Sociality impacts brain resource allocation in spiders

The evolution of social behavior may change the cognitive requirements of individual organisms. The distributed cognition hypothesis proposes that the social group decreases reliance on individual cognitive processing, leading to relaxed selection for individual brain investment. The predictions of the distributed cognition hypothesis have been supported in taxonomically diverse social groups, including eusocial paper wasps and naked mole rats. However, these comparisons come with considerable ecological differences, such as eusociality, making the direct impact of social behavior on brain investment unclear. In this work, the predictions of the distributed cognition hypothesis are tested in a cooperative social spider system. Social spiders lack behavioral and reproductive caste, which allows for minimization of ecological confounds associated with prior comparative studies. We compared measures of brain investment across adult females of 13 species that ranged from social (engaging in social behavior with other adults) to subsocial (only interacting with juvenile offspring). We found that in subsocial species, the volume of the arcuate body, or the cognitive processing center of the spider brain, was relatively larger compared to social spider species, indicating decreased reliance on individual cognition in social groups, in support of the distributed cognition hypothesis. Further work will examine sexual dimorphism in brain investment between social and subsocial spiders, to determine how selection for brain investment may be acting differently on males and females, who differ significantly in ecological roles.