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Cyrtophora Citricola: the Role of Sexual Selection and Sexual Cannibalism

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Cyrtophora Citricola: the Role of Sexual Selection and Sexual Cannibalism I Ben-Gurion University of the Negev The Jacob Blaustein Institutes for Desert Research The Albert Katz International School for Desert Studies Dispersal in the Colonial Spider Cyrtophora citricola: The Role of Sexual Selection and Sexual Cannibalism Thesis submitted in partial fulfillment of the requirements for the degree of "Master of Science" By: Na'ama Berner-Aharon October 2013 II Ben-Gurion University of the Negev The Jacob Blaustein Institutes for Desert Research The Albert Katz International School for Desert Studies Dispersal in the Colonial Spider Cyrtophora citricola: The Role of Sexual Selection and Sexual Cannibalism Thesis submitted in partial fulfillment of the requirements for the degree of "Master of Science" By Na'ama Berner-Aharon Under the Supervision of Prof. Yael Lubin Department of Desert Ecology Author's Signature …………….……………………… Date 31.10.2013 Approved by the Supervisor… ……… Date 29.10.2013 Approved by the Director of the School …………… Date ………….… III Dispersal in the Colonial Spider Cyrtophora citricola: The Role of Sexual Selection and Sexual Cannibalism Na'ama Berner-Aharon Thesis submitted in partial fulfillment of the requirements for the degree of "Master of Science" Ben-Gurion University of the Negev The Jacob Blaustein Institutes for Desert Research The Albert Katz International School for Desert Studies 2013 Abstract In this study, I investigated the role of sexual cannibalism in male mate choice and dispersal in the colonial spider Cyrtophora citricola. In group living organisms, dispersal occurs when the benefits and fitness of an individual remaining in the group are reduced, and the costs of staying increase. Breeding dispersal in search for mates can be the result of increasing competition over resources or mates and inbreeding avoidance that limits mating opportunities. Sexual cannibalism and its high reproductive costs potentially could influence dispersal decisions, since sexual selection is predicted to favor male mate choice and male-biased dispersal when male mating opportunities are limited by high costs. Sexual cannibalism in group living organisms is known so far to occur only in C. citricola. The occurrence of sexual cannibalism in a colonial spider is counter-intuitive. As opposed to other sexually cannibalistic species that experience low mate encounter rate, in colonial species, females should be readily available for males. However, if inbreeding is a risk, females might be expected to be selective and prefer to mate with more than one male and lower the risk of inbreeding. Cannibalism IV may prevent a male from copulating with both pedipalps, while leaving the female with the possibility of re-mating. This conflict of interests between the male and the female may be solved for the male through male mate choice. Many spiders, among them sexually cannibalistic spiders, are known to use sex pheromones in mate search and mate choice. These pheromones, which are found on the body or on the silk of females, may include information on the receptivity, the quality, the relatedness and the reproductive history of the female. I hypothesized that in C. citricola, sexual cannibalism selects for male choosiness, where males are able to assess a female’s reproductive state and react accordingly in terms of dispersal decisions. In order to test this hypothesis I examined the sexual behavior of C. citricola in relation to its mating behavior, mate choice and dispersal decisions under both laboratory and semi-natural condition. I found that sexual cannibalism is a fixed trait and occurs in all matings. This behavior selects for male mate-choice based on web-borne pheromones. Males are able to reach females via aerial dispersal, which may assist in mate choice, as they are able to arrive to remote but perhaps favored females. Males preferred to stay on webs of virgin females and dispersed from webs of sub-adult or mated females. However, males can mate with a mated female and females can re-mate at least once. These finding imply that although males probably have maximum 50% paternity, this is the maximal reproductive success possible under these conditions. This however reinforces the need for careful mate choice. Further research is required for fully understanding the evolution of sexual cannibalism in a colonial spider. V Acknowledgments I wish to thank Yael Lubin, my supervisor, for being a real inspiration. Thank you for opening the wonderful world of spiders to me, and for untangling the web around their fascinating behavior. I wish to thank Iris Musli and Ishai Hoffman, our technicians, for all the technical help and good advice. I wish to thank my lab members for all their useful, wise comments: Efrat Gavish-Regev, Eric Yip, Huda Al Beiruti, Iara Sandomirsky, Itai Opatovsky, Idan Kahnonitch and Vardit Makover. I wish to thank my family and friends who encouraged and supported me throughout the way. And to Shlomi, who believed in me and inspired me to follow my dreams. For the endless love I receive from you and from our little spider lover, Tal Devora VI Table of Contents 1. General Introduction………………………………………………………...1 1.1 Dispersal…………………………………………………………..…2 1.2 Sex-biased Dispersal………………………………………………....3 1.3 Sexual Conflict and Sexual Cannibalism………………………….....5 1.4 Male Mate Choice …………………………………………………...6 2. General Methods…………………………………………………………..…9 2.1 Study Species………………………………………………………....9 2.2 Collecting and Rearing………………………………………………9 2.2.1 Sampling Sites……………………………………………...9 2.2.2 Maintenance of Spiders…………………………………...10 2.3 Statistical Analysis………………………………………………….11 3. Aerial Dispersal in Cyrtophora citricola……………………………………13 3.1 Introduction…………………………………………………………13 3.2 Methods……………………………………………………………..17 3.2.1 Dispersal Trials………………………………………...…17 3.2.2 Statistical Analysis………………………………………..17 3.3 Results………………………………………………………………18 3.4 Discussion…………………………………………………………..20 4. Sexual Cannibalism and Male Mate Choice in Cyrtophora citricola…….24 4.1 Introduction…………………………………………………………24 4.2 Methods……………………………………………………………..30 4.2.1 Laboratory Experiment…………………………………………...30 4.2.1.1 Preliminary Mating Observations……………...30 VII 4.2.1.2 Female Receptivity Experiment…………………30 4.2.2 Field Experiment………………………………………………….31 4.2.2.1 Net-house Dispersal Experiment………………..31 4.3 Results……………………………………………………………….32 4.3.1 Preliminary Mating Observations………………………...32 4.3.2 Female Receptivity Experiment…………………………...35 4.3.3 Net-house Dispersal Experiment……………………….…36 4.4 Discussion…………………………………………………………...37 5. General Discussion………………………………………………………….41 6. References…………………………………………………………………...44 VIII List of Tables and Figures Table 1- Description of male and female behaviors during courtship………...52 Table 2- Summery of the female receptivity experiment……………………...52 Figure 1- Fitness as a function of group size …………………………………..5 Figure 2- Map of collecting sites in southern Israel…………………………...10 Figure 3- The proportion of juveniles, males and females showing dispersal behaviors……………………………………………………..18 Figure 4- Dispersal as a function of body length……………………………...19 Figure 5- Male tiptoeing in the laboratory apparatus …………………………20 Figure 6- The male approaches the female……………………………………54 Figure 7- Copulation…………………………………………………………..54 Figure 8- Sexual cannibalism……………………………………………….....55 Figure 9 - The proportion of males remaining-on or dispersing-from webs of females………………………………………………………...57 1 1. General Introduction Group living is a widespread phenomenon that evolved many times in the animal kingdom. The definition of a group includes social structures of related or unrelated individuals, forming various kinds of groups such as herds, harems, schools, colonies and nests, which remain together through time and space by philopatry of juveniles and adults of both or only one of the sexes (Wilson 1975, Lee 1994). Individuals living in a group interact with members of their group more than with other conspecifics. The evolution of group living is through different selective forces, all favoring the formation of a group under certain environmental and ecological conditions. Individuals in a group may benefit from this way of living by obtaining more or higher quality resources, increasing the chances of finding a mate and increasing the efficiency of defense against predators (Krause & Ruxton 2002). Additionally, relatedness is expected to be high in group living organisms due to philopatry (Hamilton 1964). Thus, benefiting from indirect fitness through the success of kin can also influence the choice to live in groups. Nevertheless, the cost of group living may be high and includes an increase in disease and parasite transmission, competition over resources and mates, kin competition and conspicuousness to predators (Rubenstein 1978, Krause & Ruxton 2002). As a result, natural selection will favor group living if the benefits from the presence of other individuals are greater than the costs (Pitcher et al. 1983); if the costs exceed the benefits, dispersal will be favored. 2 1.1 Dispersal Dispersal is the movement of individuals or propagules with potential consequences for gene flow across space. It includes both natal dispersal, where individuals leave their birthplace, and breeding dispersal where potential mates move from one reproduction site to another (Ronce 2007). Howard (1960) defines natal dispersal as “the permanent movement an individual makes from its birth site to the place where it reproduces or would have reproduced had it survived and found a mate”. Breeding dispersal,
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