Social foraging and exploitative behaviour in group-living crab spiders

When animals live and forage in groups, the food discoveries made by a few are usually shared among many individuals. Food sharing can improve the per-capita foraging success of group members, but competition over food resources may adversely reduce the payoff return. Moreover, in the absence of mechanisms that reinforce the cooperative nature of group foraging, some group members (scroungers) may exploit the efforts of food-procuring individuals (producers), leading to unequal reward distributions on the individual level. By investigating the cooperative and exploitative ways in which individuals interact when foraging in groups, studies on social foraging assess the interplay between advantages and disadvantages to group foraging - but the existing research mostly concerns non-predatory species. However, the costs that predators avoid by scrounging from the prey laboriously subdued by others, and thus the payoffs reaped from this defective tactic, should be particularly substantial. In consequence, the evolution and maintenance of group foraging - and more generally of group living - in social predators will be severely challenged by the rise of exploitative foraging behaviour. In this thesis, I study social foraging and exploitative behaviour using Australia's group-living, subsocial crab spiders as a predatory model system, in particular the species Australomisidia ergandros and furthermore Australomisidia socialis and Xysticus bimaculatus. Since these crab spiders do not build capture webs, but nonetheless attack and feed communally in offspring groups, insights gained from this study system may be projected on many other social carnivorous species. I conducted an array of laboratory foraging experiments to investigate the common pros and cons of foraging as a group at the level of species, the effects of group size and group composition on exploitative foraging behaviour at the level of groups within species, and the existence and determinants of individual specialisation in social foraging tactic at the level of individuals within groups. On the level of species, my results indicate that group formation and group retention in subsocial crab spiders are driven by competition-reducing advantages associated with maternal provisioning beyond the nutritional independence of young - and later, in offspring groups, by enhanced predatory success in groups as compared to solitary individuals. I thus disprove the presumption that non-webbuilding spiders which live in groups do not enjoy foraging benefits of grouping owing to the absence of a large communal capture web. Further, this suggests that group foraging in crab spiders is based on the same principles that promote social living as cooperatively hunting vertebrates. On group level, I provide novel empirical support for a relevant prediction of social foraging models: in A. ergandros, I demonstrate that the extent of scrounging increases with group size, mediated via shifts in the ratio of tactic-specialised producers to scroungers. Given tactic specialisation, negative frequency dependent selection is thought to limit the spread of the scrounger type, because that tactic only promises high rewards as long as producers are common. Accordingly, I show that A. ergandros scroungers adhere to their defective strategy by not sharing food in groups of their own, consequently suffering negative fitness payoffs. The tactic of producers, on the other hand, seems to be remarkably adaptive to group composition. In this respect, inherent producers are here shown to receive food shares from other producers. On individual level, I finally reveal that the evidently cooperative tendency to procure and share food is sex-dependent in A. ergandros, with males acting as providers. Based on my findings, I suggest an explanatory framework for the evolution and maintenance of group living and cooperative group foraging in social predators despite the temptation to exploit.