The distribution of a water strider, Aquarius rsmig/s, among habitat patches explained by sex specifie dispersal strategies

H. Helen Bang

Department of Natural Resource Sciences Macdonald Campus, McGiII University

August 2002

A thesis submitled ta the Faculty of Graduate Studies and Research in partial fulfillment of the requirements of the degree Master of Science

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Canada Abstract

A survey of a natural population of Aquarius remigis, a stream dwelling water strider, living in East Turkey Creek, Arizona, USA, revealed that they were mostly found in either pairs consisting of one male and one female, or in small female biased groups. Chi-squared analysis confirmed that this finding was not due to random chance. The sex ratios were manipulated to further test this observation, that is, that the water striders would retum to the most commonly observed sex ratio combinations even after being deliberately rearranged. Pairs of water striders or female biased sex ratios were observed in the experimental pools after a 24 hour period. Behavioural experiments conducted in the laboratory were performed to investigate the possible behavioural interactions that could influence the sex ratios observed in the field. Artificial pools with three water striders, in four sex ratio combinations, and four water striders, in five sex ratio combinations, were used. Male water striders were active in ail experimental pools regardless of sex ratio treatment. Female water striders were less active in male biased pools. Females were, however, more aggressive and active in pools containing only females. Résumé

Un aperçu d'une population normale des patineurs Aquarius remigis vivant dans East

Turkey Creek, Arizona, Etas-Unis, a indiqué qu'ils ont été trouvés la plupart du temps soit en paire composant d'un mâle et d'une femelle, ou parmi des petites groupes ayant plus de femelles que de mâles. L'analyse chi carré ajusté a confirmé que ces observations n'était pas dû à la chance aléatoire. Les rapports de sexe ont été manipule pour examiner plus profondement l'observation que les patineurs renverraient aux combinaisons le plus généalement observées de rapport de sexe même après une réarrangement délibéré. Après une période de 24 heures, les paires de patineurs ainsi que les groupes de patineurs ayant des rapports décentrés femelles de sexe dans les piscines expérimentales ont été observés. Des expériences comportementales ont été exécutées dans le laboratoire afin d'étudier les interactions comportementales possibles qui pourraient influencer les rapports de sexe observés sur le terrain. Des expériances exploitant des piscines artificielles avec trois patineurs dans quatre combinaisons de rapport de sexe, et quatre patineurs dans cinq combinaisons de rapport de sexe ont été élaborés. Les patineurs masculins étaient actives dans toutes les piscines expérimentales indépendamment du traitement de rapport de sexe. Les patineurs femelles étaient moins actives parmi les piscines avec unplus grand nombres de patineurs par mâle. Les femelles étaient cependant plus agressives et en actives dans les piscines contenant seulement des femelles.

11 Table of Contents

Abstract i

Resume il

Table of contents iii

List of tables ...... v

List of figures vii

Acknowledgments viii

Introduction ...... 1

Literature review ...... 3 General description ...... 3 Distribution ...... 4 Feeding behaviour 5 Territoriality 6 Mating system of Aquarius remigis 7 Mating dynamics: the conflict between male and female water striders 11 The effect of food on mating behaviour ...... 14 Sex ratio, density and dispersal-predictions based on literature ...... 15

Methods 16 East Turkey Creek ...... 16 Survey of the natural population 17 Effect of sex ratio on dispersal - field experiments 17 Dispersal from groups of three water striders 17 Dispersal from groups of four water striders 18 Comparison of male and female water striders ...... 19 Behavioural observations - laboratory component 19 Marking techniques 21 Data analysis ...... 22

Results 25 Dispersal experiments: a survey of natural populations 25 Effect of sex ratio on dispersal from groups...... 27 Effect of sex ratio on dispersal of males 28 Effect of sex ratio on dispersal of females ...... 28 Effect of sex ratio on occurrence of non-experimental water striders 29

Hl Laboratory behavioural experiments ...... 30 Effect of sex ratio on behaviour offocal males 30 Effect of sex ratio on behaviour of focal females ...... 32 The relationship between behavioural and dispersal experiments 33

Discussion 54 Male dispersal ...... 54 Female dispersal 56 Dispersal and natural distribution of water striders among pools 58

Conclusion 61

Literature cited 62

IV List of tables

Table 1: Observed frequencies and Chi-squared analysis for the movement of water striders, Aquarius remigis, in pools along East Turkey Creek in southeast Arizona 46

Table 2: Observed frequencies and Chi-square analysis of the movement of water striders, Aquarius remigis, where no new individuals arrived, in ail pools along East Turkey Creek in southeast Arizona ...... 46

Table 3: The analysis of the movement of focal male water striders, Aquarius remigis, along East Turkey Creek in southeast Arizona 47 a) fram ail pools b) fram ail pools in which no new individuals arrived c) fram those pools in which no new individuals arrived and ail original water striders remained

Table 4: The analysis of the movement of focal female water striders, Aquarius remigis, along East Turkey Creek in southeast Arizona ...... 48 a) fram ail pools b) from those pools in which no others arrived c) fram those pools in which no others arrived and ail originally marked individuals remained

Table 5: The analysis of newly arriving water striders, Aquarius remigis, along East Turkey Creek in southeast Arizona ...... 49 a) newly arriving males and females in ail pools b) newly arriving males in ail pools c) newly arriving females in ail pools

Table 6: The effect of sex ratio treatment on the number of attacks made on the focal male by otherwater striders in an artificial pool during a one hour observational Ume period 50

Table 7: The effect of sex ratio treatment on the number of attacks made on the focal male by other male water striders in an artificial pool during a one hour observational time period ...... 50

Table 8: The effect of sex ratio treatment on the number of attacks made on other water striders present in an artificial pool by the focal male during a one hour observational time period 51

v Table 9: The effect of the sex ratio treatment on the attack rate on females by the focal male water strider (per minute of time the male was single) in an artificial pool during a one hour observational time periad ...... 51

Table 10: The effect of sex ratio treatment on the attack rate made on the focal female by other water striders (per minute of time the female was single) in an artificial pool during a one hour observational time period ..... 52

Table 11: The effect of sex ratio treatment on the number of attacks made on other water striders by the focal female in an artificial pool during a one hour observational time period 52

Table 12: The effect of sex ratio treatment on the attack rate on the focal female water strider by males (per minute of time the female was single) in an artifici al pool during a one hour observational time period , 53

VI List of figures

Figure 1: The percentage of pools containing various numbers of water striders, and the number of individuals per pool of th.e water strider, Aquarius remigis, along East Turkey Creek in southeast Arizona ...... 35

Figure 2: The index of pool size versus the number of the water strider, Aquarius remigis, residing in East Turkey Creek in southeast Arizona 37

Figure 3: The percentage of female biased, male biased and equal sex ratios of the water strider, Aquarius remigis, and the number of individual water striders inhabiting pools along East Turkey Creek in southeast Arizona 39

Figure 4: The observed versus the expected frequency of various sex ratio combinations of the water strider Aquarius remigis found inhabiting pools along East Turkey Creek in southeast Arizona ...... 41

Figure 5: The effect of male harassment on the dispersal behaviour of female water striders Aquarius remigis, in pools along East Turkey Creek in southeast Arizona ,...... 43

Figure 6: The effect of available mating opportunities on the dispersal behaviour of male water striders, Aquarius remigis, in pools along East Turkey Creek in southeast Arizona ...... 45

vu Acknowledgements

! would like to thank my co-supervisor Dr. Piotr Jablonski for sharing his passion and enthusiasm for science with me, and for his continued support and encouragement throughout this project, both as a mentor and a friend.

1would Iike to thank my supervisor Dr. David Lewis for his input, advice and support for this project, as weil as Dr. Rodger Titman for sitting on my committee and offering critical and constructive comments.

Thank you to Dr. Wade and Mrs. Emily Sherbrooke at the Southwestern Research

Station of the American Museum of Natural History (SWRS) for making SWRS not only a great place to conduct research, but also a home away from home for the past three years, and to Cookie and Graham Johnson, Fred and Nancy Gehlbach and Jerry and

Esther Brown for completing the family.

Thank you to the volunteers at SWRS (Lesley Hayward, Andrew McCormick, Kristen

Bott, Taryn Hall, Sarah Froehler, Doris Hausieitner, Amy Gemmel, Ofer Ambalo and

Michelle Klukosky) and Jared Ogao for assisting with insect collecting and marking, and for making a tedious task so much fun.

Thank you to my friends Pete St. Onge, William Lee, Jelena Plecas, Tiffany Keller, and

Lyris Autran for words of encouragement and support to help me see this project through to the end.

A special thank you to my parents, grandmother and sister, Sandra, not only for their financial and continued emotional support, but for always believing in me. 1could not have made it this far without you.

Funding was received from the SWRS Student Support Fund of the American Museum of Natural History.

viii Introduction

The mating system of water striders (Hemiptera: Gerridae) is influenced

by a variety of factors including population size, habitat features, population sex

ratio, and competition for mates and resources (Rowe et. aL, 1994). Some water

strider populations provide model systems that answer questions relating to

factors that shape the distribution of a population (metapopulation) into smaller

groups (sub-populations) of interacting individuals inhabiting habitat patches of fragmented landscapes. Given their complex mating behaviour (reviewed below)

and evolutionary conflict between the sexes, studies of these animais may

illustrate the effect of mating systems, mate competition and sexual conflict on aspects of metapopulation structure.

During the mating season, Aquarius remigis pairs can be seen in aggressive encounters, often displaying very physical and dramatic conflicts.

Females are able to store viable sperm for at least ten days (Rowe et aL, 1994), yet they mate throughout the mating season, often several times a day (Arnqvist,

1989a, 1992b; Rowe, 1992; Sih et aL, 1990). Extra copulations, therefore, are not necessary, yet they continue to occur.

Superfluous matings are costly to females because females use energy stores to fight off male mating attempts; mating also increases theif predation risk

(Arnqvist,1989b; Rowe et aL, 1994). Mate guarding performed by the male after Insemination is also costly to the female since the pair remain in tandem, with the female carrying the male on her back, subsequently reducing her skating speed and increasing the energy spent on locomotion that could otherwise be

1 allotted to other activities. To avoid such costs, females shorten guarding

duration by dislodging the males, adjust their levels of foraging activity, adjust

their distribution within a pool, and increase the use of pool edges when the

proportion of males in the population increases (Arnqvist, 1997). An alternate

cost reduction strategy for females may be to disperse in search of pools with

lower mating frequencies.

Male reproductive success increases with the number of copulations that

take place with reproductive females and with the effectiveness of mate

guarding. An alternative strategy for a male that maximizes fertilization success through multiple copulations may be to leave a pool with male-biased sex ratios

in search of other pools with fewer males. The resolution of the evolutionary conflict between male and female water striders depends on the balance between, 1) the cost of male harassment and the cost of physically fighting off multiple copulation attempts, and 2) the energy cost of carrying a male and increased predation risk. The resolution also depends on the costs to males of the pre-copulatory struggles and sex-ratio-dependent costs of mate searching, as weil as on the behefits to males from mate guarding. It is possible that these costs and benefits affect the dispersal behaviour of the sexes, influencing the changes in number and sex ratio of water striders in local pools.

The objectives of this study were to survey the natural sex ratio composition, the behavioural interactions and dispersal behaviour of a population of A. remigis residing in pools along East Turkey Creek, southeast Arizona.

2 Uterature review

General description

Water striders belong to the order Hemiptera, family Gerridae. The

monophyly of the infraorder Gerromorpha has been firmly established (Andersen,

1982) and has been very weil studied (Andersen, 1982; Spence and Andersen,

1994). Gerridae is one of the few families of hemipterans that is semiaquatic. It

is comprised of approximately 1500 species that rely on water surfaces as their

primary adaptive zone (Spence and Andersen, 1994). They are often found

"skating" on the surface of the water, in both lentic and lotie environments.

Hemipterans are distinguished from other insect orders is based on three outstanding features: (1) the mouthparts are modified to form a piercing/sucking beak; (2) the anterior part of the first pair of wings are leather at the base, but are membranous at the apex; the second pair of wings are entirely membranous; and (3) metamorphosis is simple and graduai (Pennak, 1978).

Most species live in freshwater systems, although some lineages have developed ways of adapting to marine habitats (Spence and Andersen, 1994).

Gerrids range in size from 2-15 mm in length, with body shapes ranging from short and oval, to somewhat long and cylindrical. Males and females are easily distinguished in the field; the abdomen of the male ends in a pointed tip because of the adeagus (intromittent organ), whereas in comparison the female water strider's abdomen is rounded. Gerrids have the ability to stay on the surface of the water with the aid of a silvery white, velvety, waterproof pile that covers their

3 underside (Swan and Papp, 1972), and body secretions that repel the water.

Many species display five nymphal instars (Andersen, 1982; Foster, 1989;

Vepsalainen and Krajewski, 1986; Zimmermann, 1984) although there are a few

exceptions. Immature water striders appear as miniature versions of the adults; they behave in the same manneras the adults, but are sexually immature. The

development time of juvenile gerrids ranges from 40 to 65 days in both temperate and tropical environments, with the stage between the egg and the last two nymphal instars being the longest in duration (Spence and Andersen, 1994).

AquarÎus remÎgÎs, however, displays heritable, interpopulation variation in development time, which suggests an adaptation to local temperatures

(Blanckenhorn, 1991). A. remÎgÎs are polymorphic for wing development and have shown a single population containing long winged, short winged and even wingless adults (Spence and Andersen, 1994). Wing length is said to be governed by an interaction of genetic, ontogenetic and environmental effects

(Spence and Andersen, 1994). Most individuals are wingless, and rarely they have been observed in flight.

Distribution

Gerrids can be found on ail continents with the exception of Antarctica.

The greatest diversity occurrs in the Neotropics, central and west Africa, including Madagascar, and the Indo-Australian region (Andersen, 1982). The discovery of their high diversity in areas such as southern Asia, Malaysia, and

New Guinea, continues with recent work done by Polhemus and Polhemus

(1990). Their diversity in habitats, from fresh water to marine, (including five

4 species that are oceanic), displays their adaptability (Cheng, 1985, 1989; Cheng

and Wormuth, 1992). The structure of gerrid populations is wide ranging, from

very small populations in local streams (Fairbairn, 1985a) to large regional

populations that occupy many weil connected sites, supporting the movement of

individuals (Spence, 1989).

Feeding behaviour

Gerrids are predaceous, feeding on low flying insects, as weil as insects that are trapped on the water's surface. During times of food shortage, they have been known to be cannibalistic on weaker individuals or nymphs of their own species (Pennak, 1978). The principle components of their diet are small terrestrial and aquatic insects. Gerrids have modified forelegs for seizing and holding prey, while their mouthparts are used to pierce prey and ingest body fluids. The middle and hindlegs have become long stilts, with claws located some distance from the last tarsal segment, ensuring that water striders do not break the water surface (Swan and Papp, 1972). Single water striders prey on smaller insects, while larger groups of water striders may be better able to subdue and share a larger prey item (Erlandsson, 1988; Nakasuji and Dyck,

1984). This group sharing of prey is thought to dilute the individual risk of predation and explains why group members feed longer while a predator is approaching (Dili and Ydenberg, 1987). Males engaging in post copulatory mate guarding have been seen to share/feed on the prey captured by the female of the tandem (Vepsalainen, 1985; Vepsalainen and Nummelin, 1985).

5 Territoriality

Territorial behaviour in water striders is demonstrated through the

guarding of resources such as food (Rubenstein, 1984), breeding sites and mates (Koga and Hayashi, 1993). In mating populations of A. remigis, the best feeding sites are usually defended near the head of the pool by both sexes

(Rubenstein, 1984). Foraging success of territory holding insects depends on the patterns of prey arrivai and prey handling time (Blanckenhorn, 1991 b).

During the mating season, males tend to accumulate in areas with relatively few refuge sites for females; these areas of high density, male biased sex ratio and high female mating frequency are called "hot spots" (Rowe, et. al., 1994).

Experiments performed on these sites show that when additional sites for female refuge are included, thereby reducing female availability to males and decreasing mating opportunities, males tended to leave these spots and move on to other

"hot spots" (Rowe, et al., 1994).

Natural dispersal of water striders was examined by Fairbairn (1985b) in order to determine if there was preferential colonization upstream in an effort to compensate for continuous downstream drift. During the summer and fall, non­ reproductive adults tended to disperse upstream, but relatively few animais moved and the distances were relatively short (Fairbairn, 1985b). The greatest amount of movement was shown by reproductive adults, in an effort to disperse reproductive effort throughout the stream (Fairbairn, 1985b).

6 Mating system ofAquarius remigis

Mating and oviposition continue throughout the reproductive part of an

adult's life, which lasis from one to three months (Arnqvist, 1997). During this

period, the female can oviposit anywhere from 3 to 15 eggs per day, depending

on her size. She lays eggs close to the water's edge on floating debris or vegetation.

Traditionally, courtship and mating behaviour between males and females were considered harmonious, with both members co-operating to produce offspring. However, this is not necessarily the case. With respect to reproduction, male and female evolutionary interests may be asymmetrical and the conflicting behaviours seen. are the outward manifestations of these conflicts

(Arnqvist, 1997). Conflicts can occur over many aspects of mating, such as parental investment (Trivers, 1972, 1974), monogamous or polygamous mating systems (Alatalo et al., 1981), and mating frequency and duration (Parker, 1979;

Hammerstein and Parker, 1987; Arnqvist, 1989a, Thornhill and Sauer, 1991).

Water striders are ideal for studies involving mating systems since they display obvious conflicts and struggles during the mating season (Arnqvist, 1988;

Weigensberg and Fairbairn, 1994; Jablonski and Vespalainen, 1995). These apparent struggles make it easy to observe the start of the mating attempt, the duration ofcopulation, and the termination of copulation.

The process of mating is plastic from both the male and the female perspective. Male mating behaviour is fluid in water striders, being dependent, to

7 some extent, on factors such as the behaviour and density of other males in the

surrounding environment, ecological factors (Spence and Andersen, 1994), and seasona! changes in both habitat and reproductive status of females (Hayashi,

1985). Mating frequencyand duration are related to the operational sex ratio, which has a different effect on males than on females (Clark, 1988; Arnqvist,

1992a; Rowe, 1992). As suggested by Arnqvist (1997), most water strider mating activities can be broken down into two categories: Type 1and

Type II.

Type 1mating is characterized by very apparent conflicts and struggles between the sexes. In Type 1mating, the following five phases can be distinguished (Arnqvist, 1988; Rowe, 1992):

1. male lunging towards the female;

2. pre-copulatory struggle;

3. copulation;

4. post-copulatory contact guarding; and

5. post-copulatory struggle.

A male either sits and waits, or actively seeks out a mate. When another water strider is in sight, the male lunges at it in an attempt to mount and insert his genitalia. Male water striders are indiscriminate, and attempt to mount both conspecific or heterospecific males (Arnqvist, 1997). If successful, the mounted male tries to insert his adeagus, while grasping the female with his forelegs. The

8 female invariably responds with some form of resistance. The male meets

resistance since the female is trying to dislodge him; this displays the

asymmetrical and conflicting interests between males and femaies. One method

of resistance often displayed by the female is that of somersaulting (Arnqvist,

i 989b; Rowe, 1992). With the male mounted on the female's back, the female

rears up on her midlegs; this causes an imbalance in the pair, sending them falling backwards or sideways. The female also attempts to break the grasp of the male's forelegs with her own forelegs. Somersaulting behaviour is often combined with jumping and abrupt movements to try to further dislodge the male.

Precopulatory struggles range fram seconds to minutes, and in many species, females are successful in dislodging the male. Precopulatory struggles in A. remigis are successful in more than half of the observed cases (Weigensberg and Fairbairn, 1994). When the female desists, the male inserts his genitalia.

This copulation period varies widely fram species to species, and in A. remigis copulation can continue for hours (Clark, 1988; Fairbairn, 1988).

After insemination, the male remains coupled (in tandem) with the female, but his genitalia are retracted; this marks the beginning of the post-copulatory contact guarding period (Arnqvist, 1997). This period is also highly variable depending on the species and ecological factors. The mating is terminated in the same manner as it was initiated, with a visible struggle. In the post-copulatory struggle, the female again tries to dislodge the male, while he tries to remain in tandem; the duration of this struggle is highly variable amongst different species

(Arnqvist, 1988; Rowe, 1992; Weigensberg and Fairbairn, 1994; Jablonski and

9 Vespalainen, 1995), and under a variety of enviranmental factors (Weigensberg

and Fairbairn, 1994; Vespalainen and Savolainen, 1995).

Type Il mating behaviour is distinguished fram Type 1behaviour by the

persuasive courtship behaviours which precede physical contact. The males

appear to "court" females by producing ripple signais (males using their legs to

produce waves on the water surface which are detected by females in close

proximity) while maintaining a territory in a suitable oviposition site. Type Il mating involves little aggression and conflict. Males choose and defend territories which are ideal for oviposition in an attempt to attract females.

Receptive females appear to be attracted to these ripple signais (Wilcox, 1972;

Nummelin, 1988), and once they are within the territory, the males alter their signais in a manner to repel other males. Copulation begins after the female inspects the oviposition site, deems it suitable and allows the male to insert his genitalia. After copulation, the male dismounts and the female subsequently oviposits. During this time, the male displays post-copulatory non-contact guarding; he does so by staying within close praximity to the female while she is ovipositing, guarding her from other males. Oviposition lasts anywhere fram 10 to 20 minutes. After successful copulation and oviposition, the female leaves the site and the male resumes courtship to attract other females. In Type Il mating systems, there is a direct association between copulation and female oviposition

(Arnqvist, 1997).

10 Mat/ng dynamics: the confliet between male and fema/e water sfriders

Mating activity is high for bath male and female water striders, although costs and benefits differ for each sex. Females and males have been said ta mate fram "a few" to "several" times per day, throughout the reproductive part of their lives (Arnqvist, 1989a, 1992b; Rowe, 1992; Sih et aL, 1990). Male reproductive success is based on the number of females with which he is able ta mate. The last male to inseminate the female fertilizes the majority of her eggs

(Weigensberg, 1996; Rubenstein, 1989; Arnqvist 1988). As a result, each additional mating for males is advantageous. For females, on the other hand, this is not necessarily the case; a single mating may generally be enough to achieve success (Arnqvist, 1992). There is greater benefit for males, but not for females, to search for additional mating opportunities (Parker, 1979). Multiple mating in females also increases sperm competition among males, leading ta prolonged mating and mate guarding (Alcock, 1994). Female water striders are able ta store viable sperm for at least 10 days after copulation and it has been found that there is no direct benefit for thefemale to mate several times

(Arnqvist, 1989a; Rubenstein, 1989). Therefore, for females, ail other copulations may be considered superfluous. This subsequently leads to conflicts in the overall mating process.

For water striders, there is a very apparent conflict over mating decisions and mating duration, demonstrated by both pre-copulatory struggles and post­ copulatory struggles (Arnqvist, 1997). The cost of mating ta males is quite low in

11 terms of the energetic cost of gamete production, and the time and energy devoted to mating (Daly, 1978; Dewsbury, 1982; Andersen, 1982).

Sixty-five percent of a female's eggs of A. remigis are fertilized by the last male with whom she copulates, and the degree of sperm displacement is partly related to the duration of copulation (Rubenstein, 1989). This favours attempts by males to reduce sperm displacement through prolonged mating and mate guarding tactics. By guarding a female after copulation, he reduces the risk of having another male mate with her before she oviposits (Arnqvist, 1997). The cost of mate guarding to the male water strider is the potential loss of other mating opportunities (Parker 1979; Alcock, 1994; Jablonski and Kaczanowski,

1995).

The overall cost of mating to female water striders is much greater than that to males. Each additional mating costs her time, energy, increased predation risk and the risk of injury or disease transmission (Arnqvist, 1997).

Females are able to store viable sperm for long periods of time without further reduction in fertility, with females of A. remigis being able to store viable sperm for over 24 days (Arnqvist, 1988, 1989b, Rubenstein, 1989). The predation risk to females increases with mating (she is positioned underneath the male);

Arnqvist (1989b) showed that the risk of predation to female water striders of

Gerris odontogaster by backswimmers (Hemiptera: Notonectidae) increased by a factor of three when mating. In A. remigis, Fairbairn (1993) demonstrated that the risk of predation by frogs doubled for mating females versus single females.

The risk of predation is increased for mating females due to an increase in

12 visibility and a decrease in her ability ta escape when being attacked (Fairbairn,

1993; Rowe, 1994). in attacks made on mating pairs, females are more often taken as prey than males (Arnqvist, 1989b; Rowe, 1994). The physical struggles caused by male harassment and mating attempts are energetically costly ta females. Females also suffer fram reduced mobility during mating

(Arnqvist, 1989b; Fairbairn, 1993). As weil, since females constantly move around on the surface of the water in search for food, being in tandem results in reduced foraging success for equal effort, or a higher energetic effort for equal foraging success, compared with single females (Arnqvist, 1989a; Fairbairn,

1993).

How is it then that females resolve this conflict? Arnqvist (1992b) showed that female water striders make the "best of a bad job" and resist matings, but only up ta a certain point. There is a trade off between the costs of mating (i.e., lack of foraging succèss and increased predation rate and the costs of rejecting harassing males (i.e., energy cost of the struggle and increased predation).

When the level of male harassment is high, as it is in the presence of many males, females do not resist mating as intensely. Thus, pre-copulatory struggles are short in the presence of many harassing males (Arnqvist, 1997), and mating frequencies and mating duration increase. In other words, mating frequencies and mating duration increase in high density male situations.

However, if harassment rates are low, a female that is able ta dislodge a maie does not participate in copulation and is not harassed as frequently. These longer intervals between episodes of harassment allow for increased foraging

13 time. Under such conditions, females struggle more intensely in order to dislodge

males (Arnqvist, 1997). Under conditions of high male density, Wilcox (1984)

demonstrated that paired females foraged more effectively than a single female,

as a consequence of reduced harassment rates. Changes in the female's

behaviour, whether to avoid a copulating male or to give in, depends greatly on the operational sex ratio. Therefore, female reluctance is predicted to be a declining function of the rate of male harassment (Rowe, et aL, 1994).

The operational sex ratio affects not only the female's reluctance to mate, but also the level of male persistence. In female biased situations, sperm displacement does not occur as often as in male biased situations. In female biased situations, the duration of mate guarding is shortened in order to increase the male's benefit, allowing him to search for and gain other mating opportunities.

From the female's point of view, given a lack of harassing males, the cost of carrying a malè in tandem is greater than the energy cost of dislodging the male.

Therefore, mating frequency is lower, as is mating duration. Indeed, it has been demonstrated that females accept longer matings when the rate of harassment

(density of males) is high; mating duration increased with increased sex ratio

(Clark, 1988; Arnqvist, 1992b; Rowe, 1992; Jablonski and Vepsalainen, 1995;

Vepsalainen and Savolainen, 1995).

The effect offood on mating behaviour

Several experiments conclude that hunger has little effect on the mating behaviour of male water striders (Hayashi, 1985; Rowe, 1992). Clark (1988)

14 found that there was no effect on female hunger on mating duration or frequency.

A reduction of food availability for both males and females did not significantly effect mating frequency or duration (Rowe, et. aL, 1994). In A. remigis, mating females are not harassed as intensely as single females, and therefore, have increased foraging time and foraging success (Wilcox, 1984; Rubenstein, 1984;

Krupa, et aL, 1990).

Sex ratio, density and dispersal- predictions based on literature

Different sex ratios result in different consequences for male and female water striders. For female water striders, a pool with a male biased sex ratio may be less beneficial to them due to increased harassment. Therefore, one could expect that female dispersal would be higher in these pools. For males, a pool with a male biased sex ratio may result in more competitors for mates; therefore, one would also expect that male water striders would leave male biased pools more often, in search of pools with a lesser male biased sex ratio, where competition with other males would be lower. Density is almost the same factor as the amount of food resources available (higher density resulting in less available food resources); it is known that food resources influence dispersal decisions. Therefore, we could expect a positive correlation between pool size and numbers of water striders. In fact, Nummellin (1988) found that with a decrease in local food availability, female gerrids increased their dispersal rates, while the males did not. Upon removal of the females from the area, males increased their dispersal rates.

15 Methods

Ali experiments were conducted in the Coranado National Forest, located

in the southeastern corner of Arizona, USA. Field experiments were carried out

along a 40 m stretch of East Turkey Creek (31. r N, 109.2° W). Laboratory

experiments were conducted in the Mammalogy Laboratory, at the Southwestern

Research Station (SWRS) of the American Museum of Natural History, in Portal,

Arizona, USA.

East Turkey Creek

East Turkey Creek flows at an elevation of 1935 m in the Chiricahua

Mountains of southeastern Arizona, U.S.A (31.rN, 109.2°W). It has a baseflow of >1 Llmin and is a permanent, spring fed stream with first-order drainage (Lytle,

1999, 2000); the underlying stream substrate consists mostly of bedrack with cobbles, boulders and leaf detritus (Lytle, 1999, 2000). The natural vegetation found along this montane stream consists of pine-oak-juniper woodland (Hutto,

1985). The biotic community is characterized as being between the Transition

Life-zone and the Canadian Life-zone (Lowe, 1985). Major species of trees found at this elevation include silverleaf oak (Quercus hypoleucoides), trembling aspen (Popu/us tremu/oides), Douglas fir (Pseudotsuga menziesJ), and alligator juniper (Juniperus deppeana) (Lowe, 1985).

Precipitation in southeast Arizona falls mostly in the summer and winter; annual amounts of precipitation vary fram 30 to 55 cm of rain (Lowe, 1985).

16 SUNey ofthe natural population

A survey of the naturally dispersed population was conducted on 11 May,

2000. A series of 76 pools, with 283 individual water striders, was examined.

The number and sex of individuals present in each pool were recorded. An index

of pool area was calculated by multiplying the length by the width, and was used

in the correlation between pool size and number of water striders.

Effect ofsex ratio on dispersal œ field experiments

Field experiments were conducted in order to investigate how the dispersal decisions of individuals may affect the organization of a population into smaller groups, with certain sex ratios, in a natural situation. A series of small pools of water, approximately 2 m in diameter and isolated from each other, was used. These isolated pools were connected by streams of water, at least 0.5 m in length, ensuring that movement between the pools was not random. The isolated pools were distanced from each other to ensure that movement out of one pool would not affect the activities of the adjacent pool. East Turkey Creek was chosen for dispersal experiments for two reasons: (1) the presence of water striders, A. remigis; and (2) the presence of a series of small isolated pools.

Dispersal from groups of three water striders

From 14 May, 2000 to 10 June, 2000, a series of 18 isolated pools were chosen for study, measured and numbered with biodegradable flagging tape.

Individual water striders were collected from these pools, as weil as from two pools upstream and two pools downstream, to ensure that most of the insects

17 observed were part of the experiment. Individuals were then marked on the pronotum with enamel paints for identification (Blanckenhorn and Perner, 1996;

Rowe, et al., 1996); nine possible colours in combinations of one to four dots were used. Once marked, the insects were returned to the pools in specified sex ratios (male:female, 3:0, 2:1, 1:2,0:3). To avoid a possible bias due to the use of a particular pool for only some sex ratios, ail four sex ratios were represented at least once in each of the 18 pools used; therefore, a minimum of 72 trials was performed. Twenty-four hours after the original set up of each pool, the following were noted: the sex ratio of the water striders in the pool, original individuals remaining and any new individuals present. The 24 hour time period was used based on preliminary data (not presented here) done previously with two individuals in isolated pools, which showed that most of the initial movement occurred in the first 24 hours. Each group was composed of different individuals, therefore, results from each group were treated as an independent data point in subsequent statistica! analyses. Once again, the pools used in this experiment were distanced apart fram each other to ensure that the activities of one pool did not affect those of the other surrounding pools.

Dispersal trom groups oftour waier sir/ders

From 25 May, 2000 to 11 June, 2000, a series of 26 isolated pools was chosen in East Turkey Creek for dispersal experiments. The same procedures that are outlined above were repeated; however, four water striders were used instead of three, in specified sex ratios (male:female; 4:0, 3:1,2:2, 1:3, 0:4). Ali

18 five sex ratios were represented at least once in each of the 26 isolated pools to ensure that there was no bias for a sex ratio in any given pool. After 24 hours, changes due to dispersal were noted. Although some of the pools were used in both the three individual and four individual dispersal experiments, the insects themselves were used only once.

Groups of three and four water striders were used based on the results of the natural survey. Larger groups of water striders were not used in the experiments in order to keep them more akin to what was observed in nature.

Using a larger sample size with just one treatment group size would not give any explanation as to why water striders were most often found in pairs, with a female bias, nor would the results reflect the possible behaviours influencing this distribution.

Comparison ofmale and female water striders

The effect of sex ratio on the probability of dispersal was subsequently examined using the method outlined in Zar (1984), the double dichotomy contingency table. This effect needed to be examined in order to see if the sex of a water strider affected dispersal behaviour. The null hypothesis examined was that sex (either male or female) does not influence dispersal. A contingency table was done for each of the sex ratios.

Behavioural observaiions • laboratory component

Experiments were performed in the laboratory to determine the types of interactions that may have occurred during the 24 hour time period that

19 influenced certain water striders to leave the pools in certain sex ratio

combinations and not in others.

From 10 May, 2000 to 23 June, 2000, behavioural observations were

recorded in artificial pools in the Mammalogy Laboratory at SWRS. Four pools were set up (1 m x 1 m) on fiat tables; water froma nearby creek was used to fill the pools to a depth of 10 cm. A water current was produced on the surface of the pool using an air pump with flexible plastic tubing submerged below the surface of the water. The pump was left running during the entire experiment.

Although the laboratory itself is a roofed and enclosed structure, due to the absence of insulation, heating, and air conditioning and the open nature of the structure, the internai temperature of the laboratory was very similar to amb.ient temperature.

Two sets of experiments were conducted, the first with three water striders in four different sex ratio combinations (male:female; 3:0, 2:1, 1:2, 0:3) and the second with four individuals in five different sex ratio combinations (male:female;

4:0, 3: 1, 2:2, 1:3, 0:4). Unmarked water striders were collected each morning from East Turkey Creek to ensure that the population being studied in the behavioural experiments was similar to that used in the field studies.

Similar to the field experiments, insects were identified with coloured dots on their pronotum using enamel paints. The insects were sexed and then placed into the pool in agiven sex ratio combination. Two pools were set up simultaneously since that was the number of pools that could be monitored by one experimentor. The insects were left for one hour to acclimate to the plastic

20 pools. These artificial pools were devoid of any floating debris so as to prevent the insects from resting; this pramoted continuous interaction, so that the trials were comparable to the field experiments with respect to the number of insects that potentially interacted with each other. The observation period began when ail insects behaved normally (i.e., moving around the pool, bodies lifted fram the water surface, leg rubbing, seemingly unaware of human presence). Each sex ratio combination was watched continuously during several one hour periods; ten replicates were made for each sex ratio. Behavioural observations were timed in seconds using a stopwatch. Observations included: number of attacks, which were considered as any copulation attempts made by males on other males and females, or any aggressive behaviours made by females towards other water striders (male on male, male on female, female on male, female on female), duration of attacks, and duration of copulation, if it occurred successfully. These actions were recorded fram the point of view of the aggressor. The results were analyzed in order to explain observations fram the field manipulation experiment.

Marking techniques

Identification of individual water striders for experimental purposes can be achieved by marking the insects on their dorsum with coloured paints (Wilcox,

1984; Jablonski and Scinski, 1999) or liquid paper (Clark, 1988); it is thought to have no affect on the behaviour or survival ofthe insect (Cooper, 1984).

21 Data analyses

Observed frequencies of sex ratios (male:female; 0:2, 1: 1, 2:0, 0:3, 1:2,

2:1, 3:0,0:4, 1:3,2:2, 3:1,4:0,0:5, 1:4,2:3, 3:2,4:1, 5:0) were compared with

calculated expected frequencies using the method outlined in Zar (Figure 4). Chi

squared analysis was performed on the results of the survey of the natural

population ta determine whether the number of individuals found living in pools of approximately the same size (1 m x 1m) were occurring at random or not.

For the dispersal experiments, contingency tables and Chi-Squares (or its equivalent G- calculated using logarithms) were used often, since the data collected in the dispersal experiments were in the form of counts (i.e., did a water strider leave the pool, yes or no). Three analyses were conducted. First, the group of individuals in the pool was treated as a focal subject and any changes in its sex ratio composition were noted. Second, for each group (pool), a focal male was randomly chosen and the frequency of focal males leaving a pool was analyzed. Third, for each group, a focal female was randomly chosen and the frequency of focal females leaving a pool was analyzed. The last two analyses were performed to illustrate how sexes differ in their dispersal decision, in response to local sex ratios within a group. These analyses were used to establish how the dispersal decisions of the two sexes shaped the frequency of changes in group composition. Chi squared analysis was also performed on the results of the dispersal experiments to see if the distribution of the water striders into specifically observed sex ratios (male biased, female biased, or equal sex ratio) was by random chance. The analysis on which the conclusions were

22 based for the focal males and females was fram the pools in which ail experimental water striders, except for the randomly chosen focal individual, remained in the pool for 24 hours. This was to ensure that if the focal individual left the pool, the pool still contained the original set of water striders.

Subsequently, only a subset of pools were analyzed and a different subset for females and males were used. In addition to analyzing the data gathered fram experiments performed on ail pools (section A), two subsets were further analyzed; one included only pools where no other water striders arrived (section

B) and the other included pools in which no water striders arrived and ail non­ focal water striders remained for the 24 hour experimental period (section C).

For the purpose of discussion and simplification, results from only section B will be discussed. This is because results fram section A could also be due to new water striders arriving and remaining in the experimental pools, which would change the sex ratio being examined and therefore, change the interaction between the sexes. Results fram section C would be stricter and more ideal for use in analysis, however, due to the small sample sizes, the results may not be the most reliable. Section A and C were included in order to reflect the trend that was seen in the experiments performed. The analysis should not be treated as independent results, but rather three different views on the same set of results.

ln the analyses of behavioural experiments, a focal individual was randomly chosen from each pool and followed throughout the one hour observation period. Kruskal-Wallis ANOVA by ranks was performed to analyze the behavioural data, since the results were not normally distributed (Zar, 1984).

23 ANOVA was chosen over Mann-Whitney tests because the number of groups being compared was greater than two (Zar, 1984). Statistical analyses were performed using Statistica for Windows, Release 4.5 by StatSott© Inc, 1993.

24 Results

Dispersal experiments: a survey ofnatural populations

A survey of a population of A. remigis living in small pools in East Turkey

Creek was performed. The survey showed that of the pools surveyed, A. remigis

most commonly resided in groups of less than five water striders (Figure 1). 51 %

of the pools surveyed contained less than five water striders. 29% of the pools

sampled contained a pair of water striders (a pair in this experiment was defined

as a male and a female, any other group of two was defined as a dyad).

Although groups of more than five water striders were found, they were not common in the size of pool being examined (1 m x 1m on average) (Figure 2).

The distribution of pool sizes represents the characteristic pool size for these streams. It is common to find only a few larger pools amongst numerous smaller pools. The experiment performed and the results given are not biased toward analyzing only smaller pools while excluding larger pools, but are in fact representative of the pool size present in these streams.

The number of water striders living in pools ranged from one insect to eighteen insects (Figure 2). Examining the index of pool size (Iength x width), it can be seen that a majority of the pools in East Turkey Creek ranged between zero to five square meters. The most common range of numbers of water striders living in pools of this size was between one and four.

Of the groups containing five water striders and less, a majority of them lived in equal sex ratio combinations or in groups of female biased sex ratios

(Figure 3). In pools containing only two water striders, almost 90% of them were

25 in a pair and the remaining percentage contained two femaies. A pool containing

two males was never observed. Pools containing a male biased group was

observed in pools of three and four water striders, however, even in these two group sizes, female and equal sex ratio groupings were most common. Only in groups of five water striders and greater was the male biased sex ratio more commonly seen, and their frequency was similar to the frequency of the female biased groups. Behavioural experiments, performed later in the laboratory, helped shed some light on why this non-random distribution of sexes among the small pools may have occurred.

The observed frequencies of the sex ratio possibilities for two and three individuals differed from the expected frequencies. Chi squared analysis, to test the null hypothesis that the distribution of water striders was random, shows that the null hypothesis could be rejected for the groupscontaining two water striders

(X2=12.SS0 for X20.05.2=S.991 , df=2, p

The absence of a group of two male water striders, and a group of two male water striders and one female water strider, was not just due to random chance.

The observed frequency of one male and one female is almost double that of the expected, as weil as the combination of one male and two female water striders.

As the group sizes increase to four and five water striders, the observed and expected frequencies become closer. There is a complete absence of groups of four males, five females, and five males. The results of the chi squared analysis

26 for four (X2=1.333 for X2 0.05,4=9.488, df=4, p<0.05) and five water striders

(X2=2.412 for X2 0.05,5=11.070, df=5, p<0.05) does not allow us to reject the null

hypothesis that distribution was due to random chance.

Effect ofsex ratio on dispersal from groups

The treatment (sex ratio combination) had a significant effect on the movement of individuals fram pool to pool (Table 1 and 2). Table 1 reflects the results of ail pools that were manipulated. Table 2 shows the same observations as Table 1, but only those pools where no new water striders arrived (and therefore not allowing any new sex ratio combinations to interfere with the original set up) were considered.

For both the three water strider and four water strider treatments, the male biased pools (i.e., 3:0,2:1,4:0, 3:1) had a very high frequency (>82%) of pools from which at least one water strider left during the 24 hour observation period.

Similarly, a high frequency of dispersal was observed for pools with four females, but not for pools with three females, for which the lowest frequency (42%) was recorded. Consequently, the 2:2 sex ratio compositions were the most stable over the 24 hour observation period «61 % of pools had at least one water strider disperse during the 24 hour period).

27 Effect ofsex ratio on dispersal ofmales

Treatment had a significant effect on the movement of male water striders

(M-L Chi Square=6.63, df=2, p=O.036, M-L Chi-Square=11.94, df=3, p=O.008)

(Table 3). In male biased pools, there was more movement out of the pool by the focal male than in the female biased pools. In pools containing three males, the focal male (chosen randomly) left in 72.2% of the trials, and in pools containing four males, the focal male left the pool in 55.6% of the trials. In pools where the focal male was the only male, the male left only in 29.4% of the trials, in the 1:2 sex ratio pool, and in only 15.8% of the trials in the 1:3 sex ratio pool.

Effect ofsex ratio on dispersal offemales

There was no significant difference between the sex ratio treatment of a pool in female dispersal behaviour, for pools containing three water striders (M-L

Chi-Square=4.19, df=2, p=O.123) or for pools containing four water striders (M-L

Chi-Square=4.78, df=3, p=O.189) (Table 4).

The effect of sex ratio on the probability of dispersal was examined. The null hypothesis examined was that sex (either male or female) does not influence dispersal. A contingency table was done for each of the sex ratios. The null hypothesis could be rejected in the 2: 1 sex ratio pool (X2=5.385 for X2

0.05,1 =3.841, df=1, p

(X2=0.4.98 for X2 0.05,1=3.841, df=1, p

X20.05,4=9.488, df=4, p

28 p<0.05). Taking into account the fact that multiple comparisons were performed

here, it was assumed to be safe to conclude that the null hypothesis could not be

rejected and that sex does not influence dispersal. Hence, there is no evidence

that there are significant differences between males and females with respect to

their general tendency to disperse.

Effect ofsex ratio on occurrence ofnon-experimental water !)friden;

Newly arriving water striders were also recorded (Table 5). A new male was found in 33.3% of the trials in the 1:2 sex ratio pools and in 34.6% of the trials in the 0:3 sex ratio pools (Table Sb). In the pools set up with four water striders, newly arriving males remained in the pool 42.3% of the trials performed in the 4:0 sex ratio and in 41.2% of the trials performed in the 1.3 sex ratio pools

(Table Sb). The numbers above are interesting in comparison to those reflecting the movement of newly arriving females. In the 4:0 sex ratio pools, newly arriving females remained in 46.2% of the cases, and in the 0:4 sex ratio pool, newly arriving females remained in 38.5% of the cases (Table Sc). What is interesting to note is the difference between the sexes in the 0:3 sex ratio treatment pools. The trend is smaller, but in the same direction for the 0:4 sex ratio treatment pools. It is, however, larger in the 0:4 sex ratio treatment pools.

Hence for female biased treatments, new, non-experimental males were observed more often than new, non-experimental females.

The sex ratio treatment of a pool had a significant effect on whether or not newly arriving males and newly arriving females remain in a pool, for both pools

29 containing three and four water striders(M-L Chi-Square=8.77, df=2, p=0.033 and

M-L Chi-Square=21.16, df=4, p=O.003 for newly arriving males, and M-L Chi­

Square=7.88, df=2, p=0.049 and M-L Chi-Square=25.05, df=4, p<0.0001 for

newly arriving females) (Table 5).

Laboratory behavioural experiments

Effect ofsex ratio on behaviour offocal males

The effect of sex ratio treatment on the number of attacks made on the focal male by other water striders in an artificial pool, during a one hour observational time period, was examined (Table 6). There was a significant effect of sex ratio treatment, and of the number of attacks made on the focal male by others present in the pool (K-W H2.32=21.291, p<0.001 for 3 water striders, K-W H3.40=30.967, p<0.001 for 4 water striders). In the 1:2 sex ratio pool where the focal male was the only male, the male was never attacked by the others present in the pool. However, the rate of attack increased to a median value of 7.5 in the 3:0 sex ratio pools. The same trend was observed in the experimental pools set up with four water striders. In pools where there were greater numbers of males, the focal male was attacked more often than in those pools where there were no other males. In the pool where the focal male was the only male (1 :3), the focal male was not attacked at ail.

The effect of sex ratio treatment on the number of attacks made on the focal male by other male water striders was examined (Table 7). In the 3:0 sex ratio pools, the attacks made on the focal male by other males was higher than

30 the rate of attack on the focal male in the 2: 1 sex ratio treatment This trend of

increasing numbers of attacks on the focal male by other males was further

magnified in the pools where four water striders were present ln the 4:0 sex

ratio pools, the median number of attacks was 7.5, whereas the median was only

1 in the 2:2 sex ratio pools. There was a significant effect of sex ratio treatment

on the number of attacks made on the focal male by other males (K-W

Hi ,Z1 =3.782, p=0.052 for pools with three water striders, and K-W Hi ,zo= 10.038,

p=0.002 in pools with four water striders).

The number of attacks made by the focal male on others present in the pool showed that there was no significant effect of sex ratio treatment on the number of attacks in pools with three water striders (K-W Hz,3z=0.02, p=0.99)

(Table 8). The focal male made approximately the same number of attacks, regardless of the sex ratio treatment present in the pool for the experimental set ups with three individuals.

It was found that there was a significant effect on the number of attacks made by the focal male on others in the pool in the experiments containing four water striders (K-W H3,4Q=8.514, p=0.037). The focal male made more attacks on others in the 1:3 sex ratio pool, in comparison to the other sex ratios.

There was a significant effect of sex ratio treatment on the number of attacks made on the female by a focal male in pools containing four water striders (K-W Hz,30=13.017, p=0.002) (Table 9). There were a greater number of attacks made by the focal male in the pool where he was the only male present. ln pools with three water striders, the number of attacks made on the females by

31 the focal males increased with a female biased sex ratio, but this increase was

only marginally significant (K-W H1,2f=3.655, p=0.056).

Effect ofsex ratio on behaviour of focal temales

The effect of sex ratio treatment on the number of attacks on the focal female by others was significant in pools with three water striders (K-W

H2,32=8.373, p=0.015), although not in pools with four water striders (K-W

H3,40=6.039, p=0.109) (Table 10). However, in both experimental set ups, the focal female was attacked more often in pools where there was a higher number of male water striders present. The effect of sex ratio treatment on the number of attacks made by the focal female on others was significant for both the experimental set ups with three water striders and four water striders (K-W

H2,32=8.1448, p=0.017 and K-WH3,4o=18.768, p=0.0003) (Table 11). The focal female made attacks on others in the sex ratio pools containing only females

(i.e., 0:3 and 0:4).

The effect of sex ratio treatment on the attack rate on the focal female, by the males present, showed that there was no significant effect (K-W H1,21=OA04, p=0.525, K-W H2,32=1.1 08, p=0.575) (Table 12). The sex ratio treatment of the pool did not effect whether or not the focal female would be attacked more or less often by the males present in the pool.

32 The relationship between behaviouraJ and dispersal experlments

Behavioural data collected should help ta explain some of the observed dispersal behaviour. Figure 5 shows the effect of harassment by males on female dispersal fram pools. There is no significant effect of harassment on female dispersal (R2=D.185, p=D.185). There is no clear relationship between harassment by males and the dispersal behaviour of females.

Figure 6 shows that there is a marginally significant correlation between mating opportunities for males and male dispersal tendency (R2=D.434, p=D.D64). With an increase in mating opportunities, the percentage of males leaving the pool decreases.

33 Figure 1: The percentage of pools containing various numbers of water striders, and the number of individuals per pool of the water strider, Aquarius remigis, along East Turkey Creek in southeast Arizona

34 35

30

25

20 .... !:: III t: III I:l. 15

10 -fi

5

o 1 2 3 4 5 6 7 8 9 10 11 12 Number of Individual Water striders Figure 2: The index of pool size versus the number of the water strider Aquarius remigis residing in East Turkey Creek in southeast 8 Arizona (p=1. 755x1 0- )

36 18 l , • 16 y ::: O.5024x + 1.8321 2 R ::: 0.4437 14

12 - t!! w 1:1 'b: -~ 10- w lU -~ '0 "- 8 W .CI E ::l Z 6

4 2 -, ....• • o ~ r 1 0.00 5.00 10.00 15.00 20.00 25.00 Index of Pool Size Figure 3: The percentage of female biased, male biased and equal sex ratios of the water strider, Aquarius remigis, and the number of individual water striders inhabiting pools along East Turkey Creek in southeast Arizona

38 100

90

80

70 -

60 l: CI) f 50 CI) a..

40

30

20

o r"" '·::'w·;·'·'·;·'·'·;·'·;· 5 >5 2 3 Number of wateA;triders in a pool Figure 4: The observed versus the expected frequency of various sex ratio combinations of the water strider, Aquarius remigis, found inhabiting pools along East Turkey Creek in southeast Arizona

40 Frequency ...... I\J 0 I\J .j::>. 0) 00 0 I\J .j::>. 0) 00 0

.. 1.:.: FF

MF

MM

FFF

MFF

MMF

MMM th (1) FFFF >< :;JlJ III.... ër MFFF n 0 3 0" MMFF :Ji' ....III S' ::3 (II MMMF

MMMM

FFFFF

MFFFF

MMFFF

MMMFF

MMMMF

MMMMM Figure 5: The effect of male harassment on the dispersal behaviour of female water striders, Aquarius remigis, in pools along East Turkey Creek in southeast Arizona

42 50 l llIII 45 -~

llIII '0- 40 - o Q" llIII tU 35--

Q) =-;;; 30- Q) --«S E 25- ~

$ 20­ llIII --«S , ~ Q) 15- llIII Q" tA Ci 10-

5 -

o 1 ------,------r- 1 ,------1 o 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 harrassement by males (attacks by males on females per minute) Figure 6: The effect of available mating opportunities on the dispersal behaviour of male water striders, Aquarius remigis, in pools along East Turkey Creek in southeast Arizona

44 lIIIII V 'IF"" m -...... (1) ~ E N J!! i 'IF"" c: 0 m ...... (1) ~ 0 E 'IF"" ...... ~ (,) .2 ..c>. m CO ~ (,) ~ ~ ~ -m te (1) E lIIIII c: ::::s ~ 0 Q. V Q. 0 œ c: ;; ~ :li lIIIII N r lIIIJi 1 llllJii 1 0 0 0 0 0 0 0 0 0 0 00 t--- te 1.0 V (If) N 'IF"" (Iood a4J val Je4~ salew le:lO~ ~o 0/0) leSJads!o Table 1 Observed frequencies and Chi Squared analysis for the movement of water striders, Aquarius remigis , in pools along East Turkey Creek in southeast Arizona

No. Individuals Sex Ratio(M:F) % left N dt fi 3:0 100 20 2:1 82.1 28 3 25.59 3 <: 0.0005 1:2 63.3 30 0:3 42.3 26 4:0 84.6 26 3:1 93.3 30 4 2:2 60.9 23 13.76 4 0.008 1:3 70.6 34 0:4 92.3 26

% left is the percentage of wa!er striders that left the pool after 24 hours

Table 2 Observed frequencies and Chi Square analysis of the movement of water striders, Aquariu$ remigis , where no new individuals arrived, in aU pools along East Turkey Creek in southeast Arizona

No. Illdividuais Sex Ratio (M:F) % left N dt fi 3:0 100 18 2:1 85 20 3 29.25 3 <: 0.0001 1:2 64.7 17 0:3 25 16 4:0 100 9 3:1 100 24 4 2:2 55 20 22.71 0.0001 1:3 70 20 0:4 88.9 9

% left is the percentage of water striders that left the pool after 24 hours Table 3 The ana!ysis of the movement of focal male water striders, Aquarius remigis , along East Turkey Creek in southeast Arizona A From ail pools B From those pools in which no new individuals arrived C From those pools in which no new individuals arrived and ail original water striders remained

No. Individuals Sex Ratio (M:f) % left N X2 df P 3:0 70 20 3 2:1 42.9 28 6.8 2 0.033 1:2 33.3 30 A 4:0 38.5 26 3:1 60 30 4 12.15 3 0.007 2:2 34.8 23 1:3 18.2 33

3:0 72.2 18 3 2:1 50 20 6.63 2 0.036 1:2 29.4 17 4:0 55.6 9 3:1 62.5 24 4 11.94 3 0.008 2:2 30 20 1:3 15.8 19

3:0 100 1 3 2:1 66.7 12 15.5 2 0.0004 1:2 0 10 C 4:0 100 1 3:1 50 6 4 9.13 3 0.03 2:2 30.8 13 1:3 0 8

% let! is the percentage of water striders that let! the pool after 24 hours Table 4 The analysis of the movement of focal female water striders, Aquarius remigis , along East Turkey Creek in southeast Arizona A From ail pools B From those pools in which no others arrived C From those pools in which no others arrived and ail originally marked individuals remained

No. Individuais Sex Ratio (M: F) % left N )(2 df P 0:3 30.8 26 3 1:2 40 30 2.37 2 0.306 2:1 21.4 28 A 0:4 30.8 26 1:3 33.3 33 4 2.71 3 0.438 2:2 17.4 23 3:1 36.7 30

2:1 20 20 3 1:2 47.1 17 4.19 2 0.123 0:3 18.6 16 B 3:1 41.7 24 2:2 15 20 4 4.78 3 0.189 1:3 36.8 19 0:4 44.4 9

2:1 8.3 12 3 1:2 40 10 4.67 2 0.097 0:3 7.7 13 C 3:1 66.7 6 2:2 0 13 12.65 3 0.005 1:3 25 8 0:4 50 2

% left is the percentage of water stridérs that left the pool after 24 hours Table fi The analysis of newly arriving water striders, Aquarius remigis , along East Turkey Creek in southeast Arizona A Newly arriving males and females in ail pools B Newly arriving males in ail pools C Newly arriving females in ail pools

No. Individuals Sex Ratio (M:F) %newind N )(2 df P 3:0 10 20 2:1 28.6 28 3 7.76 2 0.051 1:2 43.3 30 0:3 38.5 26 A 4:0 65.4 26 3:1 20 30 4 2:2 13 30 27.18 4 < 0.0001 1:3 41.2 34 0:4 65.4 26

3:0 5 20 2:1 17.9 28 3 8.77 2 0.033 1:2 33.3 30 0:3 34.6 26 B 4:0 42.31 26 3:1 10 30 4 2:2 4.35 23 21.16 4 0.003 1:3 41.18 34 0:4 50 26

3:0 5 20 2:1 25 28 3 7.88 2 0.049 1:2 20 30 0:3 3.85 26 C 4:0 46.15 26 3:1 16.67 30 4 2:2 8.7 23 25.05 4 < 0.0001 1:3 5.88 34 0:4 38.46 26

% new ind. is the percentage of new individuals that arrived and remained in the pool after 24 hours Table 6 The effect of sex ratio treatment on the number of attacks made on the focal male by other water striders in an artificial pool during·a one hour observational time period

No. Individuals Sex Ratio (M:F) Median Min Max lower Quartile Upper Quartile Kruskal~Waiiis p 3:0 7.5 0 16 2 10.5 3 2:1 1.5 0 18 1 4.5 21.291 < 0.0001 1:2 0 0 1 0 0 4:0 7.5 4 24 4 9 3:1 4 1 8 2 4.5 4 30.967 < 0.0001 2:2 0.5 0 5 0 1.5 1:3 0 0 0 0 0

Table 1 The effect of sex ratio treatrnent on the number of aUacks made on the focal male by other male water striders in an arl:ificial pool during a one hour observational time period

No. Individuals Sex Ratio (M:F) Median Min Max lower Quartile Upper Quartile Kruskal~Waiiis p 3:0 7 2 16 4 11 3 3.782 0.052 2:1 1 0 18 1 4.5 4:0 7.5 4 24 4 9 4 3:1 4 1 7 2 4.5 10.038 0.002 2:2 1 0 5 0 1 Table 8 The effect of sex ratio treatment on the number of attacks made on other water striders present in an artificial pool by the focal male during a one hour observational time period

No. Individuals Sex Ratio (M:F) Median Min Max lower Quartile Upper Quartile Kruskal~Wallis p 3:0 6 0 29 2 11 3 2:1 6 1 13 2 8 0.02 0.99 1:2 4 0 13 2 7 4:0 5 0 18 1 9.5 3:1 9 5 53 6 12.5 4 8.514 0.037 2:2 7 4 14 4 9 1:3 14 7 18 8 15.5

Table 9 The effect of the sex ratio treatment on the attack rate on females by the focal male water strider (per minute of time the male was single) in an artifical pool during a one hour observational time period

No. Individuals Sex Ratio (M:F) Median Min Max lower Quartile Upper Quartile Kruskal-Wallis p 2:1 2 0 8 0 4.5 3 3.655 0.056 1:2 4 0 9 2 7 3:1 5.5 0 14 2 8 4 2:2 5 2 13 2 5.5 13.017 0.002 1:3 14 7 18 8 15.5 Table 10 The effect of sex ratio treatment on the attack rate made on the focal female by other water striders (per minute of time the female was single) in an artifical pool during a one hour observational time period

No. Im:iividuals Sex Ratio (M:F) Median Min Max Lower Quartile Upper Quartile Kruskal-Wams p 0:3 1 0 6 0 1 3 1:2 2 0 9 1 5 8.373 0.015 2:1 3 0 14 0 8.5 0:4 1 0 5 0 1 1:3 7 2 9 3 7 4 6.036 0.109 2:2 5 1 13 1 6.5 3:1 10 3 20 5 13.5

Table 11 The effect of sex ratio treatment on the number of attacks made on other water striders by the focal female in an artificial pool during a one hour observational time period

No. Indivic.iI..Ials Sex Ratio (M:F) Median Min Max Lower Quartile Upper Quartile Kruskal-Wallis p 0:3 2 0 5 0 3 3 1:2 0 0 1 0 1 8.144 0.017 2:1 0 0 1 0 0.5 0:4 4 0 18 1 9.5 1:3 0 0 2 0 1 4 18.768 0.0003 2:2 0 0 1 0 0.5 3:1 0 0 1 0 0 Table 12 The effect of sex ratio treatment on the attack rate on the focal female water strider by males (per minute of time the female was single) in an artificial pool during a one hour observational time period

No. Individuals Sex Ratio (M:F) Median Min Max lower Quartile Upper Quartile Kmskal-Wallis p 2:1 0.073 0 0.237 0 0.142 3 0.404 0.525 1:2 0.071 0 0.15 0.033 0.117 3:1 0.167 0.063 0.339 0.083 0.227 4 2:2 0.083 0.017 0.217 0.017 0.109 1.108 0.575 1:3 0.117 0.033 0.15 0.05 0.119 Discussion

Male dispersal

During the mating season, male water striders are driven by the need to find as many mating opportunities as possible and are less focused on the task of finding other resources (Clark, 1988). Behavioural experiments performed in the lab showed that male water striders were very active in ail experimental set ups regardless of sex ratio. Male water striders were constantly swimming and attempting to copulate with any other water striders that they detected in the vicinity (personal observation). Water striders do not use any visual cues to distinguish between the sexes (Arnqvist, 1997) and therefore will attempt to mate with both males and females alike. In ail male pools, there were a large number of copulation attempts made by the males in the pool. Increased movement around the poolled to increased interaction. This increased activity may have enabled male water striders to ascertain that there were no mating opportunities present. In the dispersal experiments involving experimental pools set up with males only, dispersal out of these pools occurred in almost 100% of the cases.

The absence of mating opportunities may have led to dispersal behaviour. If male water striders remain in a pool during the mating season strictly for mating opportunities, this should be reflected in their dispersal activity from pools with female water striders present. Indeed, as the number of females present in the pool increased, male water striders decreased their tendency to leave the pools.

Male reproductive success is based on the number of successful copulations (Parker, 1979). Success is also based on a last male mating

54 advantage (Arnqvist, 1988) where a last male fertilizes at least 65% of a female's

eggs (Rubenstein, 1989). Male water striders therefore increase mate guarding

tactics in order to prevent sperm displacement (Arnqvist, 1997). Female water

striders are able to store viable sperm for at least ten days after copulation

(Arnqvist, 1989a). Any additional mating results in the loss of time and energy,

increased predation risk, risk of injury and disease transmission (Arnqvist, 1997) and increased visibility to predators (Fairbairn, 1993). Therefore, females resist mating attempts and long guarding times by maies. Female A. remigis are flexible in their levels of resistance during the mating season. When the level of male harassment is high, females will lower their resistance. It would be less costly for females in this situation to carry a male in tandem and forage freely, than to struggle with each males' attempt to copulate. Indeed, females in tandem forage more efficiently than single females (Wilcox, 1984). Female reluctance is dependent on the operational sex ratio and is predicted to be a declining function of the rate of male harassment (Rowe, et. al., 1994). Male water striders must deal with female reluctance in two ways, either to increase copulation attempts and effort, or disperse in search of other mating opportunities. With an increase in the number of females in a pool, males must try harder to overcome female reluctance and mate successfully. On the other hand, with an increased number of males present in the pool, even though female reluctance is low, males must increase mate guarding after copulation to decrease the chance of sperm displacement. Given longer guarding time, the males have less time for mate searching. Thus, an increase in the number of males decreases mating

55 opportunities not only due to the decrease in number of females available per

male, but also due to the decrease in the amount of time the male has for mate

searching. Therefore, as seen in Figure 6, these lower mating opportunities

seem to cause higher male dispersal from male biased pools. Dispersal for male

water striders out of female biased pools is the lowest in comparison to ail other

sex ratio combinations. Although female reluctance may be higher in these

instances, there are still mating opportunities available to male water striders

without competition from other males.

Female dispersal

ln pools set up with male biased sex ratios, females experienced higher

levels of harassment by males that were attempting to mate. These high levels of harassment should result in high dispersal of females out of male biased pools.

Female water striders are motivated to find resources rather than mating opportunities asstated by Nummelin (1988). Nummelin found that dispersal rates of female water striders increased when food availability decreased. However, food availability did not affect the dispersal rate of male water striders while removal of female water striders did increase their dispersal rates. Nummelin's findings results are reflected in the dispersal results of this experiment. If females were interested only in finding resources, then a large group of females living in a small pool would create an increase in competition for available resources. Indeed, in pools with 0:3 and 0:4 experimental sex ratios, the

56 dispersal rate of females is higher than in pools with an intermediate sex ratio.

Accordingly, the behavioural data fram ail female pools (0:3 and 0:4 sex ratio pools) shows an increase on the rate of attacks made by a focal female in a pool

on other femaIes present in the pool. The number of attacks decrease with the presence of at least one male in the pool, indicating that a level of competitive aggression between females decreases in those pools with a male biased sex ratio. Hence, higher levels of female-female aggression may cause female dispersal from female biased pools. This prediction is contrary to the assumption that females avoid high harassment by males searching for mating opportunities.

It is not surprising that most of the results for focal females in the dispersal experiment were non-significant. The only significant result (Table 4c, for four individual pools) shows, as expected from the above, that high dispersal occurs in the extreme male and the extreme female biased pools.

One of the lowest rates of female dispersal should also be seen in pools with an even sex ratio. This is because females in tandem can still forage without being overly harassed and without competition fram single females.

Further, with respect to males, competition is lower in the absence of other males, giving single males mating opportunities. The dispersal experiments showed that the two lowest rates of dispersal are seen in the female biased pools and the pools with an even sex ratio. The behavioural experiments showed that the pools with even sex ratios had fewer incidents of males attacking males and males attacking females. These results may be due to the fact that since males

57 and females are equal in number, there is enough opportunity for mating and foraging without competition.

Dispersal and natural distribution of water striders among pools

If the behavioural and dispersal results are accurate, then the pools of A. remigis populations seen in the field should be pools of female biased sex ratios or pools with an even sex ratio. A survey of small pools in East Turkey Creek,

Arizona showed that most of the pools sampled contained equal or female biased sex ratios. In pools where there were only two water striders, the two consisted of one male and one female. In pools where there were three water striders, over 90% of those pools were female biased. Equal sex ratios allow for ample mating opportunities as weil as foraging opportunities. Female biased sex ratios also allow for these opportunities, with females experiencing minimal harassment fram both males and females.

Although its significance was not examined in this experiment, in the sampled population, water striders lived in small groups, in pools appraximately 1 m by 1 m in size. As shown in Figure 4, some of the expected frequencies of the sex ratio combinations were not seen in the field, including the combination of two males, two males and one female, four males, five females and five males.

However, based on the behavioural and dispersal data of this experiment, these combinations, although expected, were not seen. Pools containing only males would not last very long in the population, as the active males may quickly ascertain the absence of mating opportunities and thus move on to another pool.

58 ln pools containing only females, competition for resources, resulting in

increased female-female aggression, may be high and result in female dispersal from these pools. In the field, the occurrence of observed female and equal sex

ratio pools dominated over the expected frequencies.

With respect to mating, the conflict between male and female A. remigis is evidenced by both their pre- and postcopulatory struggles (Arnqvist, 1997).

Mating activity is high for both males and females throughout the reproductive period, with each sex mating up to several times per day (Arnqvist, 1992b, Sih et al., 1990), with some matings lasting several hours (Weigensberg and Fairbairn,

1994). As stated in the literature, females must make the "best out of a bad situation" (Arnqvist, 1992b), by weighing the cost of struggles against the cost of carrying a male. They must then behave accordingly and mate or disperse. To my knowledge, there have been no studies that have surveyed the sex ratios present in a natural population, then examined their distribution within the population and attempted to explain why they are present in these particular sex ratios. My experiments showed that there is a link between mating behaviour

(copulation attempts, successful copulations, female harassment and female competition), dispersal behaviour (to remain or disperse from a pool) and the sex ratio of a pool, of a given population of water striders living in the field.

Bath male and female. water striders must weigh the mating opportunities present in a pool against the cost of spending energy on persisting with a copulation aUempt (male) or fighting off a copulation attempt (female). They must also weigh the possibility of mating opportunities against predation and the

59 benefit of a successful copulation attempt, leading to foraging opportunities.

Subsequently, A. remigis are able to find a balance between the costs and the measure of successful mating. Hence, they live in pools where a balance is met.

60 Conclusion

This study attempts to explain the composition and sex ratios of a natural population of A. remigis, in the field, based on behaviourai and dispersal data obtained through experiments.

The costs and benefits of mating and harassment lead to whether or not males and females alike will disperse fram pools. Dispersal behaviour ceases and water striders remain in a given pool where there is mating opportunity along with foraging opportunities and minimal competition.

Most previous studies conducted on the effects of sex ratio on behaviour in this system were performed in artificial situations with closed pools. These numerous studies pravided the basis on which predictions were made on what might happen in a natural situation if dispersal were possible. Further experiments conducted in the field, using an open system, would be a useful extension to previous studies.

61 Uterature cited

Alatalo, RV., A. Carlson, A. Lundberg, and S. Ulfstrand. 1981. The conflict between male polygamy and female monogamy: the case of the pied flycatcher, Ficedula hypo/euca. Am. Nat. 117: 738-753.

Alcock, J. 1994. Postinsemination associations between males and females in insects: The male-guarding hypothesis. Annu. Rev. Entomol. 39:1-21.

Andersen, N. M. 1982. The Semiaquatic Bugs (Hemiptera, Gerromorpha). Phylogeny, Adaptation, Biogeography, and Classification. Klampenborg, Denmark: Scandinavian Science. Pp. 455.

Arnqvist, G. 1988. Mate guarding and sperm displacement in the water strider Gerris lateralis Schumm. (Heteroptera: Gerridae). Freshwater Biology. 19 :269-274.

__ 1989a. Sexual selection in a water strider: the function, nature of selection and heritability of a male grasping aparatus. Oikos. 56 :344-350.

__ 1989b. Multiple mating in a water strider: mutual benefits or intersexual conflict? Anim. Behav. 38 :749-756.

__19928. The effects of operational sex ratio on the relative mating success of extreme male phenotypes in the water strider Gerris odontogaster (Zett.) (Heteroptera: Gerridae). Anim. Behav. 43:681-83.

__ 1992b. Precopulatory fighting ina water strider: intersexual conflict or mate assessment? Animal. Behav. 43 :559-567.

__1997. The evolution of water strider mating systems: causes and consequences of sexual conflicts. In, The Evolution of Mating Systems in Insects and Arachnids. Choe, J.C., and B.J. Crespi (eds). Cambridge University Press, Cambridge.

Blanckenhorn, W.U. 1991a. Fitness consequences offood-biased territoriality in the water strider, Gerris remigis. Anim. Behav. 42:147-49.

__1991 b. Foraging in groups of water striders (Gerris remigis): effects of variability in prey arrivais and handling times. Behav. Ecol. Sociobiol. 28: 221-26.

Blanckenhorn, W. U., and D. Perner. 1996. Life history dependent behavioural variation in water striders, Aquarius remigis. Ethology. 102: 993-1007.

62 Cheng, L. 1985. Biology of Halobates (Heteroptera: Gerridae). Annu. Rev. Entomol. 30: 111-35.

__1989. Factors limiting the distribution of Halobates species. Pp. 357-62. In Reproduction, Genetics and Distribution of Marine Organisms. Ryland, J., and P. Tyler (eds.). Fredensborg, Denmark: Olsen & Olsen.

Cheng, L, and J. H. Wormuth. 1992. Are there separate populations of Halobates sericeus (Heteroptera: Gerridae) Mar. Biol. 72:303-9.

Clark, S. J. 1988. The effects of operational sex ratio and food deprivation on copulation in the water strider (Gerris remigis Say). Behav. Ecol. Sociobiol. 25: 317-22.

Cooper, S. D. 1984. The effects of trout on water striders in stream pools. Oecologia. 63:376-379.

Daly, M. 1978. The cost of mating. Am. Nat. 112 :771-774.

Dewsbury, D.A. 1982. Ejaculate cost and male choice. Am. Nat. 119 :601-610.

Dili, L.M., and R.C. Ydenberg. 1987. The group size-flight distance relationships in water striders (Gerris remigis). Cano J. Zool. 65:223-26.

Erlandsson, A. 1988. Food-sharing vs. monopolizing prey: a form of kleptoparasitism in Velia caprai (Heteroptera). Oikos. 53:203-6.

Fairbairn, D. J. 1988. Sexual selection for homogamyin the Gerridae: an extension of Ridley's comparative approach. Evolution. 42: 1212-1222.

___1993. The costs of loading associated with mate-carrying in the water strider Gerris remigis. Behav. Ecol. 4: 224-231.

--1985a. Comparativeecology of Gerris remigis (Hemiptera; Gerridae) in two habitats: a paradox of habitat choice. Cano J. Zool. 63:2594-2603.

__1985b. A test ofthe hypothesis of compensatory upstream dispersal using a stream-dwelling waterstrider, Gerris remigis Say. Oecologia. 66: 147­ 153.

Foster, W.A. 1989. Zonation, behaviour and morphology of the intertidal coral­ treader Hermafobates (Hemiptera: Hermatobatidae) in the south-west Pacific. Zool. J. Linn. Soc. 96:87-105.

63 Hammerstein, P. and G.A. Parker. 1987. Sexual selection: games between the sexes. In Sexual Selection: Testing the Alternatives. J.W. Bradbury And M.B. Andersson, (eds). John Wiley, Chichester.

Hayashi, K. 1985. Alternative mating strategies in the waterstrider Gerris elongatus (Heteroptera, Gerridae). Behav. Ecol. Sociobiol. 16:301-6.

Hutto, R. L. 1985. Seasonal changes in the habitat distribution of transient insectivorous birds in southeastern Arizona: competition mediated? The Auk. 102:120-132.

Jablonski, P., and M. Scinski. 1999. Water striders are prescient foragers: use of sensory information for patch assessment in food-based territoriality of Aquarius remigis (Gerridae, Heteroptera). Pol. J. Ecol. 47(3): 247-256.

Jablonski, P., and S. Kaczanowski. 1995. Influence of mate-guarding duration on male reproductive success: an experiment with irradiated water strider (Gerris lacustris) males. Ethology 98:312-317.

Jablonski, P., and K. Vepsalainen. 1995. Conflict between sexes in the water strider Gerris lacustris: a test of two hypotheses for male guarding behaviour. Behav. Ecol. 6:388-392.

Koga, T., and Hayashi, K. 1993. Territorial behavior of both sexes in the water strider Metrocoris histrio (Hemiptera: Gerridae) during the mating season. J. Insect Behav. 6:65-78.

Krupa, J. J., and A. Sih. 1993. Experimental studies on water strider mating dynamics: spatial variation in density and sex ratio. Behav. Ecol. Sociobio!. 33: 107-120.

Krupa, J.J., Leopold, W.R., and A. Sih. 1990. Avoidance of male giant water striders by females. Behaviour. 115: 247-253.

Lowe, C. H. 1985. Arizona's Natural Environment: Landscapes and Habitats. Tucson: The University of Arizona Press. Pp. 93.

Lytle, D. A. 1999. Use of Rainfall Cues by Abedus herberti (Hemiptera: Belostomatidae): A Mechanism for Avoiding Flash Floods. J. Insect Behav. 12: 1-12.

__,2000. Siotic and abiotic effects of flash flooding in a monane desert stream. Arch. Hydrobio!. 150: 85-100.

64 Nakasuji, F., And V. A Dyck. 1984. Evaluation of the role of Microvefia doug/asi atrolineata (Bergroth) as predator of the brown planthopper Nilaparvata /ugens Stal. Res. Popul. Ecol. 26:134-49.

Nummelin, M. 1988. The territorial behaviour of four Ugandan water strider species (Heteroptera: Gerridae): a comparative study. Ann. Entamol. Fenn. 53: 17-22.

Parker, G. A. 1979. Sexual selection and sexual conflict. Pp. 123-166. In Sexual Selection and Reproductive Competition in fnsects. Blum, M.S., and N.A, Blum (eds). Academie Press: New York.

Pennak, R. W. 1978.·Freshwater fnvertebrates ofthe United States, 2nd Edition. John Wiley and Sons, NY.

Polhemus, J.T., and D. A Polhemus. 1990. Zoogeography of the aquatic Heteroptera of Celebes: regional relationships versus insular endemism. Pp. 73-86. In Insects and the Rain Forests of South East Asia (Wallacea). Knight, W.J., and J.D. Holloway (eds). Longdon: R. Entomo!. Soc.

Rowe, L 1992. Convenience polyandry in a water strider: foraging conflicts and female control of copulation frequency and guarding duration. Anim. Behav. 44: 189-202.

__1994. The costs of mating and mate choice in water striders. Anim. Behav. 48:1049-1056.

Rowe, L, and G. Arnqvist. 1996. Analysis of the causal components of assortative mating in water striders. Behav. Eco!. Sociobio!. 38 :279-286.

Rowe, L, Arnqvist, G, Sih, A, and J.J. Krupa. 1994. Sexual conflict and the evolutionary ecology of mating patterns: water striders as a model system. TREE. 9(8): 289-293.

Rowe, L, Krupa, J. J., and A. Sih. 1996. An experimental test of condition­ dependent mating behavior and habitat choice by water striders in the wild. Behav. Eco!. 7(4): 474-479.

Rubenstein, D. 1. 1984. Resource acquisition and alternative mating strategies in water striders (Gerris remigis). Am. Zoo!. 24:345-353.

__1989. Sperm competition in the water strider Gerris remigis. Anim. Behav. 38:631-636.

65 Sih, A., J.J., Krupa, and S. Travers. 1990. An experimental study on the effects of predation risk and feeding regime on the mating behaviour of the water strider. Am. Nat. 135:284-290.

Spence, J.R 1989. The habitat templet and life history strategy of pond skaters (Hemiptera: Gerridae): reproductive potentiai, phenology and wing dimorphism. Cano J. Zool. 67 :2432-47.

Spence, J.R, and N.M Andersen. 1994. Biologyofwater striders: interactions between systematics and ecology. Annu. Rev. Entomol. 39: 101-28.

Swan, L. A., and C.S. Papp. 1972. The Common Insects of North America. Harper and Row, Publishers. New York: New York. pp109-113.

Thornhill, Rand KP. Sauer. 1991. The notai organ of the scorpion fly (Panorpa vulgaris): an adaptation to coerce mating duration. Behav. Ecol. 2: 156-164

Tramontin, A. D., and A. Sih. 1995. Experiments on the. effects of food and density on voltinism in a stream-dwelling water strider (Aquarius remigis). Freshwater Biol. 34:61-67.

Trivers, RL. 1972. Parental investment and sexual selection. In Sexual Selection and the Descent of Man. B. Campbell (ed.). Aldine, Chicago.

___1974. Parent-offspring conflict. Am. Zool. 14: 249-264.

Vepsalainen, K 1985. Exclusive female vs. male territoriality in two water strider (Gerridae) species: hypotheses of function. Ann. Entomol. Fenn. 51 :45-49.

Vepsalainen, K, and S. Krajewski. 1986. Identification of the waterstrider (Gerridae) nymphs of northern Europe. Ann. Entomol. Fenn. 52:63-77.

Vepsalainen, K, and M. Nummelin. 1985. Female territoriality in the water striders Gerris najas and G. cinereus. Ann. Zool. Fenn. 22:433-39.

Vepsalainen, K, and R Savolainen. 1995. Operational sex ratios and mating conflict between the sexes in the water strider Gerris lacustris. Am. Nat. 146:869-880.

Weigensberg, 1., and D.J. Fairbairn. 1994. Conflict of interest between the Sexes: a study of mating interactions in a semiaquatic bug. Anim. Behav. 48: 893-901.

66 __1996. The Sexual Arms Race and Phenotypic Correlates of Mating Success in the water strider, Aquarius remigis (Hemiptera: Gerridae). J. of Insect Behav. 9(2): 307-319.

Wilcox, R. S. 1972. Communication by surface waves: mating behaviour of a water strider (Gerridae). J. Comp. Physiol. 80: 255-266.

__1984. Male copulatory guarding enhances female foraging in a water strider. Behav. Ecol. Sociobiol. 15:171-174.

Zar, J. H. 1984. Biostatistical Analysis. Prentice-Hall. Englewood Cliffs, New Jersey.

Zimmerman, M. 1984. Population structure, life cycle and habitat of the pondweed bug Mesovelia furcata (Hemiptera, Mesoveliidae). Rev. Suisse. Zool. 91: 1017-35.

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