drift and preventing
35 diluting effects by invading of exogenous individuals (Maynard Smith, 1966; Dillon, 1984;
36 Palumbi, 1994; Nosil, Vines and Funk, 2005; Hankison and Ptacek, 2008). However,
37 population divergence can also happen in the face of gene flow (Maynard Smith, 1966;
38 Maynard Smith and Price, 1973; Jiggins et al., 2001; Miyagi and Terai, 2013). Divergence of
39 traits display among populations that are not genetically segregated is often ascribed to
40 phenotypic plasticity, where a genotype give rise to multiple phenotypes under different
41 environmental conditions (Oliveira, Taborsky and Brockmann, 2008; Taborsky and
42 Brockmann, 2010). If environmental changes occur rapidly and unpredictably through an
43 individual‟s life-time, behavioural phenotypic plasticity should be beneficial, as it allows
44 individuals to rapidly adjust their responses (Oliveira, Taborsky and Brockmann, 2008;
45 Dingemanse et al., 2010; Wright et al., 2010). Moreover, costs and benefits associated to a
46 behavioural response have been shown to vary according to environmental conditions
47 (Oliveira, Taborsky and Brockmann, 2008). Individuals might thus develop a set of possible
48 responses they can adopt when exposed to rapid changes in the environment, and the relative
49 frequency of responses will be reflected in their behavioural reaction norm (Charmantier et
50 al., 2008; Oliveira, Taborsky and Brockmann, 2008; Chevin, Lande and Mace, 2010;
51 Dingemanse et al., 2010; Wright et al., 2010). For instance, male field crickets Teleogryllus
52 oceanicus have been shown to either sing from their burrows to attract females, or to behave
53 as satellites intercepting females approaching singing males (Rowell and Cade, 1993). Males
54 rapidly switch between behaviours depending on changes in environmental factors, such as
55 variation in female density (Rowell and Cade, 1993).
31 56 When behavioural plasticity occurs while competing for mates, it may result in alternative
57 reproductive tactics (ARTs) (Oliveira et al., 2008). Behavioural ARTs consist of traits that
58 differ from the prevailing trait adopted by same-sex competitors to obtain inseminations
59 (Tuttle, 2003; Oliveira, Taborsky and Brockmann, 2008). Such traits can be adopted
60 consistently throughout an individual reproductive life, or represent plastic traits,
61 conditionally expressed depending on internal and external environments (Brockmann and
62 Penn, 1992; Gross, 1996; Engqvist, 2007; Locatello et al., 2007; Simmons and Kotiaho,
63 2007). Plastic behavioural traits that are expressed within individuals might be sequential, i.e.
64 linked to a particular life stage of individuals (Brockmann and Penn, 1992; Locatello et al.,
65 2007; Cardoso, Teles and Oliveira, 2015), or labile, so they can be expressed facultatively and
66 reversibly (Engqvist, 2007; Oliveira, Taborsky and Brockmann, 2008). Since fitness pay-offs
67 associated to a particular tactic might vary depending on the environment in which this tactic
68 is adopted (Schluter, 2001; Schradin, 2013), ARTs are likely to occur at different frequencies
69 among populations (Oliveira, Taborsky and Brockmann, 2008; Monroe et al., 2016).
70 Differential selective pressures might therefore promote intra and inter-population phenotypic
71 divergence in tactic display, also under the presence of gene flow (Leinonen et al., 2006;
72 Hankison and Ptacek, 2008).
73 A study organism that shows behavioural ARTs is represented by the nursery-web spider
74 Pisaura mirabilis, where males express alternative tactics to acquire mates (Bilde et al., 2006;
75 Albo et al., 2011; Ghislandi et al., 2014). These tactics involve the use of a nuptial gift, which
76 might occur in the form of a genuine or a worthless gift offered to females during courtship
77 (Bristowe, 1958; Albo et al., 2011; Ghislandi et al., 2014). Genuine gifts consist of arthropod
78 prey wrapped in silk, whereas worthless gift are often prey leftovers, partially or entirely
79 consumed, plant buds, pollen or small pieces of wood which are offered to females after
80 wrapping in white silk (Albo et al., 2011; Ghislandi et al., 2014). Also, males can mate
32 81 without offering any gift at all (Stålhandske, 2001; Albo et al., 2011). Even if existing data
82 suggest that individual males may be able to perform either tactic (Albo et al., 2011), there is
83 limited quantitative evidence for the occurrence of behavioural plasticity in the use of
84 different tactics. Empirical data suggest that the frequency in male use of different gift types
85 may vary among populations (Prokop and Maxwell, 2012; Ghislandi et al., 2017). One
86 possibility is that trait expression varies in response to prey availability, sexual selection and
87 male body condition (Ghislandi et al., 2014; Chapter II). If different populations show
88 variation in the expression of male tactics, this could be in response to variation in such
89 ecological factors among populations, and their influence on pay-offs of male tactics.
90 Pisaura mirabilis males can also perform another interesting plastic trait, the so-called
91 thanatosis behaviour or “death feigning” (Bilde et al., 2006; Hansen et al., 2008). Thanatosis
92 consists of a very stereotyped posture in which males stretch their legs out hanging motionless
93 onto the nuptial gift when the female attempts to run away with the gift in her mouthparts
94 (Bilde et al., 2006; Hansen et al., 2008). This position allows the male to be dragged around
95 on the substrate, without impeding the female‟s movements, and once the female stops and
96 (re)start feeding on the gift, the male “reanimate” and initiate copulation (Bilde et al., 2006;
97 Ghislandi et al., 2014). Thanatosis behaviour has been shown to function as a male mating
98 effort, producing longer copulations and increasing male mating acceptance and re-acceptance
99 chances, with males also able to feign death between mating insertions of their copulatory
100 organs (Hansen et al., 2008; Ghislandi et al., 2014). Furthermore, thanatosis has been
101 proposed as a male strategy not to lose the gift to females: female spiders are often larger than
102 males and a struggle over the gift would most likely result in the male losing the donation. If
103 the female has full control over the gift, the male‟s likelihood of subsequently being accepted
104 or re-accepted is extremely low (Andersen et al., 2008; Ghislandi et al., 2014). Therefore,
105 thanatosis might play an important role for male mating success. Previous studies showed that
33 106 males in a Danish population facultatively express thanatosis during mating encounters,
107 suggesting that this is a plastic labile trait (Bilde et al., 2006; Hansen et al., 2008). Given the
108 benefits to males of enacting thanatosis and the fact that it is not systematically performed, it
109 is possible that there are costs associated with the performance of thanatosis (Bilde et al.,
110 2006; Hansen et al., 2008). If there is variation in potential costs in response to environmental
111 conditions, thanatosis is may be employed at different frequencies in different populations.
112 Also, the degree of plasticity in thanatosis display might vary among populations, with
113 individual males their plastic responses to external factors.
114 The aims of the present study are 1) to examine variation in the display of gift tactic and
115 thanatosis among populations, as this might be a consequence of variation in genetic, i.e.
116 internal, or ecological, i.e. external, factors, and 2) to understand whether males can
117 plastically adopt either of the gift traits or thanatosis within and between populations. We
118 collected data on the frequency in tactic use and behavioural plasticity in tactic performance
119 in laboratory trials of P. mirabilis males from five different populations.
120
121 Methods
122 Collecting and rearing
123 Immature Pisaura mirabilis specimens were collected from 5 European populations over a
124 period from April 2013 to April 2016. Spiders were collected from populations in 1) Italy
125 (Merlino, 45°25'18.0"N 9°27'34.8"E) 2) Germany (Grosshadern, 48°06'34.0"N 11°28'16.9"E)
126 3) Slovakia (Novaky, 48°43'25.9"N 18°32'18.0"E), 4) United Kingdom (Nottingham,
127 52°56'47.9"N 1°13'05.3"W) and 5) Denmark (Mols, 56°13'36.8"N 10°34'45.5"E). The
128 German and the Danish populations were chosen because of previous knowledge of these
129 spiders (i.e. populations‟ location, preliminary data on tactics display and average individuals
34 130 abundance in the field) (Albo et al., 2011; Ghislandi et al., 2017), the Italian and the British
131 because of their geographical isolation the Alps and the English Channel, and the Slovakian
132 population because of the reported difference from the Danish population in the frequency of
133 display of gift tactics and thanatosis (Prokop and Maxwell, 2012; Prokop, pers.comm.).
134 Spiders were raised in the laboratories at Ludwig Maximilian University, Munich, Germany
135 (German, Italian and Slovakian population), and at Aarhus University, Aarhus, Denmark
136 (Danish and British population) under standardised conditions.
137 Specimens were individually housed in plastic vials labelled with individual ID (5 cm
138 diameter X 10 cm height), with moss on the bottom to maintain humidity and topped with a
139 foam stopper in order to assure oxygen supply. Moss was sprayed with water twice a week
140 and spiders were located at room temperature of approximately 23C°, under natural
141 photoperiod. Animals were fed once every two days with insect prey consisting of Drosophila
142 spp. fruitflies, Musca spp. and Lucilia spp. flies and Gryllus spp. cricket nymphs. Spider
143 moulting events were noted to keep track of individual age, and 12-14 days after the last
144 moult to adulthood, males were tested for gift their wrapping and thanatosis propensity in
145 standardised trials. Tactic display could be influenced by body condition. In Pisaura
146 mirabilis, body weight correlates positively with size (male cephalothorax width), used to
147 calculate residual body indexes in spiders (Jakob, Marshall and Uetz, 1996), and to residual
148 body index itself (Ghislandi, unpublished). We therefore used body mass as a proxy for male
149 body condition. Before starting each trial, we measured male body mass as a to the nearest
150 0.01 g using a digital scale (KERN PKT, KERN & SOHN GmbH, Balingen, Germany, and
151 SARTORIUS, Sartorius AG, Göttingen, Germany).
152
153
35 154 Gift tactics
155 We investigated the propensity of males for producing a genuine and a worthless gift. In total,
156 204 males were assayed, 26 from the German population, 67 from the Danish, 42 from the
157 Italian, 28 from the Slovakian and 41 from the British. Experiments were conducted in
158 transparent plastic terraria (7 cm height × 10 cm width × 10 cm depth) with a bottom layer of
159 absorbent paper. To elicit male gift-wrapping behaviour, we allowed a sexually mature female
160 belonging to the same population, to move freely in the experimental terrarium for 30
161 minutes, prior to male introduction. When walking around, females deposit silk draglines, to
162 which males respond by showing sexual excitement, often leading to gift construction
163 (Ghislandi et al., 2017). As soon as a male started vibrating their abdomen, indicating
164 initiation of courtship, we randomly offered the male either a genuine gift item consisting of a
165 live prey of known weight and size (a cricket nymph or a fly), or a worthless item consisting
166 of a dried prey (a cricket nymph exuvia or a dehydrated fly carcass). Fly carcasses were dried
167 out in an oven at 60 °C for 3 hours (UF30, MEMMERT GmbH, Schwabach, Germany and
168 THERMO SCIENTIFIC, Thermo Electron LED GmbH, Langenselbold, Germany) prior to
169 the experiment. This procedure mimicks what happens in nature, where worthless gifts often
170 consist of empty arthropod exoskeletons (Bristowe, 1958; Nitzsche, 1988; Ghislandi et al.,
171 2014). Body length of live prey and dried prey was measured to the nearest 0.01 mm using
172 digital callipers (AEROSPACE, China, and TOOLMATE, Denmark).
173 Live prey was simply released in the terrarium by placing it with forceps in the vicinity of the
174 male. If a male did not catch the prey within 20 minutes, the trial was interrupted and the male
175 returned to the vial. Also, trials were interrupted when males caught the prey but did not
176 construct a nuptial gift within 20 minutes. Dried prey was offered with forceps by gently
177 moving it in front of the male. This was repeated every 5 minutes. If a male did not accept the
178 worthless item within 20 minutes, thus after 4 offers, they were returned to their vial.
36 179 Gift wrapping occurred when a male accepted the offered item by grabbing it in their
180 mouthparts (chelicerae) and started depositing silk on it to produce a nuptial gift. Each
181 wrapping event was considered finished when males stopped circling around the gift to
182 envelop it and collected it from the substrate with their chelicera. After 20 minutes from the
183 last wrapping event, the experiment was terminated, gifts were gently removed with forceps
184 from the male‟s mouthparts, and males were returned to their vials.
185 Each male was tested twice per gift type (i.e. genuine and worthless) for the Italian, Danish
186 and Slovakian populations, up to 3 times per gift type for the German population and once per
187 gift type for the British population. Males were randomly assigned to produce a genuine or a
188 worthless gift in the first trial. Males tested for genuine gift wrapping were subsequently
189 tested for worthless gift wrapping and vice versa, systematically reversing trials order for all
190 populations. To estimate the percentage of males producing a gift for each population, and the
191 propensity of males to plastically employ both tactics, we used only the first trial for each
192 male for each gift type.
193
194 Thanatosis
195 In total, 128 males were assayed, 27 from the German population, 27 from the Danish, 21
196 from the Italian, 17 from the Slovakian and 36 from the British. Thanatosis behaviour was
197 examined in two successive mating trials, each conducted by pairing males with a random
198 novel female. Experiments were performed in transparent plastic terraria (19 × 13 × 14 cm)
199 provided with absorbent paper on the bottom. At first, a female belonging to the same
200 population as the male was placed in the terrarium allowing her to deposit draglines to elicit
201 male sexual excitement, as described above. After 30 minutes, the female was removed and
202 the male introduced in the mating arena. When the male started vibrating its abdomen, a live
37 203 cricket nymph or a fly of known weight and size was placed with forceps in the male‟s
204 proximity. Crickets body length was measured as described above. If the male did not catch
205 the prey or did not wrap it within 20 minutes the trial was terminated, and the male was
206 returned to its vial. When a male accepted the gift item, we allowed them to produce a nuptial
207 gift through one or multiple wrapping events. Wrapping events were considered concluded
208 when males stopped enveloping the gift and collected it from the bottom with their chelicera.
209
210 After 10 consecutive minutes from last gift wrapping event, the female was returned to the
211 mating arena. If a male initiated courtship and was not accepted within 1 hour, the trial was
212 interrupted, and each individual was returned to its vial to be tested again on the following
213 day. Once the female accepted the gift by fixing it in her mouthparts, we recorded thanatosis
214 occurrence (yes/no). Thanatosis was also recorded when occurring between mating
215 interruptions. When the mating ceased, i.e. when the couple spontaneously divided, males and
216 females were returned to their own vials. The trial was repeated with the same male and a
217 different female the following day, to get an estimation of individual plasticity. Each male
218 was tested up to 5 times for the Italian, German and Danish populations, 4 for the Slovakian
219 and 2 for the British.
220
221 Data analysis
222 The frequencies of males producing genuine and worthless gifts among populations was
223 compared with Chi-square models with gift production (Y/N) as response variable and
224 population as predictor, and male body mass and the interaction between male body mass and
225 population as co-variates. We calculated individual male behavioural plasticity as the
226 proportion of males facultatively performing thanatosis and compared male plasticity among
227 populations using a Chi-square test setting plasticity (Y/N) as response variable and
38 228 population, male body weight and the interaction between population and body weight as
229 explanatory variables. We used the 2 trials for each males in this analysis.
230 To study whether variation male behavioural plasticity varies with the number of trials
231 performed, we used a Chi-square model setting plasticity for thanatosis (Y/N) as response
232 variable and population, n° of trial, body weight and the interaction between population and
233 n° of trial as explanatory variables. For thanatosis, we included only the German, Danish and
234 British populations, due to low sample size for the Italian and Slovakian populations.
235 All statistical analysis were performed using the software JMP 13.0 from SAS Institute Inc.,
236 Cary, NC.
237
238 Results
239 Males assessed for gift production differed in their body mass across populations, with
240 individuals from the Slovakian population being the heaviest (one-way ANOVA, F=10.59,
241 p<0.0001, mean and SD “German” 96.08±2.99, “Danish” 82.43±1.82, “Italian” 86.70±2.35,
242 “Slovakian” 102.96±2.93, “British” 89.79±2.38).
243 Gift tactics
244 The percentage of males producing a genuine gift varied among populations (overall model:
245 χ2 test, χ2=22.19, p=0.0083). Genuine gift production did not vary significantly with body
246 mass (χ2 test, χ2=0.05, p=0.82), while high variation among populations was present (χ2 test,
247 χ2=19.09, p=0.0008, (Fig. 1). The lowest proportion of males producing a genuine gift was
248 seen in the German population, (60%), while the highest proportion was seen in the British
249 population (97,6%) (Fig.1). We found no significant body mass and population interaction (χ2
250 test, χ2=3.08, p=0.54).
39 251 No significant population differences in the percentage of males producing a worthless nuptial
252 gift was detected (χ2 test, χ2=11.16, p=0.26, Fig. 2). No significant effect of body mass (χ2
253 test, χ2=0.1909, p=0.66) and/or population (χ2 test, χ2=6.00, p=0.20, or their interaction χ2 test,
254 χ2=4.54, p=0.33) was detected.
255 In all populations, males showed behavioural plasticity in gift production (i.e. individual
256 males capable of producing genuine and worthless gifts), with marginally non-significant
257 variation in male plasticity among populations (Fig. 3, overall model, χ2 test, χ2=16.57,
258 p=0.055), effect of population (χ2 test, χ2=7.51, p=0.11), and body weight (χ2 test, χ2=0.2,
259 p=0.88), with a significant interaction between population and body weight (χ2 test, χ2=10.60,
260 p=0.0031).
261 Some males were tested more than once per gift type (German, Italian, Danish and Slovakian
262 populations). When we analysed the relationship between plasticity and trial number, we
263 found that males were less likely to show plasticity with increasing trial number (χ2 test,
264 χ2=8.32, p=0.0039, Fig. 4). Interestingly, the more trials we conducted, the less likely we were
265 to find that males are plastic for gift type production. Since this might be due to male
266 preference for producing genuine gifts, we tested this hypothesis using a χ2 model with male
267 acceptance Y/N as response variable and gift type Genuine/Worthless as explanatory variable.
268 The overall model was significant (χ2 test, χ2=153.746, p<0.0001), with males being much
269 more likely to produce a genuine (80%) rather than a worthless gift (20%).
270 Thanatosis
271 The overall model to analyse variation in the percentage of males performing thanatosis was
272 statistically significant (χ2 test, χ2=19.51, p=0.0073). The percentage of males performing
273 thanatosis varied significantly among populations (Fig. 5, χ2 test, χ2=12.27, p=0.0081), and
274 according to male body mass, with smaller males having a higher propensity of performing
40 275 thanatosis (χ2 test, χ2=4.14, p=0.038), but not with the interaction between population and
276 body mass (χ2 test, χ2=2.46, p=0.46).
277 In each of the investigated populations, males showed behavioural plasticity in performing
278 thanatosis (Fig. 6, overall model, χ2=9.3, p=0.097). Plasticity in performing thanatosis varied
279 among populations (χ2=7.84, p=0.019), but not with body weight (χ2 test, χ2=7.84, p=0.60)
280 and the interaction between population and body weight (χ2 test, χ2=0.78, p=0.67).
281 When we analysed the relationship between plasticity and trial number, we found that males
282 were more likely to show plasticity with increasing trial number (χ2 test, χ2=19.72, p=0.0006,
283 Fig. 7). The more trials we conducted, the more likely we were to find behavioural plasticity.
284 However, we did not detect significant variation among populations (χ2=4.05, p=0.13), or
285 with body weight (χ2=2.22, p=0.12), or their interaction (χ2=1.37, p=0.24).
286
287 Discussion
288 In our study, we investigated whether variation in the frequency of behavioral reproductive
289 tactics occurs among populations of the gift-giving spider Pisaura mirabilis. Our results show
290 that males expressed both genuine and worthless gift traits are in all the studied populations,
291 and that there is variation in tactics frequency occurs among populations. We found variation
292 in the percentage of males producing a genuine nuptial gift among populations. Our study
293 however, did not detect significant variation in the worthless gift tactic among populations.
294 This could perhaps be explained by a clear difference in male propensity to produce the two
295 different gift types. We found pronounced preference for genuine gifts, with 79.89% of all
296 males producing a genuine gifts, whereas worthless items were accepted for wrapping in
297 20.11% of cases among all males (all populations). Remarkably, the worthless gift trait also
298 occurred in the Slovakian population, where these were not previously reported (Prokop and
41 299 Maxwell, 2012). Frequencies of a tactic might vary to the extent of this tactic being displayed
300 by the largest majority of males in a population, with extremely few individuals performing
301 alternative tactics (Plaistow et al., 2004; Hendrickx, Vanthournout and Taborsky, 2015). The
302 relatively low incidence of worthless gift production may explain why previous studies on the
303 Slovakian population detected males exclusively carrying genuine fresh gifts (Prokop and
304 Maxwell, 2012). We also investigated whether male weight, as a proxy for male body
305 condition, affects male tactic, as males in good condition could be expected to be more likely
306 to produce a genuine gift. However, we did not find variation in the frequency of genuine and
307 worthless gifts to be explained by variation in male body weight. The choice of nuptial gift
308 tactic may therefore rely on other factors, possibly extrinsic to individuals, rather than
309 representing a condition-dependent tactic.
310 Genuine and worthless gift tactics were found to be plastic labile traits, which males can
311 alternatively and reversibly adopt (Oliveira, Taborsky and Brockmann, 2008; Taborsky and
312 Brockmann, 2010). Behavioural plasticity for nuptial gift production was indeed present in all
313 the populations included in our study. However, when investigating whether behavioural
314 plasticity varied among populations, we did not detect any significant relationship. However,
315 this could be because of lack of power, as population varied marginally non-significantly
316 between 10 and 30%. The stronger preference in males producing a genuine gift, is likely to
317 affect the expression of plasticity. Further support to this hypothesis is provided by the fact
318 that males did not show increasing plasticity in producing both gift types with additional
319 trials. In our study indeed, 77.5% of males tested never produced a worthless gift, while only
320 19.05% never produced a genuine gift. Among males that only produced a single gift type, the
321 percentage of males exclusively producing a worthless gift was 1.05%, while males chose to
322 solely produce a genuine gift in 64.01% of cases (Table 1). These numbers suggest that males
42 323 are much more reluctant to produce worthless than genuine gifts, perhaps suggesting that
324 worthless gift production is an opportunistic behaviour.
325 Our results also showed that males perform thanatosis in all studied population, and the
326 percentage males performing thanatosis varies among populations. Differently from the
327 nuptial gift tactics, we found some evidence for male body mass to affect thanatosis
328 performance, with lighter males being more likely to perform this behavioural tactic.
329 Although our results are not very conclusive on this point, it is possible that smaller or lighter
330 males use this tactic towards resistant females (Hansen et al 2008), where larger and heavier
331 males are able to directly overcome female resistance. Individual body condition has been
332 identified as an important factor in performing fitness enhancing behaviours (Lorch et al.,
333 2003; Webb, Shine and Christian, 2006), particularly when competing for mates (Burkhardt,
334 de la Motte and Lunau, 1994; Wiklund and Kaitala, 1995). Here we seem to have an opposite
335 effect, supporting the idea that thanatosis may indeed be an alternative reproductive tactic.
336 We detected variation in plasticity of performing thanatosis among populations, but we found
337 no effect of body mass on male plasticity for thanatosis. In contrast to plasticity in gift tactic,
338 thanatosis plasticity increased with number of trials. This shows that all males are capable of
339 performing thanatosis. The fact that not all males perform thanatosis, despite the benefits of
340 this behaviour (Bilde et al 2006), may nevertheless suggests costs associated to the trait‟s
341 expression. The expression of nuptial gift tactics in the field correlates with environmental
342 conditions that include demographic densities, with higher nuptial gift occurrence positively
343 correlating with increasing number of available females and increasing prey availability
344 (Chapter II). Since thanatosis has been proposed as a male mating effort (Bilde et al 2006),
345 and costs are likely associated to the trait, such tactic might be strategically displayed
346 depending on female densities. Under high competition for mates, males may be more likely
347 display a trait that increases mating success (Yoccoz et al., 2002; Oliveira, Taborsky and
43 348 Brockmann, 2008). Note, however, that thanatosis is performed in direct response to
349 encounters with females, in contrast to gift production, where gifts are produced prior to
350 encounters with females. While gift tactic may be expressed in response to extrinsic factors
351 (Chapter II), it is perhaps less likely that this is the case for thanatosis.
352 The fact that there is variation in tactic expression and also in plasticity of trait expression
353 among populations, suggests that populations vary in ecological or environmental factors that
354 may influence trait expression. (Oliveira, Taborsky and Brockmann, 2008; Taborsky and
355 Brockmann, 2010; Monroe et al., 2016). While we have some evidence that intrinsic factors
356 (male body weight) might influence the expression of tactic, other studies have recovered
357 little evidence for condition-dependent ARTs in P. mirabilis (Chapter II and III). A
358 population genetic study of P. mirabilis populations showed very little genetic variation
359 among populations (Lauridsen 2015). It is therefore unlikely that population differences are
360 genetically based. We showed in Chapter II that tactic expression within a population varies
361 with environmental factors along the reproductive season. If populations vary in ecological
362 and environmental factors that influence tactic expression, this could explain the population
363 differences observed in this study.
364
365
44 366 Figures and tables
367 Table 1- Percentages of males wrapping a genuine or a worthless gift. In line 1 is reported the percentage of males
368 that produced one or the other gift when provided with the possibility to produce a nuptial gift. Line 2 reports the
369 percentage of males never producing one or the other gift among all trials. Line 3 reports the percentage of males that
370 produced exclusively one gift type.
371
Genuine Worthless
Percentage of males producing a gift 79.89 20.11
Percentage of males never producing a gift type 19.05 77.5
Percentage of males exclusively producing a 64.01 1.05
single gift type
372
373
45 374 Figures
375
376 Fig.1-Percentage of males wrapping a genuine gift in different populations. Males differ significantly among
377 populations in producing a genuine nuptial gift
378
379
380 Fig.2-Percentage of males wrapping a worthless gift in different populations. Males did not differ significantly among
381 populations in producing a worthless nuptial gift
382
46 383
384 Fig.3-Percentage of males producing both nuptial gift among populations. Males did not differ significantly in
385 showing plasticity of producing both nuptial gifts among populations
386
387
388
389 Fig. 4-Percentage of males producing both nuptial gift in relation to trial number.
390
47 391
392 Fig. 5-Percentage of males performing thanatosis in different populations. Males differ significantly in adopting a
393 “death feigning” behaviour among populations.
394
395
396 Fig. 6-Percentage of males that were plastic for thanatosis among populations.
397
48 398
399 Fig. 7-Percentage of plastic males performing thanatosis in relation to trial number.
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55
Chapter II
Resource availability, mating opportunity, and sexual selection intensity influence the expression of male alternative reproductive tactics
Ghislandi P.G., Pekár S., Matzke M., Schulte-Doinghaus S., Bilde T. & C. Tuni
Re-submitted to The Journal of Evolutionary Biology
56 1 Resource availability, mating opportunity, and sexual selection intensity influence the
2 expression of male alternative reproductive tactics
3
4 Paolo Giovanni Ghislandi* a, Stano Pekár b, Magdalena Matzke c, Sarah Schulte-
5 Doinghaus c, Trine Bilde a✚, Cristina Tuni c✚
6
7 a Department of Bioscience, Aarhus University, Aarhus, Denmark
8 b Department of Botany and Zoology, Masaryk University, Brno, Czech Republic
9 c Department of Biology, Ludwig Maximilian University, Munich, Germany
10 * Corresponding author: [email protected]; Ny Munkegade 116, 8000 Aarhus C,
11 Denmark
12 ✚ These authors contributed equally.
13
14 Running title: Alternative mating traits in gift-giving spiders
15
16
17 Acknowledgements
18 We thank Alexander Hutfluss for valuable help in collecting spiders, André Walter and 19 Frederik Hendrickx for thoughtful discussion, and Niels Dingemanse for logistic support. We 20 are grateful for helpful comments on a previous draft of the manuscript from Caitlin Stern, 21 Jessie Barker, André Walter and Thomas Rejsenhus. We also thank Rainar Nitzsche for 22 providing useful information. The study was supported by The Danish Council for 23 Independent Research grant number 4002-00328B to TB.
24
25
26 Conflict of interests
27 There are no conflict of interests for the present manuscript
57 28 Abstract
29 Variation in the expression of plastic alternative reproductive tactics may depend on
30 environmental cues that vary spatially or temporally. For example, variation in resources such
31 as food or competition for mates is expected to influence the pay-off of each tactic and shape
32 the decision of which tactic to employ. Males of the nuptial gift-giving spider Pisaura
33 mirabilis can adopt three tactics: offering a genuine prey gift, a ‘worthless’ non-nutritious
34 gift, or no gift. We propose that food resources and/or male body condition, mating
35 opportunities, and sexual selection intensity, vary over the course of the mating season to
36 shape the expression of alternative traits. We quantified these variables in the field over two
37 seasons, to investigate the correlation between environmental variables and tactic as the
38 mating season progresses. We hypothesize that 1) males become more likely to employ a gift-
39 giving tactic, and 2) the likelihood of switching from worthless to genuine gifts increases with
40 the availability of prey and /or mates. Prey availability increased over the season and co-
41 varied with the propensity of males to employ the gift-giving tactic, but we found no support
42 that males in good condition are more likely to produce genuine gifts. Males responded to an
43 increase in female availability by increasing their mating effort (gift production). The
44 frequency of genuine gift use increased with the progress of season, which is consistent with
45 increasing sperm competition intensity with time. Our results suggest that the frequency of
46 alternative tactics is shaped by seasonal changes in ecological factors and possibly post-
47 copulatory sexual selection. This leads to relaxed selection for the gift-giving tactic early in
48 the season when females are less choosy and resources more scarce, and increased selection
49 for genuine gifts later in the season driven by mating opportunity and risk of sperm
50 competition.
51
52 Key words: alternative reproductive tactics, nuptial gifts, opportunistic mating, sexual
53 selection, prey availability, female availability
58 54 Introduction
55
56 Intra-sexual competition over mate acquisition generates strong selection on morphological
57 and/or behavioural traits that increase male reproductive success, and is an important factor in
58 promoting phenotypic variation in sexual traits (Cogliati et al., 2014; Conroy & Gray, 2014;
59 Fraser et al., 2014; Outomuro et al., 2014). Phenotypic variation in a sexual context may
60 result in alternative reproductive tactics (ARTs), where several phenotypes are expressed
61 simultaneously in a population (Emlen, 2008; Oliveira et al., 2008; Outomuro et al., 2014).
62 ARTs can be genetically determined (Moczek & Emlen, 2000; Hunt & Simmons, 2001), or
63 context-dependent (plastic) and expressed in response to external or internal conditions
64 (Brockmann 2001; Smith et al. 2013). The maintenance of ARTs within populations is often
65 ascribed to frequency dependent selection or density dependence (Oliveira et al., 2008). For
66 example, a model developed by Hendrickx et al. (2015) showed that a high-investment male
67 phenotype is constrained in mating opportunities, because the costs of developing this
68 phenotype leads to extended development time and maturation later in the season. This
69 maturation delay leads to a vacant mating niche in the beginning of the mating season, which
70 favours a low-investment male phenotype that matures early to exploit the availability of
71 unmated females. The high-investment and low-investment strategies are maintained by
72 negative frequency-dependent selection (Hendrickx et al., 2015). Variation in ecological
73 factors is expected to play a key role in shaping the evolution and maintenance of alternative
74 traits, because changes in the environment alter the costs and benefits associated with trait
75 expression (Emlen, 2008; Oliveira et al., 2008). This may be particularly relevant for plastic
76 and simultaneous tactics, where fluctuating resource availability influences trait expression
77 (Oliveira et al., 2008; Bell, 2010).
78 The use of nuptial gifts in the form of nutritious donations offered to females to
79 facilitate male reproductive success is a common strategy, particularly in insects and spiders
59 80 (Boggs, 1995; Vahed, 1998; Lewis & South, 2012). Males of the nuptial gift-giving spider
81 Pisaura mirabilis (Clerck, 1757) adopt three different strategies: they can offer a genuine
82 nuptial gift consisting of an insect prey wrapped in silk (Bristowe & Locket, 1926; Nitzsche,
83 1988; Bilde et al., 2007), they can offer a ‘worthless’ gift consisting of prey remains, empty
84 exoskeletons, or even plant parts wrapped in silk (Nitzsche, 1988; Albo et al., 2011b;
85 Ghislandi et al., 2017), and they can mate without offering any gift at all (Stålhandske, 2001;
86 Albo et al., 2011b). Females exert strong preference for males that offer a nuptial gift
87 (Stålhandske, 2001; Albo et al., 2011b). While the donation of a nuptial gift dramatically
88 increases male mating success, gift production is likely to be costly (Simmons, 1990). This
89 could be, for instance, through increased exposure to predators while hunting for a prey, use
90 of venom to kill the prey, metabolic effort involved in prey capture and gift construction, lost
91 foraging opportunities, or energetic costs of carrying the gift while searching for a mate.
92 These potential costs might explain the coexistence of gift and no-gift tactics in the
93 population.
94 Interestingly, males that offer worthless gifts are equally likely to be accepted by
95 females as males offering a genuine gift, at least when females are unmated (Albo et al.,
96 2011b). If worthless gifts are less costly to produce, this should favour the worthless gift-
97 giving tactic in the population, as seen in dance flies (Empididae) (Preston-Mafham, 1999;
98 LeBas & Hockham, 2005). However, both gift-giving strategies are observed, and this is
99 likely because males with worthless gifts experience shorter copulations, as the female
100 interrupts the copulation sooner, which results in reduced sperm transfer and fertilization
101 success (Albo et al., 2011b). Since females are polyandrous (Tuni et al., 2013) the cost of
102 offering worthless gifts to mated females is amplified, as these males lose out in sperm
103 competition to males that offer genuine gifts and thereby acquire longer copulations to
104 transfer more sperm. This effect exerts selection for the genuine gift-giving tactic.
105 Interestingly, we observe all three mating tactics simultaneously in any given population
60 106 (Albo et al 2011; XY, unpublished data), prompting the question of what maintains the co-
107 existence of alternative strategies in this system (Ghislandi et al., 2014). Preliminary data
108 suggests that ARTs of P. mirabilis are plastic, as individual males are capable of producing
109 both genuine and worthless gifts (XY, unpublished data). It is therefore possible that males
110 switch among tactics in response to environmental or intrinsic factors, in order to adopt the
111 most advantageous tactic at any given time (Maynard Smith, 1982; Gross, 1996). The
112 interplay between environmental and/or intrinsic factors may result in differential pay-offs for
113 each of the male mating tactics at different times of the season thereby promoting the
114 simultaneous expression of tactics (Ghislandi et al., 2014; Hendrickx et al., 2015; Taborsky
115 and Brockmann 2010).
116 A number of external factors can influence gift tactic: Gift occurrence (gift vs. no gift) -
117 When considering the occurrence of a gift tactic (any gift, genuine or worthless) compared to
118 a no gift tactic, one possibility is that construction of a nuptial gift may depend on prey
119 availability, which provides males with the opportunity for gift construction. This generates
120 the expectation that the proportion of males producing any gift (genuine or worthless)
121 increases over the course of the mating season because prey becomes more abundant (Hatley
122 & Macmahon, 1980; Clarke & Gaston, 2006), and prey acquisition would require lower
123 effort. If gift construction is condition-dependent (Albo et al., 2011a), and male condition
124 improves with increasing prey availability over the season, we would expect a higher
125 frequency of the no-gift strategy early in the season, as only males in good condition can bear
126 the cost of producing a gift. If gift construction depends on sexual selection intensity, we
127 expect that the no-gift tactic is predominantly used early in the mating season. Firstly, because
128 sperm competition is lower at the beginning of the season, and should favour low sperm
129 investment tactics (Parker & Pizzari, 2010). Females are known to mate with multiple males
130 in the wild and broods often represent at least 4 sires (Lauridsen et al., 2016), justifying the
131 assumption that sperm competition increases as the season progresses. Second, preliminary
61 132 data show that only virgin females accept males courting without a gift (XY, unpublished
133 data), suggesting a possible role for variation in female choice over the season in influencing
134 tactic use.
135 Gift type (genuine vs. worthless gifts) - Once males construct a nuptial gift, they can produce
136 either a genuine or worthless gift. We expect the proportion of males producing worthless
137 gifts to be higher earlier in the mating season. When prey availability is low, males may face
138 greater effort in prey capture and rely on prey for their own feeding prior to gift construction.
139 Moreover, males may be in lower body condition and unable to bear the costs of genuine gift
140 construction (Albo et al., 2011a). Finally, post-copulatory sexual selection is expected to
141 increase as the mating season progresses, creating selection on males to offer genuine gifts
142 that facilitate longer sperm transfer and improved performance in sperm competition
143 (Drengsgaard & Toft, 1999).
144 To examine how male reproductive tactics vary in relation to several ecological factors
145 we performed a 2 year field study where we i) determined the relative use of three male
146 tactics (no gift, genuine gift, worthless gift) over the mating season, and ii) investigated how
147 male tactic expression co-varies with prey availability, male body condition, and intensity of
148 sexual selection (measured as operational sex ratio (OSR), and changes in the availability of
149 adult females) over the season (Ghislandi et al., 2014). Overall these data allow us to
150 investigate 1) whether males become more likely to employ a gift-giving tactic compared to
151 no-gift as the season progresses, and 2) whether the likelihood of switching from worthless to
152 genuine gifts increases as the mating season progresses.
153
154 Materials and Methods
155 Pisaura mirabilis
156 The nursery-web spider Pisaura mirabilis belongs to the Family Pisauridae (World Spider
157 Catalog) and has a Paleartic distribution. The species typically occurs in grassy meadows and
62 158 forest areas and is a generalist predator, feeding on arthropod prey (Nentwig, 1986). The life
159 cycle is annual (Bristowe & Locket, 1926), with spiderlings hatching from late spring to early
160 summer depending on the latitude (XY, pers. obs.), and reaching adulthood the following
161 spring. The mating season occurs in May-June and females produce eggsacs in June-July,
162 which they carry in their mouthparts. Prior to hatching, females produce a tent shaped
163 nursery-web, where they suspend the eggsac and guard their brood until dispersal.
164
165 Field-collected spiders and nuptial gifts
166 Pisaura mirabilis spiders were collected in XY, during spring 2015 and 2016 over the
167 duration of the mating season, from the 11th of May to the 12th of June in 2015 and from the
168 21st of May to the 22nd of June in 2016. Spiders were primarily collected from grassy
169 meadows and forest edge, consisting mainly of Urtica spp. bushes. On each day of collection,
170 all specimens found were caught and placed in transparent plastic vials (10 x 4 cm or 10 x 2
171 cm) closed with foam lids. Once collecting ended, we scored the developmental stage and sex
172 of all individuals (juveniles, subadult males and females, and adult males and females) and
173 counted the total number of individuals collected per stage. If adult males carried a nuptial
174 gift, the gift was immediately removed using forceps, placed in an Eppendorf tube, and
175 tagged with the male ID. Males and gifts were brought to the laboratory at XY University,
176 XY. Juveniles, subadult and adult females were released after identification. A few
177 individuals that escaped during the survey were counted if their sex and life history stage
178 could be correctly identified. We avoided the risk of multiple counting by sampling in one
179 direction only.
180
181 Determination of gift type
182 In the laboratory, nuptial gifts from 2015 were opened under a Zeiss stereomicroscope W-PI
183 10 x 23 binocular using a Zeiss KL 1500 LCD light by gently removing the silk cover with
63 184 micro-dissecting forceps. Gift content was inspected and the items found inside counted. The
185 gift was defined as genuine if it contained fresh and intact prey, and worthless if it contained
186 dried prey or prey leftovers (i.e. pieces of body parts or exoskeletons), seeds, pollens or plant
187 parts. Gifts that contained more than one prey were classified as genuine when at least one of
188 the items consisted of fresh prey.
189 Nuptial gift type in 2016 was determined based on their weight, as we used data from 2015 to
190 estimate the relationship between gift type (genuine or worthless) and gift weight using a logit
191 linear calibration model fitted by GLM with binomial error structure. We determined the
192 threshold between genuine and worthless gifts based on 155 field-collected gifts (93 from
193 2015 and 62 collected from other experiments) that were weighed and opened. The obtained
194 model of the form ln(p/1-p)=1.983+-0.287weight, where p is the probability of genuine type,
195 estimated a 50% probability of the genuine gift type to be at 6.9095 mg. Thus, gifts heavier
196 than this value were considered genuine, and gifts lighter than this value were considered
197 worthless.
198
199 Male condition
200 In the laboratory, male body mass was measured using a digital scale KERN PKT (KERN-
201 SOHN GmbH, Germany) accurate to 0.001 g. Cephalothorax width was measured using a
202 digital calliper (AEROSPACE, China) accurate to 0.01 mm by gently covering males with a
203 transparent plastic sheet or a mosquito net to immobilize them. The measure was taken three
204 times per male. To evaluate male body condition we calculated Residual Body Index (RBI),
205 by saving residuals from a simple linear regression of body weight on mean cephalothorax
206 width, calculated on the three repeated measures (Jakob et al., 1996). All males were released
207 either immediately or, on a few occasions on the following day, in the same location as they
208 were collected.
209
64 210 Prey availability
211 To measure prey availability in the mating season of 2015, we used three different capture
212 methods: pitfall traps, sticky sheets, and sweeping net. In 2016, we did not use sweep nets as
213 sweeping yielded the lowest prey capture.
214 Pitfall traps were used to catch prey moving on the ground. They consisted of plastic cups
215 (0.2 l, 10x7 cm) placed approximately 10 m from each other, covered by a plastic mesh (grid
216 6.44 mm x 12.81mm). Each cup was placed in a hole dug in the ground and filled with a
217 solution of water and liquid soap that reduces the water surface tension, preventing arthropods
218 from walking on the fluid and escaping the trap. Due to its size, the plastic mesh prevented
219 larger organisms from falling in while permitting relevant-sized prey to slide down the waxed
220 knitting. We placed a total of 72 pitfall traps in 2015 and 40 in 2016. This was because in
221 2015 we sampled both a forest and a meadow area, but found that spiders were mainly present
222 in the meadow, therefore the forest area was not sampled in 2016. Traps were replaced every
223 2-3 days in 2015, and every 4 days in 2016. Sticky sheets were used to catch flying prey.
224 They consisted of transparent A4 format sheets placed approximately 5 m from one another.
225 Each sheet was covered with a thin layer of odourless and pheromone free, transparent,
226 weather-resistant insect-glue (Temmen Insekten-Leim, Temmen GmbH, Germany). In the
227 field, they were hung on branches of bushes and trees (1.5 m above the ground on average)
228 with sisal ropes (3 mm diameter) fixed on two pierced holes in the corner of the sheet.
229 Removed sticky traps were covered with clean wrapping plastic film, brought to the
230 laboratory and frozen until the content was scored. We placed a total of 74 sticky sheets in
231 2015 and 32 in 2016 (see explanation above).
232 Sweeping was used to catch prey sitting on grass blades or flying immediately over the
233 vegetation level and was performed with a sweeping net (32 cm diameter, 72.5 cm length,
234 grid 0.72 mm x 0.72 mm). We used a standardized procedure consisting of sweeping once
235 every step moved, for a total of five sequential steps walking in one direction. This was
65 236 repeated three times. The net content was emptied in plastic bags and prey samples frozen
237 until identification was performed. We swept a total of 70 times.
238 All collected arthropods were sorted and identified to order in the laboratory using a
239 Zeiss stereomicroscope W-PI 10 x 23 binocular and a Zeiss KL 1500 LCD light. Arthropods
240 groups scored as prey were Diptera, Coleoptera, Hymenoptera, Hemiptera, Arachnida,
241 Lepidoptera and minor Pterygota groups. We excluded from our analysis the groups that are
242 known not to represent a trophic resource for Pisaura mirabilis, such as Mollusca, Chilopoda,
243 Myriapoda, Odonata, Hymenoptera (Formicidae), and Hemiptera (Aphididae) (Toft, 1997;
244 Bilde & Toft, 2001). We discarded Isopoda in the 2015 analysis as we did not find them
245 wrapped in nuptial gifts, but we considered them in 2016 as they occurred in a few cases as
246 wrapped prey.
247
248 Measures of sexual selection intensity
249 To evaluate the relationship between operational sex ratio and gift-trait, we measured the
250 Operational Sex Ratio (OSR) on each day of sampling, calculated by dividing the total
251 number of adult males by the sum of adult males and females, only including individuals with
252 reproductive capability. Therefore, adult females with eggsacs or with a nursery-web were
253 excluded from the analysis, since these females are not sexually receptive while taking care of
254 their eggs/broods (Nitzsche, pers. comm.). We also calculated how the change in availability
255 of mature females over the season co-varied with male reproductive tactic, by estimating the
256 proportion of adult females relative to the sum of adult females plus sub-adult females during
257 the mating season.
258
259 Statistical Analyses
260 Prey availability, male condition, sexual selection intensity over the mating season
66 261 We first explored how prey availability, OSR, proportion of maturing females, proportion of
262 males with a gift (N males with a gift/total number of males), and proportion of worthless
263 gifts (N males with a worthless gift/total number of males with a gift) varied over the mating
264 season using Generalized Additive Models (GAM), that allow fitting both linear and non-
265 linear relationships to temporal trends (Wood, 2006). When investigating seasonal trends,
266 because the effects of many (not measured) environmental variables, such as temperature,
267 precipitation, sun isolation, etc., fluctuate and have strong effect on the study response
268 variables, smoothed curves to temporal trends (non-linear relationships) provide a better fit.
269 Thin-plate splines with automatic selection of their dimension were used with non-linear
270 terms. Depending on the distribution and variance-to-mean ratio of the response variable, we
271 used Gamma error structure (GAM-g) to analyse effects of season on amount of prey, and
272 binomial errors (GAM-b) for OSR, proportion of maturing females, proportion of males with
273 a gift, and proportion of worthless gifts.
274 When investigating variation of Residual Body Index (RBI) over the season we conducted a
275 GAM model using Gaussian errors (GAM), with both season and prey availability as
276 explanatory variables. We also used standard linear model (LM) to explore the effect of prey
277 availability on RBI.
278
279 Proportion of males with a gift (genuine or worthless), and with a worthless gift
280 To explore the relationships between prey availability, OSR and proportion of mature females
281 respectively with the proportion of males with a gift and with the proportion of males with a
282 worthless gift we used Generalized Linear Models (GLMs) with binomial errors structure.
283
284 Analyses were performed separately for each year because there were major differences
285 in prey number and other factors among years, but the effect of year per se was not an
286 objective of the study. Adequacy of models was evaluated by assessing the distribution of
67 287 residuals and homogeneity of variance using standard diagnostic tools. All analyses were
288 performed in R 3.0.0 (R Core Team 2015) using functions from the mgcv package (Wood,
289 2006).
290
291 Results
292 In total we collected 2511 spiders, 1327 in 2015 and 1184 in 2016, of which 387 were adult
293 males in 2015 and 241 in 2016 (Figure S1). In 2015, 93 carried a gift (24%), of which 42
294 were worthless (45.1%), while in 2016, we collected 56 males carrying a gift (23.2%) of
295 which 25 were classified as worthless (44.6%).
296
297 Prey availability, male condition, sexual selection intensity over the mating season
298 We caught, counted and identified 14729 arthropods, 12851 collected in 2015 and
299 1878 collected in 2016. In 2015, there was on average 5.4 times more prey per male available
300 than in 2016 excluding data from sweep nets in 2015. In 2015, the amount of prey (suitable
301 prey/male/day) increased significantly during the mating season (GAM-g, F1,19 = 4.6, P =
302 0.045, Figure S2a). In 2016, the effect of season on prey availability was also significant, but
303 showing a different pattern, prey availability fluctuated and increased only slightly over
304 season (GAM-g, F5.8,6.0 = 3.7, P = 0.02, Figure S2b).
305 ‘Season’ (time) had significant effect on Male Residual Body Index (RBI) in both
306 years. In 2015 RBI fluctuated with no clear trend with the progress of the mating season
307 (GAM, F7.2,8.2 = 6.2, P < 0.0001, Figure S3a). In 2016 a slightly negative overall relationship
308 was present (GAM, F6, 7.2 = 6.9, P < 0.0001, Figure S3b), with male body condition
309 decreasing as the season progressed. Male RBI increased significantly with prey availability
310 in 2015 (LM, F1,378 = 3.8, P = 0.02, Figure S4a), and increased significantly initially with
311 prey availability and then decreased in 2016 (LM, F1,173 = 16.5, P < 0.0001, Figure S4b).
68 312 In 2015, OSR (the number of adult males divided by the sum of adult males and
313 females) became more female-biased over the mating season but showing a fluctuating
2 314 pattern (GAM-b, X 4.75 = 19.6, P = 0.003, Figure S5a). In 2016, no significant relationship
2 315 between season and OSR was found (GAM-b, X 5.6 = 11.1, P = 0.12, Figure S5b). The
316 proportion of maturing females showed a significant increase over the season in both years
2 2 317 (2015: GAM-b, X 3.8= 5.8, P = 0.0035, Figure S6a; 2016: GAM-b, X 2.2 = 18.7, P < 0.0001,
318 Figure S6b).
319
320 Proportion of males with a gift (genuine or worthless)
321 We found a significant increase over time in the proportion of males carrying a gift in 2015
2 322 (GAM-b, X 1 = 6.6, P = 0.018, Figure 1a) and in 2016 (GAM-b, F1,16 = 5.2, P = 0.04, Figure
323 1b). The proportion of males carrying a gift and prey availability were significantly positively
2 324 related in 2015 (GLM-b, X 1 = 7.7, P = 0.005, Figure 2a), whereas we did not observe the
2 325 same relationship for 2016 (GLM-b, X 1 < 0.1, P = 0.95, Figure 2b). The proportion of males
326 carrying a gift was not significantly affected by male body condition in 2015 (GLM-b, F1,86 =
327 0.4, P = 0.52) but it was negatively affected in 2016 (GLM-b, F1,159 = 5.4, P = 0.021; Figure
328 S7). We found no effect of OSR on the proportion of males carrying a gift for both 2015
2 2 329 (GLM-b, X 1 = 1.5, P = 0.21) and 2016 (GLM-b, X 1 < 0.5, P = 0.50). We found a positive
330 significant relationship between the proportion of males carrying a gift and the proportion of
2 331 maturing females, for both 2015 (GLM-b, X 1 = 4.3, P = 0.03, Figure 3a) and 2016 (GLM-b,
2 332 X 1 = 4.4, P = 0.036, Figure 3b).
333
334 Proportion of males with a worthless gift
335 In 2015, the proportion of males carrying a worthless gift varied over time (GAM-b, F1.9,2.0 =
336 7.0, P = 0.043, Figure 1c): early in the season the proportion was relatively high, then it
337 declined with males offering proportionally more genuine gifts, and at the end of season the
69 338 proportion of worthless gifts again increased. In 2016, the proportion of worthless gifts had a
339 rather similar U-shaped pattern (GAM-b, F1.9,2.0 = 7.5, P = 0.041, Figure 1d).
340 For the mating season of 2015, we found a significant positive relationship between
2 341 the proportion of males carrying a worthless gift and prey availability (GLM-b, X 1 = 4.7, P =
342 0.03, Figure 2c), while this relationship was not statistically significant for 2016 (GLM-b,
343 F1,12 = 0.2, P = 0.67, Figure 2d). RBI did not predict the type of gift used by males either in
344 2015 (GAM-b, F1,86 = 0.4, P = 0.52) or in 2016 (GAM-b, F1,41 = 3.7, P = 0.063). We also
345 found a significant positive effect of OSR on the proportion of males carrying a worthless gift
346 for 2015, with a higher proportion of males carrying these donations as the relative number of
2 347 males increased (GLM-b, X 1 = 5.5, P = 0.019, Figure 4a). However, we did not detect the
348 same relationship for 2016, where the proportion of males carrying a worthless gift was not
349 significantly affected by OSR (GLM-b, F1,15 = 0.2, P = 0.63, Figure 4b). A significant
350 negative relationship between the proportion of males carrying a worthless gift and the
2 351 proportion of mature females was present in 2015 (GLM-b, X 1 = 4.8, P = 0.029, Figure 3c),
352 but this relationship was not significant for 2016, although a similar trend existed (GLM-b,
353 F1,12 = 1.7, P = 0.22, Figure 3d).
354
355 Discussion
356
357 Our result reveal that variation in prey availability, female availability, OSR and male body
358 condition show a degree of correlation with the proportion of males with a gift or a worthless
359 gift, suggesting that variation in external factors may influence the expression of male tactics.
360 This can happen if variation in resource availability, mating opportunity, and sexual selection
361 intensity over the mating season generates variation in the pay-off of different male
362 reproductive tactics, which leads to shifts in the relative expression of different tactics over
363 the season (Ghislandi et al., 2014).
70 364 We found a tendency for prey availability to increase over the mating season, in
365 accordance with the expectation of seasonal variation in temperate environments (Hatley &
366 Macmahon, 1980; Clarke & Gaston, 2006). Prey availability could influence male gift-use in
367 two ways. First, if male body condition depends on prey availability, and gift production is a
368 condition dependent trait, we expect body condition (Residual Body Index, RBI) to improve
369 with increasing prey availability, and the likelihood of males producing a nuptial gift to
370 increase with increasing RBI. We found a tendency for RBI to increase with prey availability
371 in both years (only initially for 2016, Figure S4), however, this did not translate into a general
372 improvement of male body condition over the mating season (Figure S3). In contrast to the
373 condition dependent model, we found no relationship (2015), and a negative relationship
374 82016) between the expression of nuptial gifts in the field and male RBI (Figure S7). So
375 while there is some evidence for prey availability to influence RBI positively, we have no
376 evidence that gift production is related to good body condition. If anything, we find the
377 opposite effect that males in lower body condition are more likely to produce a nuptial gift
378 (2016). Secondly, prey availability could directly influence the likelihood of obtaining a
379 nuptial gift, as we expect the effort of gift acquisition to decrease when prey becomes more
380 abundant. The data from 2015 showed that the likelihood of finding a male with a nuptial gift
381 was positively correlated with prey availability (Figure 2a). This result supports the prediction
382 that opportunity, in terms of prey availability, influences male tactic at least in some years.
383 The likelihood of prey capture and gift production may therefore co-vary, so increasing prey
384 availability over the mating season reduces the effort of acquiring a gift, thereby favouring the
385 gift-giving tactic.
386 We also expected that variation in prey availability influences which gift type (genuine
387 or worthless) males produce. We hypothesized that under low prey availability, there is higher
388 incentive to partially consume the prey and use the remains as a worthless gift, whereas males
389 would shift to producing genuine gifts when prey availability increases (Ghislandi et al.,
71 390 2014). However, the proportion of genuine gifts did not increase with prey availability, in
391 contrast the likelihood of producing a worthless gift increased with prey availability in 2015
392 but not in 2016 (Figure 2c,d). Overall, it seems that while opportunity in terms of prey
393 availability influences the likelihood of producing a gift (any gift), apparently it does not
394 predict the increased production of genuine nuptial gifts. This leads us to suggest that prey
395 availability does not have major influence on use of nuptial gift tactic.
396
397 In addition to food resources, we proposed that sexual selection plays an important role for
398 which tactic a male employs (Ghislandi et al., 2014). We assume that post-copulatory sexual
399 selection intensifies as the mating season progresses, because mated females become more
400 resistant to additional matings (Halliday, 1983; Gabor & Halliday, 1997; Bateman et al.,
401 2001; Tuni & Bilde, 2010), and this resistance may be overcome by males offering nuptial
402 gifts (Albo et al., 2011b; Hendrickx et al., 2015; Toft & Albo, 2016). Indeed, males were
403 increasingly more likely to carry a nuptial gift as the season progressed. Males without gifts
404 occurred at higher frequency early in the season, where it is possible to mate with a virgin
405 female without a gift (Stålhandske, 2001). We note that these males cannot necessarily all be
406 assumed to have chosen a no-gift tactic, as it is possible that some males are not yet ready to
407 mate, or that a male that recently mated lost his gift to the female. Nevertheless, this pattern is
408 consistent with the prediction that the use of the no-gift tactic is only successful early in the
409 season. Later in the season, the risk of sperm competition imposes strong selection on the gift-
410 giving trait (Parker & Pizzari, 2010), as males with a nuptial gift experience substantially
411 longer copulations than males with no gift (Stålhandske, 2001; Albo et al., 2011b). This is
412 because copulation duration is determined by the time it takes females to consume the gift
413 (Lang, 1996; Stålhandske, 2001), and it takes longer time to consume a genuine gift than a
414 worthless gift (Albo et al., 2011b).
72 415 We also found that males were more likely to carry a nuptial gift when the likelihood of
416 encountering a female increased. This indicates that males respond to female availability by
417 increasing their effort on gift construction. To adjust tactic according to female density in the
418 environment, males must be capable of assessing fluctuations in the number of potential
419 mates. Males spiders often use chemical cues produced by females and deposited on silk to
420 find their mates (Schulz & Toft, 1993; Roberts & Uetz, 2005; Wyatt, 2014). We know from
421 laboratory trials that male P. mirabilis respond to cues in female silk by displaying
422 behavioural sexual excitement and initiate the construction of nuptial gifts (Ghislandi et al.,
423 2017). Higher female densities in the field are likely to increase the intensity of sexual
424 chemical cues, providing a cue for males to assess female availability and/or female mating
425 status (virgin or mated) (Maklakov et al., 2003; Gaskett, 2007; Tuni & Berger-Tal, 2012).
426 Finally, we predicted the use of genuine over worthless gifts to increase as the season
427 progresses, in response to increased risk of sperm competition. Pisaura mirabilis females are
428 polyandrous (Tuni & Bilde, 2010; Tuni et al., 2013); microsatellite genotyping of offspring
429 showed that wild caught females could mate with at least four males (Lauridsen, 2016), and in
430 the laboratory females accepted up to 16 males depending on their nutritional status (Toft &
431 Albo, 2015). This degree of polyandry imposes huge sperm competition risk on males,
432 favouring strategies that prolong copulation duration and maximize sperm transfer (Parker,
433 1970; Simmons, 2001; Parker & Pizzari, 2010). In P. mirabilis, copulation duration correlates
434 positively with the amount of sperm transferred (Drengsgaard & Toft, 1999; Albo et al.,
435 2013). Female nuptial gift consumption was shown to determine copulation duration (Lang,
436 1996; Stålhandske, 2001), and copulation duration was significantly longer when males
437 offered genuine gifts compared to worthless gifts (Albo et al., 2011b). In accordance with a
438 response to increased sperm competition, we found an increase in the frequency of genuine
439 gifts over the season. Worthless gifts were relatively more frequent early in the season when
73 440 virgin females are available, while the frequency of genuine gifts increased as the mating
441 season progressed.
442
443 The co-existence of ARTs in P. mirabilis
444 We found that variation in resources affects the propensity of males to employ the gift-giving
445 strategy, as increasing prey availability over the season in one year co-varied with the
446 frequency of males producing a gift. However, we found no support for gift production to be a
447 condition-dependent trait in the sense that males in good condition are more likely to produce
448 a gift, in contrast, the result for 2016 suggest an opposite effect. Since we did not detect a
449 correlation between RBI and the probability to offer a worthless gift, we suggest that male
450 decisions underlying tactic use is more likely to be opportunistic rather than driven by
451 nutritional needs that favour prey consumption over gift production. Variation in female
452 encounter rate also influences male tactic, as males responded to an increase in female
453 availability by increasing their mating effort (producing gifts). Finally, our study supports the
454 prediction that the frequency of the genuine gift tactic increases with sexual selection
455 intensity, based on the assumption that sperm competition increases with time and favours
456 males with genuine gifts, which enhances their chances in sperm competition. Our results
457 thereby provide some support for a previously proposed model that the simultaneous
458 expression of alternative reproductive tactics is maintained by seasonal changes in ecological
459 factors and variation in sexual selection (Ghislandi et al., 2014). This leads to relaxed
460 selection for the gift-giving tactic early in the season when females are less choosy and
461 resources more scarce. Worthless gifts are also more common early in the season, where any
462 gift type facilitates male mating success. Finally, our study suggests that elevated mating
463 opportunities and sperm competition favours the genuine gift-giving tactic late in the season.
464
465 References
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76 567 selection. Behav. Ecol. 12: 691–697. 568 Toft, S. 1997. Acquired food aversion of a wolf spider to three cereal aphids: intra-and 569 interspecific effects. Entomophaga 42: 63–69. 570 Toft, S. & Albo, M.J. 2015. Optimal numbers of matings: The conditional balance between 571 benefits and costs of mating for females of a nuptial gift-giving spider. J. Evol. Biol. 28. 572 Toft, S. & Albo, M.J. 2016. The shield effect: nuptial gifts protect males against pre- 573 copulatory sexual cannibalism. Biol. Lett. 12: 43–78. 574 Tuni, C., Albo, M.J. & Bilde, T. 2013. Polyandrous females acquire indirect benefits in a 575 nuptial feeding species. J. Evol. Biol. 26. 576 Tuni, C. & Berger-Tal, R. 2012. Male preference and female cues: Males assess female sexual 577 maturity and mating status in a web-building spider. Behav. Ecol. 23: 582–587. 578 Tuni, C. & Bilde, T. 2010. No preference for novel mating partners in the polyandrous 579 nuptial-feeding spider Pisaura mirabilis (Araneae: Pisauridae). Anim. Behav. 80: 435– 580 442. 581 Vahed, K. 1998. The function of nuptial feeding in insects: a review of empirical studies. 582 Biol. Rev. 73: 43–78. Wiley Online Library. 583 Wood, S. 2006. Generalized additive models: an introduction with R. CRC press. 584 Wyatt, T.D. 2014. Pheromones and animal behavior: chemical signals and signatures. 585 Cambridge University Press. 586 587
77 588 Figure Legends
589
590 Figure. 1. Proportion of P. mirabilis males with a nuptial gift (genuine or worthless) over the
2 591 mating season of in (a) 2015 (X 1 = 6.6, P = 0.018) and (b) 2016 (F1,16 = 5.2, P = 0.04), and
592 with a worthless gift in (c) 2015 (F1.9,2.0 = 7.0, P = 0.043) and (d) 2016 (F1.9,2.0 = 7.5, P =
593 0.041). Lines are the fitted GAM-b models.
594
595 Figure. 2. Relationship between prey availability and the proportion of P. mirabilis males
2 596 carrying a nuptial gift (genuine or worthless) in (a) 2015 (GLM-b, X 1 = 7.7, P = 0.005) and
2 597 (b) 2016 (GLM-b, X 1 < 0.1, P = 0.95), and the proportion of P. mirabilis males carrying a
2 598 worthless gift in (c) 2015 (GLM-b, X 1 = 4.7, P = 0.03) and (d) 2016 (GLM-b, F1,12 = 0.2, P =
599 0.67).
600
601 Figure 3. Relationship between the proportion of adult females and (a-b) the proportion of P.
2 602 mirabilis males with a nuptial gift (genuine or worthless) in (a) 2015 (GLM-b, X 1 = 4.3, P =
2 603 0.03) and (b) 2016 (GLM-b, X 1 = 4.4, P = 0.036), and (c-d) proportion of males with a
2 604 worthless gift in (c) 2015 (GLM-b, X 1 = 4.8, P = 0.029) and (d) 2016 (GLM-b, F1,12 = 1.7, P
605 = 0.22).
606
607 Figure. 4. The proportion of P. mirabilis males with a worthless gift in relation to OSR in
608 2015 (a) and 2016 (b). The line is the fitted GAM model. No significant relationship was
609 found in 2016.
610
78 Figures
()a) ()b) t if g a h t ales wi ales m f o n o i t r o p o r P 0.0 0.2 0.4 0.6 0.8 0.0 0.1 0.2 0.3 0.4 0.5 0.6
0 5 10 15 20 25 30 0 5 10 15 20 25 30 () c) () d) t if g less h t r o a w a h t ales wi m f o
n o i t r o p o r P 0.00.20.40.60.81.0 0.0 0.2 0.4 0.6 0.8 1.0
0 5 10 15 20 25 30 0 5 10 15 20 25 30 Season (day) Season (day)
Figure. 1. The proportion of P. mirabilis males over the mating season (a-b) with a nuptial gift
2 (genuine or worthless) in (a) 2015 (X 1 = 6.6, P = 0.018) and (b) 2016 (F1,16 = 5.2, P = 0.04), and
(c-d) with a worthless gift in (c) 2015 (F1.9,2.0 = 7.0, P = 0.043) and (d) 2016 (F1.9,2.0 = 7.5, P =
0.041). Lines are the fitted GAM-b models.
79
a) b)
c) d)
Figure. 2. The relationships between prey availability and (a-b) the proportion of P. mirabilis males
2 carrying a nuptial gift (genuine or worthless) in (a) 2015 (GLM-b, X 1 = 7.7, P = 0.005) and (b)
2 2016 (GLM-b, X 1 < 0.1, P = 0.95), and (c-d) the proportion of P. mirabilis males carrying a
2 worthless gift in (c) 2015 (GLM-b, X 1 = 4.7, P = 0.03) and (d) 2016 (GLM-b, F1,12 = 0.2, P = 0.67).
80 a) b) t f i g a h t i w
s male f o
n o i t r o p o r P 0.0 0.2 0.4 0.6 0.8 0.00.10.20.30.40.50.6 0.20.40.60.81.0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 c) d)
Figure 3. The relationships between the proportion of mature females and (a-b) the proportion of P.
2 mirabilis males with a nuptial gift (genuine or worthless) in (a) 2015 (GLM-b, X 1 = 4.3, P = 0.03)
2 and (b) 2016 (GLM-b, X 1 = 4.4, P = 0.036), and (c-d) proportion of males with a worthless gift in
2 (c) 2015 (GLM-b, X 1 = 4.8, P = 0.029) and (d) 2016 (GLM-b, F1,12 = 1.7, P = 0.22).
81
Figure 4. The proportion of P. mirabilis males with a worthless gift in relation to OSR in (a) 2015
2 (GLM-b, X 1 = 5.5, P = 0.019) and (b) 2016 (GLM-b, F1,15 = 0.2, P = 0.63).
82 Supplementary Figures
(a) (b)
Figure S1. The proportion of P. mirabilis males, subadult females and adult females over the mating season of 2015 (a) and 2016 (b). The bars show the percentage of collected individuals per day of collection, the total number of individuals collected per day is given on the top of the bars. Spiders were collected on 21 days in the period of 11 May-12 June in 2015, and on 18 days from 22 May-
22 June in 2016.
83
(a) (b)
y a d / e al m / y e r p
e l b vaila A 0 5 10 15 020406080100
0 5 10 15 20 25 30 0 5 10 15 20 25 30 Season (days) Season (days)
Figure S2. Variation in prey availability over the mating season of P. mirabilis in (a) 2015 (F1,19 =
4.6, P = 0.045) and (b) 2016 (F5.8,6.0 = 3.7, P = 0.02). Lines are the fitted GAM-g models.
84 (a) (b) x e d n i y d o b al u d i s e R -0.2 -0.1 0.0 0.1 0.2 0.3 -0.4 -0.2 0.0 0.2 0.4
0 5 10 15 20 25 30 0 5 10 15 20 25 30 Season (days) Season (days)
Figure S3. Male Residual Body Index (RBI) over the mating season of (a) 2015 (F7.2,8.2 = 6.2, P <
0.0001) and (b) 2016 (F6, 7.2 = 6.9, P < 0.0001). Lines are the fitted GAM models.
85 (a) (b) x e d n i
y d o b
l ua d i es R -0.2 -0.1 0.0 0.1 0.2 0.3 -0.4 -0.2 0.0 0.2 0.4
0 20 40 60 80 100 2 4 6 8 10 12 14 Available prey/male/day Available prey/male/day
Figure S4. Relationship between P. mirabilis male Residual Body Index (RBI) and prey availability in (a) 2015 (F1,378 = 3.8, P = 0.02) and (b) 2016. (F1,173 = 16.5, P < 0.0001). Lines are the fitted LM models.
86 (a) (b) R S O 0.2 0.3 0.4 0.5 0.6 0.20 0.25 0.30 0.35 0.40 0.45 0.50
0 5 10 15 20 25 30 0 5 10 15 20 25 30 Season (day) Season (day)
2 Figure S5. Variation in OSR over the mating season of P. mirabilis in (a) 2015 (X 4.75 = 19.6, P =
2 0.003) and (b) 2016 (X 5.6 = 11.1, P = 0.12). Lines are the fitted GAM-b models.
87 (a) (b) ales m fe t f adul o
n o i t r o p o r P 0.4 0.6 0.8 1.0
0 5 10 15 20 25 30 Season (day) Season (day)
Figure S6. The proportion of adult females over the mating season of P. mirabilis in (a) 2015 (X2
2 3.8= 5.8, P = 0.0035) and (b) 2016 (X 2.2 = 18.7, P < 0.0001). Lines are the fitted GAM-b models.
Figure S7. Relationship between residual body index and the proportion of males with a nuptial gift
(genuine or worthless) in 2016. Logit GLM model is fitted.
88 Appendix
The effect of environmental variation on the expression and maintenance of ARTs
Paolo Ghislandi and Caitlin Stern, work in progress
Introduction
Theory predicts that stable ARTs can co-exist each tactic in the population is equally successful
(Maynard Smith, 1982; Gross, 1996). Evolutionarily Stable Strategy (ESS) models show, that if the average fitness return for each tactic is equal, alternative strategies cane maintained at different frequencies (Maynard Smith & Price, 1973, Gross, 1996). The empirical data collected in my thesis allows us to propose that the 3 gift tactics performed by Pisaura mirabilis are likely maintained by the interplay of intrinsic and extrinsic (environmental) factors, which we identified as male body condition, prey availability, number of receptive females and operational sex ratio. In collaboration with Dr. Caitlin Stern, I have initiated the development of a model to study under which set of conditions ARTs are expressed (individual tactic expression) and can be maintained (ESS situations for ARTs) in Pisaura mirabilis. The aim is to parameterize the model with empirical data collected in my PhD thesis, and to first model individual tactic expression based on the expected fitness outcome of each tactic depending on female encounter rates and females mating status. Female mating status has been found to play an important role for the no-gift strategy, as laboratory experiments showed that recently mated females never accept males without a nuptial gift
(Ghislandi, unpublished). Since we believe it is clear, that sperm competition plays a profound role for determining the fitness function of male tactic, this factor will also be incorporated in the model.
This work was recently initiated and is still and ongoing, therefore it is briefly presented with a short sketch of the model. (Caitlin gave pre-mature birth to twins, so the modelling work is currently on hold during maternity leave…).
89 Model
We modelled three alternative reproductive tactics that can be employed by male spiders: offering no gift, offering a worthless gift, and offering a genuine gift. We assume that any given male can express any of the three tactics (Chapter I), and can alter which tactic to use in response to environmental feedback (Chapter II).
Specifically, we consider the behaviour of males in response to different seasonal regimes, which are defined by two parameters: 1) the proportion of females encountered that are virgin rather than mated, and 2) the rate at which males encounter females. Within a seasonal period s, the proportion of females that are unmated (virgin) is given by (and thus the proportion of females that are mated is given by ). Note that this parameter is fixed for a given seasonal period. The number of females a male encounters on average within the seasonal period is given by . For example, if a seasonal period lasts for 10 days, and the encounter rate for that period is one female per day, .
Because males can change tactics in response to cues from the social environment (Chapter
II), the frequencies of each tactic can vary among seasonal periods. In seasonal period s, the probabilities that a male offers no gift, a worthless gift, or a genuine gift are , , and , respectively. Note that no other tactics are available to males, and thus .
The payoffs of each male tactic are determined by female mating status. A male that gives no gift, a worthless gift, or a genuine gift to a virgin female obtains the payoff , , or , respectively. A male that gives no gift, a worthless gift, or a genuine gift to a mated female obtains the payoff , , or , respectively.
Each male’s fitness is composed of the payoff from matings with virgin females,
, and the payoff from matings with mated females,
. Weighted by the frequencies of each female mating status type, the fitness contributions the male gains are and ( ) . The total fitness that a male gains from a given seasonal
90 period is these payoffs multiplied by the number of females encountered in that period:
( ) .
Model analysis
Using the fitness function , we can ask how the seasonal period parameters and influence the proportion of effort a male is selected to invest in each of the three alternative reproductive tactics. First, we insert biologically realistic values for the payoffs , , , , , and .
Next, we select the partial derivatives ⁄ , ⁄ , and ⁄ . By studying how and
affects these partial derivatives, we can determine how the social environment influences the optimal proportion of effort a male should invest in offering no gift, offering a worthless gift, and offering a genuine gift.
Results
We studied different sets of possible payoff values, which are summarized in Table 1 (to be extended and expanded). receives a fitness payoff of 10 as represents a male offering no gift to a virgin female, thus male is accepted in 40% of cases (Albo et al., 2011). receives a fitness pay-off of 15 as offering a worthless gift to a virgin female returns high acceptance rates, approximately of 90%. receives a pay-off of 15 as genuine gift-offering male experience high acceptance from virgin females, around 90% as well (Albo et al., 2011).
represents a male offering no gift to a mated female, which never accepts a male when recently mated. However, males might be likely to be subsequently accepted if female mating rates are low, therefore the fitness return is 5 (Ghislandi, unpublished). obtain a fitness pay-off of 10 as worthless gift-givers experience shorter copulations with already mated females as they interrupt copulation sooner when discovering gift content. They however experience the same acceptance as genuine gift-givers (Ghislandi, unpublished). receives a fitness pay-off of 15, as offering a genuine
91 gift provides prolonged copulations and high acceptance rates on mated females (Drengsgaard and Toft,
1999; Ghislandi, unpublished)
For set 1, ⁄ ( ), showing that the likelihood of offering no gift increases with both the proportion of virgin females and the number of females encountered. Similarly,
⁄ ( ), showing that the likelihood of offering a worthless gift also increases with both the proportion of virgin females and the number of females encountered. However,
⁄ , showing that the likelihood of offering a genuine gift increases with the number of females encountered but is not influenced by the proportion of virgin females.
Set
1 10 15 15 5 10 15
2
3
4
5
Table 1. Sets of mating payoff parameter values to be studied (to be expanded). , the payoff gained by a male that gives no gift to a virgin female; , the payoff gained by a male that gives a fake gift to a virgin female; , the payoff gained by a male that gives a genuine gift to a virgin female; , the payoff gained by a male that gives no gift to a mated female; , the payoff gained by a male that gives a fake gift to a mated female; , the payoff gained by a male that gives a genuine gift to a mated female.
92
Chapter III
Silk wrapping of nuptial gifts aids cheating behaviour in male spiders
Ghislandi P.G, Beyer M., Velado P. & C. Tuni
Behavioral Ecology (2017), 00(00), 1–6. doi:10.1093/beheco/arx028
93 Behavioral The official journal of the ISBE Ecology International Society for Behavioral Ecology
Behavioral Ecology (2017), 00(00), 1–6. doi:10.1093/beheco/arx028
Original Article Silk wrapping of nuptial gifts aids cheating behaviour in male spiders
Paolo Giovanni Ghislandi,a,* Michelle Beyer,b,* Patricia Velado,b and Cristina Tunib aDepartment of Bioscience, Aarhus University, Ny Munkegade 1540, DK-8000, Aarhus, Denmark, and bDepartment of Biology, Ludwig Maximilian University, Grosshaderner Strasse 2, 82152 Planegg- Martinsried, Germany *These authors share equal contribution to the work. Received 21 July 2016; revised 19 January 2017; editorial decision 20 January 2017; accepted 7 February 2017.
Sexual traits, such as nuptial gifts, are costly and often condition-dependent. Males should be under selection to reduce these costs without impairing their reproductive success. Spider gifts consist of silk-wrapped food, but may also consist of worthless (non-nutri- tive) donations that successfully lead to mating, despite yielding shorter copulations. Worthless gifts may either represent a cheaper cheating strategy or the inability to produce genuine gifts due to resource limitations (i.e. poor body condition). Unless energetic con- straints limit expenditure in silk, males should apply more silk to worthless gifts to compensate for their lower reproductive value. We ask whether in Pisaura mirabilis 1) worthless gifts are condition-dependent and 2) males strategically use silk based on gift type (genu- ine vs worthless). We tested whether male body condition explains the gift-giving strategy and compared silk amounts covering each gift type, in gifts collected from the field and produced in the laboratory by males given different feeding regimes. Our findings show that worthless gifts are not promoted by poor body condition or limited resources. They rather result from a cheating strategy evolved to opportunistically reduce the costs of genuine gifts while ensuring nutritional advantages, with cheaters gaining body mass. Males applied more silk to worthless gifts regardless of their body condition or feeding state, suggesting they can strategically adjust silk expenditure despite its costs. By masking gift contents and prolonging female feeding, silk is crucial for the maintenance of cheating, likely resulting from an evolutionary arms race between male deception and female assessment. Key words: deception, mating strategies, nuptial gifts, sperm competition.
INTRODUCTION (direct or indirect) from their mate choice, with males saving energy Theory predicts that sexual traits (e.g. courtship displays) are honest at their own reproductive advantage (Johnstone and Grafen 1993; indicators of males’ underlying quality due to the costs associated LeBas and Hockham 2005). Alongside cost-reduction in males, to their production and maintenance (Zahavi 1975). The extent to selection should favour accurate female mate assessment (Kokko which males invest in reproduction is therefore often affected by et al. 2003), leading to a potential evolutionary arms race between energetic resources, with males of higher body condition being bet- selection for cheaper deceptive traits in males and resistance to ter at expressing sexual traits (van Noordwijk and de Jong 1986). such traits in females (Arnqvist and Rowe 2013). As it is in the male’s best interest to optimize investment in current Nuptial gifts, consisting of male donations of nutritious substances reproduction in order to allocate resources to future mating or other to females (Lewis and South 2012), are sexually selected traits partic- activities, males should be under selection to reduce some of these ularly prone to male cheating (LeBas and Hockham 2005; Ghislandi costs without impairing their overall reproductive success. Males, et al. 2014). The gift may have several, non-mutually exclusive, func- for example, can use cheaper mating strategies if there are limits to tions: it is used by males to attract females, it secures sperm trans- female accurate assessment of the quality of the information they fer, and may act as parental investment through nutrient-derived convey (Dawkins and Guilford 1991). These strategies lead to male resources (Thornhill 1976a; Boggs 1995; Vahed 1998; Gwynne cheating behaviour if females do not receive the expected benefits 2008). Invertebrates show a rich variation of gift types, ranging from oral or seminal male secretions (endogenous gifts) to food items cap- tured or collected from the environment (exogenous gifts) (Lewis Address correspondence to C. Tuni, Department of Biology, Ludwig et al. 2014). Exogenous gifts are often polymorphic and may also Maximilian University, Grosshaderner Strasse 2, 82152 Planegg- consist of items with little or no nutritional value to females (i.e. plant Martinsried, Germany. E-mail: [email protected] parts, prey remains, and empty silk balloons), which nevertheless
© The Author 2017. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: [email protected] 94 Page 2 of 6 Behavioral Ecology successfully elicit female mate acceptance (Preston-Mafham 1999; condition-dependent and 2) whether males modulate silk investment Albo et al. 2011b). These types of donations generally reflect a reduc- based on their gift giving strategy (genuine vs worthless gifts). In a tion of male investment into reproduction from more costly to less first study, we collected gifts from a natural population of spiders valuable donations (from nutritious prey, to prey remains and worth- and scored them as genuine or worthless based on their contents, less donations) (LeBas and Hockham 2005; Ghislandi et al. 2014). consisting respectively of intact prey (genuine) or prey leftovers and Hence, cheaper (worthless) gifts may either represent an opportu- plant parts (worthless). We tested whether the use of worthless gifts nistic cheating strategy used by males to reduce the costs of mating is explained by variation in male body condition and compared the or a consequence of facing unfavourable ecological conditions, for amounts of silk covering each gift type (genuine and worthless). In example when resources are lacking (Engqvist 2007; Albo et al. 2014; a second study we obtained gifts during laboratory trials by pro- Ghislandi et al. 2014). Through cheaper donations, males enhance viding males with live prey, and scored whether a gift is worthless their reproductive success while minimizing the costs associated with based on the extent males fed on prey prior to silk wrapping (i.e. producing a genuine gift, such as increased risk of predation while prey weight loss). We experimentally manipulated male body condi- hunting for a prey to offer, missed foraging opportunity, or transpor- tion by allocating males to high and low feeding regimes and tested tation costs (Boggs 1995). On the other hand, males attempting to whether food availability explained worthless gift production, and use worthless gifts may incur fitness costs, derived by female mate whether silk investment varied based on gifts’ worthlessness. Unless rejection or reduced sperm transfer due to shorter copulation time the use of worthless gifts itself is condition-dependent and limits (LeBas and Hockham 2005; Albo et al. 2011b). silk investment we expect males to cover worthless gifts in greater The evolution of worthless gifts in spiders remains particu- silk amounts compared to genuine gifts to compensate for their larly intriguing. Male spiders offer a prey gift to the female dur- lower reproductive value. ing courtship and mating. Gift-offering enhances mate acquisition (Stålhandske 2001; Albo and Costa 2010) by exploiting female for- aging motivation (Bilde et al. 2007). It also increases male fertiliza- METHODS tion success by prolonging female gift-consumption and concurrent Field collected nuptial gifts sperm transfer (Drengsgaard and Toft 1999; Albo and Costa 2010; Collecting Albo et al. 2011b; Albo et al. 2013). In certain ecological contexts Males carrying gifts were collected from a natural population of the gift may also function as paternal investment (Toft and Albo Pisaura mirabilis surrounding Ludwig Maximilian University of 2015). Males wrap their gifts in dense silk layers, an advantageous Munich (Germany) during May and June 2014 and 2015. On each trait for males because it prolongs mating by increasing female day of collection, which occurred over a total period of 4 weeks feeding duration (Lang 1996; Albo et al. 2011b) and allows the at variable intervals, males were captured, placed in a vial (5 cm male to strengthen hold of the gift, preventing mating interruptions diameter, 10 cm height) covered with a sponge lid and given an (Andersen et al. 2008). Gifts may however range from edible intact individual ID. In order to avoid males’ gift manipulation after cap- prey, to worthless prey remains and plant parts (Stålhandske 2001; ture, the gift was immediately removed from the males’ mouthparts Albo et al. 2011b; Albo et al. 2014). Limited food availability and using forceps and placed in an Eppendorf tube labelled with the poor male body condition have been proposed to be the possible male ID. Males were brought to the laboratory, their body mass ecological drivers promoting the use of these worthless gifts (Albo was weighted to the nearest 0.01 g using a digital scale (KERN et al. 2014; Ghislandi et al. 2014). PKT, KERN & SOHN GmbH, Balingen, Germany) and their Disguising gift content with silk to lure females into accepting a cephalothorax width (used as a proxy for body size) measured to gift that does not confer the expected food reward is suggested to the nearest 0.01 mm using digital callipers (AEROSPACE, China). play a key role in facilitating the use of worthless gifts (Albo et al. Animals were then released back to the study site. Gifts were frozen 2011b; Ghislandi et al. 2014). In the species Pisaura mirabilis, males at −4 °C. offering worthless gifts are known to achieve similar mating success as of those offering genuine prey gifts, indicating that during court- Scoring of gift content ship females are not able to assess the value of the gift content (Albo Gifts were weighed to the nearest 0.001 g using a microbalance et al. 2011b). These males however experience on average 20% (Sartorius Supermicro S4, Sartorius AG, Göttingen, Germany.) shorter copulations and reduced fertilization success, most likely and then placed under a stereomicroscope where the silk was gen- due to female interruption of copulation once the silk layers cover- tly opened using micro-dissecting forceps. The content of field- ing inedible gift contents are consumed and the content unmasked collected gifts was inspected and the gift was defined as genuine (Albo et al. 2011b). Due to the importance of silk in serving the if containing intact prey, which did not appear to be dried out or male’s reproductive interest (Lang 1996; Andersen et al. 2008), eaten, and worthless if containing dried prey and prey leftovers (i.e. males are expected to use silk strategically. Enhancing silk invest- pieces of body parts or exoskeletons) or items such as seeds or plant ment when offering worthless gifts would be particularly advanta- parts. Gifts that contained more than one prey were classified as geous for males, not only to effectively mask worthless gift contents, genuine where at least one of the items consisted of intact prey. If but also to prolong female feeding and compensate for the otherwise possible the Taxonomical group of the arthropod prey was defined. shorter copulation achieved (Lang 1996; Andersen et al. 2008; Albo Gift contents were scored by the same observer (P.G.G). et al. 2011b). However, if gift-wrapping entails substantial energetic requirements due to costly silk proteins (Craig et al. 1999; Nentwig Experimentally produced gifts and Kuhn-Nentwig 2013), we would expect higher silk investment only in the case worthless gifts occur as a male cheating strategy. Animal rearing and treatment groups In contrast, if worthless gifts are condition-dependent limiting ener- Sub-adult spiders were collected during April 2014 from grass getic resources may also constraint silk production. meadows described above, and were brought to the laboratory. By taking advantage of the silk-wrapped nuptial gifts of male They were reared at room temperature (approx. 25 °C) under nat- Pisaura mirabilis we investigated 1) whether worthless gifts are ural photoperiod and were kept individually in plastic vials (5 cm
95 Ghislandi et al. • Males cover worthless gifts in more silk Page 3 of 6 diameter, 10 cm height) covered with a sponge lid and with a bot- nearest 0.001 g using a Sartorius microbalance. All measurements tom layer of moss that was kept wet by spraying water every sec- were taken by the same observer (M.B.). ond day. Spiders were fed regularly twice a week with 5–10 fruit flies (Drosophila spp.) or one prey consisting of a field cricket nymph Statistical analyses (Gryllus spp.) or a housefly (Musca domestica). The occurrence of We estimated male body condition by calculating a body condition molted exoskeletons in the vials was noted every second day. Upon index measured as the residuals of a regression of body mass on emergence to adulthood males were randomly assigned to one of body size (cephalothorax width) (Jakob et al. 1996). the two feeding treatments, with males in the high food treatment (HF, n = 28) being fed an insect prey (fly or cricket) twice a week Field collected nuptial gifts and those in the low food treatment (LF, n =17) being fed once a We used Logistic regression to test whether male body condition week with the same prey type. has an effect on the gift-giving strategy used (genuine vs worthless gifts). To understand whether silk investment depends on the gift Silk-wrapping trials giving strategy used by males and is affected by body resources we Males were used in experimental trials 12–14 days after reaching used ANCOVA, testing for the effects of male body condition and adulthood in order to ensure sexual maturity. Prior to each trial, gift type (genuine or worthless) on silk amounts covering gifts. male body mass and size was measured (as above). In order to elicit gift-wrapping behaviour we exposed males to female silk-draglines: Experimentally produced gifts we placed males into housing vials of sexually receptive females We tested whether the applied food treatments (LF and HF) gen- that were temporarily removed, or into transparent plastic boxes erated differences in male body condition index by using a t-test. (7 cm height × 10 cm width × 10 cm depth) with a bottom layer To analyse whether feeding treatment (LF and HF) affected silk of paper in which a female was previously housed for a minimum wrapping latency we used ANCOVA, including prey body mass as of 30 min. As soon as males showed signs of sexual stimulation a covariate in the model to control for possible effects of prey size on (i.e. abdomen jerking and rubbing of the first pair of legs against male behaviour. We estimated weather a gift is worthless in terms of each other) we placed a live cricket nymph of known weight nearby the extent to which males consumed the prey prior to silk wrapping. the spider using forceps and waited for the male to catch the prey. We therefore calculated prey weight loss as the differences in weight If males did not catch the prey within 20 min the trial was inter- between prey body mass before the trial and the content of the nup- rupted, males were returned to their own vial and the trial was tial gift after the trial (calculated as gift weight—silk weight). We repeated on the following day, for a maximum of 3 times. Similarly, used ANCOVA to test whether prey weight loss is affected by the trials were interrupted if males caught a prey but did not wrap it food treatment (LF and HF), controlling for initial prey body mass. into a nuptial gift. Overall we obtained one gift from each male, 10 To ascertain whether regression-to-mean effects explain prey weight from the LF and 24 from the HF treatment. loss in respect to prior prey weight value we used Pitman’s test for For each male that produced a gift we measured latency to the equality of variances in paired samples (Kelly and Price 2005). gift-wrapping, as the time spent from prey capture to start of silk To understand whether silk investment depends on the gift-giving wrapping, to account for possible manipulations of the prey prior strategy (worthlessness of the gift) and is affected by body resources gift construction. During gift-wrapping males move their body we used ANCOVA, testing for the effect of feeding treatment (LF in rounds keeping their silk spinning organs (spinnerets) that are and HF) and prey weight loss on silk amounts covering gifts. Linear located at the underside posterior of their abdomen attached to regression was used to assess whether the total duration of silk wrap- the gift. Males may often interrupt this process, remaining motion- ping behaviour related to the amount of silk deposited by males. We less and/or picking up the gift it their chelicerae, producing distinct also estimated the costs of gift production for males by measuring wrapping episodes. We measured the total gift-wrapping duration male body mass loss and used Linear Regression to assess whether (seconds) as the sum of the duration of all wrapping episodes to such cost is related to cheating (prey weight loss). Finally, for gifts correlate behavioural observations of silk wrapping to quantified produced overnight without direct observations we tested differences silk measures. All time durations were measured using a CG-501 in mean silk amount between LF and HF males using t-test. stopwatch (Genutek Electronics Co., Ltd, China). The trial was Statistical tests were performed using the software JMP, Version ended when males did not wrap for 10 consecutive minutes fol- 11 (SAS Institute Inc., Cary, NC). Appropriate data transforma- lowing the end of the last wrapping episode. The gift was then tions were applied when residuals did not meet assumptions of nor- removed from the male using forceps and frozen at −4 °C. mal distribution. Means are reported ± SE. At the end of the experiments, in order to evaluate male silk pro- duction without time constraints imposed by our experimental tri- als, a cricket nymph was placed inside the male’s housing vial and RESULTS on the following day we collected silk-wrapped gifts constructed overnight and stored them at −4 °C. Field collected nuptial gifts We collected a total of 113 nuptial gifts from the field (21 in 2014 Silk measurements and 92 in 2015), of which 41.6% (47) contained intact prey and Silk of nuptial gifts was measured from field-collected and experi- were scored as genuine gifts, whereas 58.4% (66) contained dried mentally produced samples. Silk was gently separated from the gift and fragmented prey leftovers and plant parts and were scored as content using micro-dissecting forceps and placed in an Eppendorf worthless gifts. Male body condition, calculated in 110 males, did tube. Silk measurements were conducted by modifying an existing not affect the mating strategy adopted (genuine vs worthless gifts) protocol for this species (Lang 1996). The removed silk was heated (Logistic regression, χ2 = 0.12, n = 110, P = 0.72). for 1–2 h in a heating block at 100 °C and then stored in a desic- Gift contents ranged from 1 to 4 items, with most gifts contain- cator (in dry atmosphere) for 12–36 h until it was weighed to the ing 1 (84%) and 2 (11.5%) items. In 79.7% of the cases, we found
96 Page 4 of 6 Behavioral Ecology arthropods (26.3% Dipteran, 21.8% Spiders, 19.1% Coleopteran, with the total amount of silk measured (log-transformed, Linear
16.4% Lepidoptera and 16.4% others), in 4.4% of plant parts regression, R2 = 0.42, F1, 27 = 21.10, P = <0.0001). and 15.9% of not identifiable items, the latter being scored as one We estimated the costs of gift construction for males in terms of item. The number of items found in gifts did not differ significantly body mass loss (male weight prior—after gift construction). After between those scored as genuine and worthless (mean N items ± gift construction 53.33% (n = 16) of the males increased their SE, genuine 1.19 ± 0.07 and worthless 1.25 ± 0.09; Wilcoxon, weight and 46.66% (n = 14) showed a decrease in body mass. We Z = 0.74; n = 113, P = 0.45). Gift weight was measured in 111 sam- found that male body mass increase correlated positively with prey ples and we found that genuine gifts were significantly heavier than weight loss (Linear regression, R2 = 0.27, F1,21 = 9.11, P = 0.0065). worthless gifts (mean ± SE (mg), genuine 13.93 ± 1.59 (n = 64) and Of the total of 45 males, 40 constructed gifts in their housing worthless 5.76 ± 0.91 (n = 47); log-transformed, t-test, t = −5.28, vials overnight, without direct observations. Gifts of HF males were df = 109, P = <0.0001). covered in higher amounts of silk compared to LF males (mean ± Due to practical unfeasibility in isolating silk in 3 genuine gifts, silk SE (mg), HF 0.38 ± 0.05 (n = 23), LF 0.17 ± 0.03 (n = 17), t-test, weight was measured from 110 samples. We found that the amount DF = 38, t = 2.68, P = 0.016). We acknowledge that our experi- of silk covering gifts varied significantly according to gift type, with mental set up may explain the differences in average silk amounts higher amounts of silk being found on worthless compared to measured from gifts produced during experimental observations genuine gifts (log-transformed, ANCOVA, Gift type F1, 104 = 4.39, (0.10 mg) and those produced without direct observations, in the P = 0.038; Body condition F1, 104 = 1.68, P = 0.19; Figure 1). laboratory (0.29 mg) and in the field (0.39 mg). Experimentally produced nuptial gifts The food treatment applied had a significant effect on male body DISCUSSION condition (mean body condition index ± SE, LF −7.49 ± 3.60 Energetic limitations have been suggested to play a major role in (n = 10), HF 3.98 ± 2.66 (n = 24); t-test; t = 2.41, DF = 32, the production of nuptial gifts (Engels and Sauer 2006; Immonen P = 0.022). Neither feeding treatment nor prey body mass (mea- et al. 2009; Albo et al. 2011a). However, unlike suggested for male sured in 27 trials) affected male latency to silk wrapping (log trans- spiders (Albo et al. 2014; Ghislandi et al. 2014), our study shows that formed, ANCOVA, Food Treatment F1, 24 = 0.06, P = 0.79; Prey limitation in male body condition or food resources does not pro- weight F1, 24 = 0.01, P = 0.93). We measured prey weight loss in 23 mote the use of worthless gifts, consisting of less valuable food dona- gifts. Whereas prey generally lost weight, in 3 cases we measured tions to females. In the field, males varying in their body condition an increase of prey body mass. This is most likely due to spiders’ were equally likely to carry genuine and worthless gifts. Similarly, injection of digestive fluids that serve for liquefying the prey during males raised under high and low food regimes in the laboratory fed feeding. We found that the feeding treatment given to males did on prey prior to gift construction to an equal extent. We can there- not significantly affect prey weight loss (worthlessness of the gift) fore overall reject the hypothesis of worthless gifts deriving from the but heavier prey lost more weight (ANCOVA, log-transformed, inability of poor quality males to produce genuine gifts. Worthless
Food Treatment F1, 20 =1.25, P = 0.27, Prey weight F1, 20 = 35.23, food gifts rather appear to result from a cheating strategy, evolved P < 0.0001) and the correlation between change in prey mass and to opportunistically reduce the costs of mating while ensuring nutri- initial prey mass was not entirely driven by regression-to-mean tional advantages. We indeed show that males producing worthless effects (Pitman’s test, t = 18.98, DF = 21, P > 0.0001). gifts increase their body mass from partially feeding on prey prior to We were not able to isolate silk from prey in 4 gifts, therefore silk gift construction. Short-term access to nutrients may promote male weight was measured from 30 samples. We found higher amounts mating success through strengthened courtship effort and vigour, of silk on the gifts that were scored as worthless (higher prey weight providing males with mating advantages that go beyond gift quality loss), but not according to male feeding treatment (LF and HF) (Kotiaho 2002; Hunt et al. 2004; Shamble et al. 2009; Albo et al.
(square-root transformed, ANCOVA, Food Treatment F1, 20 = 2.54, 2012). Using a cheaper strategy would then be particularly valu- P = 0.12, Prey weight loss F1, 20 = 4.41, P = 0.048; Figure 2). The able with increasing energy requirements necessary to overcome total time males spent wrapping a gift in silk correlated significantly resistance of already mated females (Gabor and Halliday 1997;
0.25 0.6 0.5 0.2 0.4 0.15 0.3
Silk (mg) 0.1 Silk (mg) 0.2 0.05 0.1 0 0 Genuine Worthless -5 515253545 Gift Type Prey weight loss (mg) Figure 1 Figure 2 Amount of silk (mg) covering genuine (prey) and worthless (prey leftovers The amounts of silk (mg) covering experimentally produced nuptial gifts and plants) nuptial gifts collected from a natural population of the spider correlates linearly with the extent on which males fed on the prey prior to Pisaura mirabilis. *Denotes statistical significance. silk wrapping (prey weight loss).
97 Ghislandi et al. • Males cover worthless gifts in more silk Page 5 of 6
Maklakov et al. 2003; Aisenberg and Costa 2005; Tuni and Bilde Once the gift quality is reduced, females are likely to suffer fitness 2010), or to outcompete competitors (Hack 1997; Nitzsche 2011). costs from being deceived owing to reduced direct benefits (i.e. The finding that males producing worthless gifts fed to a greater missed foraging opportunity), and are expected to be under selec- extent on prey of higher body mass, further suggests that cheat- tion to distinguish genuine from worthless gifts. In return, males ing may function to efficiently feed on larger and/or higher quality might evolve more efficient traits, such as silk wrapping to prevent prey providing males of greater hunting abilities with the oppor- accurate female assessment. Females may instead become more tunity to gain nutritional benefits, while still being able to produce resistant to the information carried by silk (Albo et al. 2012), poten- a gift that allows successful reproduction. This may also represent tially basing their reproductive decisions on other traits. Overall, an effective male strategy to optimize the decreasing reproductive this would generate an antagonistic arms race between males under payoff of prolonged copulations in relation to the costs of not feed- selection to deceive and females under selection to evolve resistance ing on the prey. Males would indeed benefit from offering larger to deception (Arnqvist and Rowe 2013). and more nutritive prey to females only if the extra copulation time Our study shows that the use of worthless gifts is well estab- achieved provides substantial fertilization advantages. If paternity lished in the population, occurring in 58.4% of the males in gain is instead a decreasing payoff function of copulation duration our field study (and 85% in laboratory trials). Similar frequen- (Parker 1970; Thornhill 1976b) feeding on larger prey and using cies are also described in natural populations of the Neotropical prey of reduced size for mating, may allow males to outweigh the gift-giving spider Paratrechalea ornata (Albo et al. 2014). These costs of a missed meal while ensuring sufficient sperm transfer for high frequencies of worthless gifts are not explained by theory, reproduction. which instead predicts cheating to occur at low incidence as the We also found that males producing worthless gifts apply more benefits of the strategy will decline with increasing frequency of silk regardless of their body condition or feeding state, suggest- cheats in the population (Dawkins and Guilford 1991; Johnstone ing that they are not energetically limited in their ability to invest and Grafen 1993). Our following reasoning may provide a pos- in the gift and can strategically adjust silk expenditure despite sible explanation for such high levels of worthless gifts. The its costs (Albo et al. 2011a). The silk cover is instrumental for non-trivial nutritional benefits gained by cheaters together with a successful cheating strategy as it prevents female pre-mating the effectiveness of the strategy derived by silk-wrapping (worth- evaluation of gift contents; greater silk amounts covering worth- less gifts are not rejected at higher rates), may allow males using less gifts will effectively disguise distasteful or inedible items to worthless gifts to maintain overall high mating rates (LeBas and females. Additional silk wrapping may also be a means of extend- Hockham 2005; Albo et al. 2011b) at the costs of reduced pater- ing inseminations in the face of the shorter matings yielded from nity gained per mating (Albo et al. 2011b). Despite the fact that worthless gifts, as extra silk layers will prolong female feeding larger inseminations promote male success in sperm competition duration and hence sperm transfer (Lang 1996). Silk itself is likely when females mate with multiple partners (Drengsgaard and Toft an important component of the nuptial gift as females ingest it, 1999; Simmons 2001), shorter copulations may not necessarily hence cheaters may provide extra silk to improve the nutritive affect male lifetime reproductive success negatively. Theoretical value of worthless items (Craig et al. 1999; Wilder 2011). Finally, models of sperm allocation predict that males should increase silk may also facilitate handling and control of smaller lower qual- sperm investment when facing sperm competition risk, but should ity gifts (Andersen et al. 2008). Because of the lack of consen- reduce sperm investment under high sperm competition intensity sus on silk acting as a visual signal enhancing mate acceptance (Parker 1970; Parker and Pizzari 2010). Therefore under environ- (Stålhandske 2002; Bilde et al. 2007; Albo et al. 2012), greater silk mental conditions in which sperm competition is most intense (i.e. layers are suggested not to promote male mating success. For all male biased sex ratios, high female re-mating rates) it may pay to the above-mentioned reasons, we suggest that males use higher reduce investment in gifts and hence sperm, as larger insemina- silk investment to compensate for the lower reproductive value tions achieved through high quality gifts may not confer the fertil- of worthless gifts. The finding that males that feed opportunisti- ization benefits necessary to counterbalance the costs of mating. cally on the prey to produce a worthless gift gain weight despite Under these circumstances males could enhance their reproduc- wrapping their gifts in more silk, further suggests that adding silk tive success by ensuring mate acquisition through cheaper mat- is not a costly compensation energy wise. We expect such com- ings (worthless gifts, shorter inseminations) rather than investing pensation in gift quality however not to be complete, as achieving largely in fewer costly encounters (i.e. genuine gifts, longer insemi- copulations equivalent in frequency and duration to those with nations), with selection favouring cheating. genuine gifts would indicate that the worthless gifts may in turn To conclude, genuine and worthless gifts may be maintained as have the potential to invade and replace genuine gifts (LeBas and alternative reproductive strategies, each of which maximizes male Hockham 2005). reproductive fitness under a specific set of circumstances (Oliveira Nuptial gifts most likely evolved in the context of female forag- et al. 2008). Whether these reflect changes in the competitive social ing, with males providing extra nutrients to their mating partners environment (e.g. sperm competition intensity) or the quality of while enhancing their reproductive success through improved fertil- available prey remain interesting avenues for research. Silk wrap- izations and increased female fecundity (Boggs 1995; Vahed 1998; ping, which increases assessment costs for females and compensates Bilde et al. 2007). Male spiders may have originally benefited from for the lower reproductive value of worthless gifts, facilitates the investing in costly nutritive gifts at their own feeding expenses (Albo maintenance of the cheating strategy. et al. 2009), with gift-giving becoming a trait strongly selected by females (Stålhandske 2001; Albo et al. 2013). Only subsequently, males may have evolved means to reduce the nutritional costs of ACKNOWLEDGEMENTS gift production by lowering content quality in order to facilitate We thank Jingzi Xu for assistance in maintaining spiders during the labora- their own reproductive interests, yet still fulfilling female mate tory trials and Sarah Schulte-Döinghaus for help in collecting spiders from choice expectations (Sakaluk 2000; LeBas and Hockham 2005). the field. Trine Bilde and anonymous reviewers gave valuable comments
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Chapter IV
Extreme male mating behaviours: anecdotes in a nuptial gift-giving spider
Ghislandi P.G., Bilde T. & C. Tuni
Arachnology (2015) 16 (8), 273–275
100 Arachnology (2015) 16 (8), 273–275 273 Extreme male mating behaviours: anecdotes in a increase sperm transfer (Vahed 1998). Gifts may consist of nuptial gift-giving spider captured prey, seminal substances transferred during copu- lation, or even somatic gifts, consisting of specialized body structures on which females feed. Male ground crickets, for Paolo Ghislandi example, possess spurs on their hind legs that females chew Trine Bilde Department of Bioscience, on during mating (Mays 1971), whereas male dwarf spiders Aarhus University, possess a dorsal cephalic elevation from which secretory Ny Munkegade 1540, glands produce a substance taken up by females during DK-8000 Aarhus C, Denmark mating (Kuntz, Garbe & Uhl 2012). Although females in email: [email protected] some species may benefit from nuptial gifts, there is much scope for sexual conflict, as males may manipulate females Cristina Tuni Ludwig Maximilians University Munich, into re-mating rates below their optima (Arnqvist & Rowe Department Biology II, 2005). Großhaderner Str. 2, Male nuptial gifts in the form of captured prey are rela- 82152 Planegg-Martinsried, Germany tively rare in spiders (Albo, Toft & Bilde 2013), and occur in the Palaeartic Pisaura mirabilis Clerck, 1757 (Araneae: Pisauridae). Males of this species offer a silk-wrapped Summary insect prey during courtship. Upon female gift consump- Nuptial gifts may serve to increase male mating success, tion, the male enters the mating position and inserts his copulation duration, and fertilization success, as is known for pedipalp into the female’s epigyne. The gift, which exploits the nuptial feeding spider Pisaura mirabilis Clerck, 1757. In the female foraging motivation in a sexual context (Bilde et this species, strong sexual selection for the gift-giving trait may al. 2007) is under strong sexual selection, as gift-less males lead to male strategies, such as gift enlargement or thanatosis behaviour (death feigning), which ultimately maximize male suffer from decreased mating success (Stålhandske 2001; fitness. In laboratory trials, we observed male gift enlargement Albo et al. 2011). The gift functions as a male mating effort by inclusion of an autosomal (self-amputated) limb, female by increasing male mating success, assuring sperm transfer, consumption of male soma during copulation, and high-injury and prolonging copulation duration, which correlates posi- risk thanatosis in which a male feigned death while still in copula and only attached to the female with his pedipalp, instead tively with male fertilization success (Stålhandske 2001; of hanging onto the gift with the chelicerae as is performed in a Albo et al. 2011). Control over the gift during mating is of typical thanatosis. Although the observations are anecdotal, we crucial importance as females often attempt to steal the gift propose functional hypotheses for these traits in the context of and then avoid or interrupt copulation. Silk wrapping facili- extreme male mating effort and cannibalism avoidance, which are characteristics of the mating system of this species. Sexual tates male gift control during courtship and copulation, and selection and sexual conflict are major evolutionary forces that reduces the risk of losing the gift to the females (Andersen shape a variety of morphological, behavioural, and physiological et al. 2008). Males may also overcome female resistance by traits (Parker 1979). Males must compete intra-sexually over performing thanatosis (death feigning) (Bilde et al. 2006; access to females or inter-sexually to attract females, and have evolved traits that serve to increase their reproductive success Hansen et al. 2008), a stereotyped behaviour in response (Andersson 1994). to female attempts to escape with the gift. When females cease eating and try to take the prey, males stretch-out their legs and hang motionless onto the gift with their chelicerae Introduction (mouthparts), while being dragged by the female. Once females resume gift consumption, males insert their pedi- In mating systems in which access to females is limited palp and resume sperm transfer (Bilde et al. 2006; Hansen and competition among males intense, males may adopt et al. 2008). These behaviours strongly suggest conflicting extreme mating strategies, in which female monopolization interests over mating rate and mating duration between the and paternity assurance is often achieved at the expense of sexes (Arnqvist & Rowe 2005). This scenario is further the male’s own survival (Schneider & Fromhage 2010). supported by the male ability to deceive females into mating Self-sacrifice, in which males feed themselves to females using worthless or inedible silk-wrapped items as gifts is one of the most severe forms of extreme male mating (Albo et al. 2011). behaviour. Male Australian red-back spiders, for example, perform a 180° somersault into the female’s mouthparts during mating, a behaviour that facilitates cannibalism, and Material and methods is adaptive for males as it increases their paternity (Andrade 1996). Breakage of male copulatory organs during mating, Here, we describe the methods adopted to raise the spiders resulting in male death or functional sterility, is also common and the observations recorded during experimental trials. in various spider species (Schneider & Fromhage 2010). We collected Pisaura mirabilis spiders at juvenile stages in Similarly, in bees and certain ants, males die by breaking April 2013 from Mols, near Aarhus (Denmark), and from off their abdomen during mating (Boomsma, Baer & Heinze Merlino, near Lodi (Italy). Specimens were housed individ- 2005). ually in 30 ml vials containing moist Sphagnum moss in the To out-compete rivals, males may adopt other mating laboratory at Aarhus University, Denmark. Individuals were strategies, such as providing nuptial gifts, which are fed three times per week with green-bottle flies Lucilia ( nutritional donations offered to females during courtship ssp.), house flies Musca( domestica) and crickets (Gryllus or mating that attract partners, facilitate copulation, and ssp.). Water was provided daily. Spiders were raised at room
101 274 Extreme male mating behaviours temperature (on average 20°C) and natural photoperiod. through the experimental arena. Notably, these males did Once individuals reached sexual maturity (approximately not disengage their pedipalp from the female’s epigyne and, 14 days after their final moult) they were assigned to mating once the female started moving, they remained suspended experiments. Mating trials took place in transparent plastic from the female’s body with the pedipalp still inserted. One terraria (19 × 19 × 15 cm) in which females were allowed of the males did not fix his chelicerae to the gift, which is to walk and leave draglines for approximately 1 hour before how males usually remain attached during thanatosis (Bilde the start of the trial in order to stimulate a sexual response et al. 2006), and was dragged solely attached by his palp. In in males. Prior to the mating experiments, males were both cases, once females stopped moving and restarted gift given a fly (Lucilia sp.) to wrap with silk as a nuptial gift, consumption, copulation was resumed and sperm transfer and were subsequently introduced to the female. During a was clearly occurring, as the haematodocha of the inserted total number of 250 experimental trials we observed the palp was pulsating. following behaviours. However, it was not possible to determine whether during these unusual thanatosis males were transferring sperm while dragged in the mating arena or if they interrupted Results sperm transfer until the females stopped moving.
Gift enlargement via male limb autotomy Discussion An adult male was observed performing autotomy (self-amputation) of one of its legs (3rd pair) by literally Limb or caudal self-amputation (autotomy) is known pulling its leg off with its chelicerae, and fixing it with as a defensive strategy to counteract predation or nega- multiple silk threads to the nuptial gift it had previously tive consequences of appendage damage in several taxa, constructed by wrapping a fly in silk. This behaviour including arthropods (Maginnis 2006; Fleming, Muller & occurred when the male was alone in its vial prior to contact Bateman 2007). In spiders, autotomy is recorded across with a female. several families and is known to have very marginal costs, Since it was not possible to determine whether the leg had by not reducing locomotor or prey-capture ability (Brueseke previous damage or was completely functional, different et al. 2001), competitive fights over prey, or development costs could have been involved in the described autotomy. (Johnson & Jakob 1999), and male’s mating success (Brau- tigam & Persons 2003). These relatively negligible costs, suggesting that one or two limbs can be sacrificed with very Soma-consumption by females during copulation little negative effect, have led to the spare limb hypothesis for this taxonomic group (Fleming, Muller & Bateman During an experimental trial in which we paired a 2007). Although autotomy has been observed in spiders non-virgin female with a gift-less virgin male, we observed in a sexual scenario, as described for Tidarren cuneolatum the female feeding on the male’s leg during copulation. As males which amputate one of their palps a few hours after previously reported for gift-less males (Albo et al. 2011), the penultimate moult and insert a single palp during copu- the male courted the female by repeatedly vibrating his lation (Knoflach & Van Harten 2000), it has never been abdomen, bouncing towards and around the female, and documented in the context of nuptial gift construction. finally pushing up the female to reach her ventral epigyne. In addition to the insect prey, Pisaura mirabilis males After four minutes of courtship the male inserted his right are known to include several elements in the nuptial gift palp in the female’s epigyne. At this stage, the female bit via silk wrapping, e.g. nutritionally worthless items such the male’s leg, and started feeding on a droplet of emerging as sucked-out insect carcasses, plant parts or seeds, or a hemolymph that was clearly visible on the female’s cheli- combination of these (Albo et al. 2011). This creates the cerae. The male did not escape but remained motionless and potential to enlarge the gift prior to encounters with females. continued copulation for 10 minutes; during this time the Since gift consumption predicts copulation duration, larger female fed on the male’s limb. gifts lead to longer copulations and consequently larger transfer of ejaculate, representing a remarkable advantage in sperm competition (Lang 1996; Stålhandske 2001; Albo Pedipalp insertion during thanatosis et al. 2011). Males should, therefore, be under strong selec- tion pressures to donate large gifts. Due to the low costs of In two mating trials we observed gift-giving males in leg loss, inclusion of an autotomized leg to the nuptial gift copula with virgin females performing thanatosis without should increase gift size and, ultimately, lead to fertilization removing their pedipalp from the female’s epigyne. In the advantages at low expense for the male. This prediction, first trial, the female attempted to avoid copulation and however, relates to a hypothesis that remains to be tested. escape with the gift once the male assumed the mating posi- What triggered leg autotomy in our laboratory trial (e.g. tion and inserted his pedipalp. In the second trial, the female disease, chemical stimulation from cues of conspecific, attempted to take the gift and interrupt copulation after inadequate size perception of the nuptial gift) also remains 20 minutes of male pedipalp insertion. In both cases, the unknown and should be subject to further investigations. female’s movements triggered thanatosis: males stretched Low costs of appendages loss may also explain female out their legs and hung motionless, anchored on the gift limb consumption during copulation. During our observa- with their chelicerae, and were transported by the females tion, leg consumption seemed to replace the function of gift
102 P. Ghislandi, T. Bilde & C. Tuni 275 consumption by allowing the male to copulate while the References female was motionless and occupied by feeding. Gift-less males are known to suffer from reduced mating success ALBO, M. J., TOFT, S. & BILDE, T. 2013: Sexual selection, ecology and evolution of nuptial gifts in spiders. In R. H. Macedo & G. and shorter copulations, which translates into lower sperm Machado (eds.), Sexual selection: perspectives and models from the transfer and fertilization success (Stålhandske 2001; Albo Neotropics. London: Academic Press: 183–200. et al. 2011). Thus, allowing the female to feed on one leg ALBO, M. J., WINTHER, G., TUNI, C., TOFT, S. & BILDE, T. 2011: may serve to prolong copulation duration, and ultimately to Worthless donations: male deception and female counter play in a increase male reproductive success. nuptial gift-giving spider. BMC Evolutionary Biology 11: 329. 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