Does body size predict foraging effort? Patterns of material investment in spider orb webs

Body size affects almost all aspects of animals' resource use, and its scaling syntheses are well established in most biological fields. In contrast, how behavioral variation scales with body size is understudied. Understanding how body size influences behavior is important as behavior responds more readily to natural selection than many other traits, and foraging effort is a critical behavioral trait. Web spiders are good models for studying foraging effort because webs are physical records of behaviors. Variability in web architectures is well documented, but how spider size scales with foraging effort and web performance is virtually unknown. Here, we investigate behavioral allometry at three phylogenetic scales - broadly across orb-weaving spiders, among a recent radiation of species, and among individuals within species. Conducting a meta-analysis across a wide range of orb weavers, we investigate how foraging effort scales with body size by measuring effort as the volumes of the three silk gland secretions used for building orb webs. We show that volumes of web material scale negatively allometrically with body size, and suggest silk investment is an important limiting factor in evolution of web performance and body size. To assess whether such broad evolutionary trends exist at finer phylogenetic scales, we investigated how foraging effort scales with body size in a group of five closely related Zygiella s.l. species (Araneidae). We find that the general scaling pattern across orb weavers is only partially confirmed. Finally, we examine patterns among individuals within each of the Zygiella species. We find different patterns of silk use in relation to body size, and show that both web architecture and silk investment need to be quantified to estimate total foraging effort. In conclusion, we find support for the prediction that behavioral traits scale differently to body size at different phylogenetic scales and at the individual level.