Complex Male Mate Choice in Marine Snails Littorina
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Complex Male Mate Choice in Marine Snails Littorina Sara Hintz Saltin Licentiate thesis Department of Marine Ecology University of Gothenburg Till Mamma, Pappa och Hanna Abstract The ability to recognise potential mates and choose the best possible mating-partner is of fundamental importance for most animal species. This thesis presents studies of male mate choice within the genus Littorina. Males of this genus are sometimes observed to initiate mating with other males or with females of other species. How such suboptimal mating patterns can evolve is the theme of this thesis. In one study we investigated pre-copulatory- and copulation behaviour in L. fabalis and between this species and its sister-species L. obtusata. We found that males preferred to mount and mate with large and more fecund females rather than small females. Males also preferred to track the largest females mucus trails even though these were trails from another species (L. obtusata) although cross-matings were interrupted before completion. In a second study we found that males of three species (L. littorea, L. fabalis and L. obtusata) preferentially followed female trails. This suggests that females add a “gender cue” in the mucus. In the forth species, L. saxatilis, males followed male and female trails at random. Along with experimental evidence for high mating costs and abilities for male L. saxatilis to detect females of a related species, this suggests a sexual conflict over mating frequency. To reduce number of matings females avoid advertising their sex by disguise their mucus. The reason for the different species strategies is that L. saxatilis lives in much denser populations than the other species and therefore are the least likely to be sperm-limited. Instead, females probably get more than enough matings and disguise their trails in order to reduce the number of the costly matings, thus letting the males search blindly for mates. Key words: Mate choice, size preference, fecundity, Littorina fabalis, Littorina obtusata, Littorina saxatilis, intraspecific mating, interspecific mating, reproductive barrier, trail- following, mating behaviour, sexual conflict Populärvetenskaplig sammanfattning Förmågan att känna igen och välja en lämplig partner är viktig för djur eftersom det påverkar dess möjligheter att fortplanta sig. Detta är därmed en viktig del av evolutionen av arter. Den här avhandlingen är en studie av partnerval inom familjen Littorina, som är en grupp marina snäckor (strandsnäckor). Hanar i den här familjen kan ibland inleda parning med andra hanar eller honor av en annan art. Hur sådana märkliga beteenden kan uppstå är temat för den här avhandlingen som består av två studier. I den första studien undersökte vi parningsbarriärer mellan systerarterna L. fabalis och L. obtusata genom att studera deras parningsbeteende. Vi fann att hanar föredrog att bestiga och para sig med stora och mer fertila honor hellre än små honor. Hanarna följde också gärna slemspåren från stora honor trots att de kom från honor av en annan art, L. obtusata. Därför verkar det som att hanarna inte kan avläsa, från spåret, vilken art som har lagt spåret. Däremot så var parningarna mellan arterna färre och kortare vilket tyder på att det finns någon artigenkännings mekanism som verkar vid närkontakt. I den andra studien så presenterar vi data som tyder på att det finns en sexuell konflikt mellan könen hos arten L. saxatilis. Vi fann att hanarna hos övriga tre svenska arter (L. littorea, L. fabalis och L. obtusata) kunde skilja på hon- och hanspår och att de hellre följde honspår. Det tyder på att det finns någon signal i honornas slemspår som hjälper hanarna att identifiera deras spår och därmed lättare hittar en hona att para sig med (genom att följa slemspåret). Hos den fjärde arten, L. saxatilis, följde hanarna däremot alla spår slumpmässigt, vilket kan förklaras med att honorna av denna art inte avger någon ”honsignal” i slemspåren och då kan inte hanarna skilja på hon- och hanspår. Anledningen till de olika strategierna är att L. saxatilis lever i mycket tätare populationer och risken att få för lite parningar är minimal. Tvärt om så får honorna antagligen mer än nog med parningar och genom att låta bli att skylta med att de är honor så kan de minska kostnaden genom att reducera antalet övertaliga parningarna som en följd av att hanarna söker i blindo, bland både han- och honspår, efter en partner. Littorina saxatilis. Foto: Patrik Nilsson This thesis is based on the following papers: Paper I: Saltin S. H, Blom E. L, Johannesson K (2010) Pre-mating behaviours in males of a marine snail; the largest female may not be the best. Manuscript. Paper II: Johannesson K, Saltin S. H, Duranovic I, Havenhand J. N, Jonsson P. R (2010) Indiscriminate Males: Mating Behaviour of a Marine Snail Compromised by a Sexual Conflict? PLoS ONE 5(8): e12005. doi:10.1371/journal.pone.0012005 Table of contents Introduction .......................................................................................................................2 Mate choice ............................................................................................................................................................................2 The study system of Littorina ........................................................................................................................................3 Reproductive barriers between closely related species.....................................................................................3 Sexual conflicts.....................................................................................................................................................................5 Conclusions ........................................................................................................................6 References .........................................................................................................................8 Acknowledgements..........................................................................................................10 ………………………………………………………………………………………………. Paper I Paper II Introduction Evolution of reproductive strategies and species recognition are central to all sexual reproducing species. In most species mate choice is part of these processes. This thesis will focus on male mate choice in the genus of Littorina and the aim is to try to explain how the sometimes seemingly suboptimal male mating behaviours can evolve. As will be presented here male mate choice can be complicated by size-preference, which can result in maladaptive interspecific copulations between sister-species. Females that disguise their gender due to a sexual conflict can also hamper male mate search, resulting in males following and initiating copulation with other males. Mate choice within the genus of Littorina is as far as we know today exclusively male mate choice. Males choose both which female to track, which to mount and which to mate with, while the female stays more or less passive during the process. A complete lack of female choice is very unexpected and this observation will be investigated further in future studies. In this thesis I will present the main results of two studies (Paper I and II) and discuss these results in the context of conceptual models and earlier empirical findings. Starting with a brief introduction of the relevance of studying mate choice and the biological characteristics of the genus of Littorina. Mate choice From an evolutionary perspective, mate choice can be the most important decision in an individual’s life. Some animals spend their whole life with one partner (Harris 1973) while most animals (not least invertebrates) meet their partner only briefly during mating. Either way choice of mate is fundamental, in particular if the entire life reproduction is invested in one or a few mating opportunities (Foellmer & Fairbairn 2003). Traditionally researchers have been focusing on female mate choice. The reason is that females invest more in each mating in terms of size of the gametes (Bateman 1948) and sometimes parental care (Trivers 1972). With higher investment follow choosiness since the loss from a bad choice will increase with increasing investment. A good mate choice, on the other hand, can increase the fitness of the offspring through enhanced survival and reproductive success for sometimes several generations. Besides investments, there can also be costs involved in mating ranging from costs of mate search to increased mortality during mating (Daly 1978). These costs are not exclusive for the female; the male also often suffer from costs of mating and both male and female tend to be more choosy if there is a high cost involved in mating (Ridley 1983). Discriminate mating (sexual selection) have given rise to many of the most remarkable characters we can see in nature with numerous examples from the avian family, e. g. the tail of the peacock (Marion et al. 1991) and the widowbird (Andersson 1982), or extensive colouration and body-shape in families of teleost fishes (Kodric-Brown 1985; Quinn & Foot 1994). During the evolution of species, sexual selection has been a major selection mechanism influencing morphology, physiology and behaviour traits in many species (Andersson 1994). Of course a fundamental and obvious strategy in mate choice must be to mate with individuals of the opposite sex and of the same species. To achieve this, mate recognition can sometimes be a complex process involving