Hunting Tactics in a Cobweb Spider (Araneae-Theridiidae) and the Evolution of Behavioral Plasticity

Although behavior is being gradually incorporated into phylogenetic studies, the understanding of the evolution of one of its main characteristics, plasticity or adjustment to environmental features, is still a challenge, mainly due to the lack of comparative data. In this paper we focus on the plasticity of the predatory repertoire of Achaearanea digitus, analyzing its responses to two prey types, and discuss the evolution of behavioral plasticity based on a comparison to the foraging repertoire of other araneoid sheetweavers and on a review of hunting tactics among the larger group of orbweavers. Contrasting with what was observed for other families, spiders of the family Theridiidae (among them A. digitus) show a small set of attack tactics, and a quite stereotyped predatory performance. These spiders regularly attack their prey with a typical sticky silk wrapping tactic, but fail to use other tactics such as bite-wrap or bite-pull out prey, which are commonly used in other spider families. We show that this stereotypy is typical of the foraging repertoire of araneoid sheetweavers. Plotting the mean size of the attack tactics repertoire on the phylogeny of Orbiculariae shows that high predatory stereotypy is a plesiomorphy of the whole araneoidea group, and that evolutionary increases in plasticity occur independently two times in the group, among Araneidae and Nephilidae. The maintenance of a plesiomorphic, stereotyped predatory behavior among theridiids is probably due to the evolution of a special behavioral technique, which includes the simultaneous use of the fourth legs during wrapping attacks. Since the individuals in the species of this group of sheetweavers face less variable environments than do orbweavers, they should indeed evolve more stereotyped phenotypes, but nevertheless their evolved predatory stereotypy contrasts with plasticity in other aspects of their foraging behavior (web building). Since, in this case, both stereotypy and plasticity result from a simplification in the mechanisms underlying behavioral expression, we suggest that less variable environments select not exactly for behavioral stereotypy, but rather to the simplification of these information processing mechanisms.