Volumetric Comparison of Overall Brain and Neuropil Size Between Social and Non-social Spiders: Exploring the Social Brain Hypothesis.

The social brain hypothesis predicts that the relative size of specific brain regions is driven by the cognitive capacity required to manage complex (social) situations. Spiders are intriguing models to test this hypothesis, as sociality is rare in this usually solitary and aggressive group. Here, we used microCT to compare the central nervous system and brain volumes between social and solitary females of the species in two taxonomic groups, huntsman and crab spiders. Overall, we found no difference in relative CNS and brain volume between social and solitary species. However, social huntsman spiders Delena cancerides had larger arcuate and mushroom bodies than the solitary huntsman species Isopeda villosa and Heteropoda jugulans. Social crab spiders Xysticus bimaculatus had larger visual neuropils than the solitary species Thomisus spectabilis and Tharrhalea evanida. Social huntsman spiders exhibit intricate social behavior, including prey sharing and kin recognition, which could explain the higher investment in brain structures that are related to cognitive integration. They also had smaller venom glands, possibly due to their prey-sharing behavior. In social crab spiders, the low-light leafnest may have driven enlarged visual neuropils. Some variations in specific brain regions between solitary and social species were consistent with the social brain hypothesis, but the patterns differed between lineages. Thus, it is likely that other ecological drivers affect the development of specific brain regions in spiders. Our study provides the essential knowledge platform to conduct experimental manipulations of social and environmental conditions on these spiders to directly test their impact on brain structures, coupled with tests of relevant behavior.