Brain investment in Anelosimus spiders
Authors:   , Caponera Virginia , O'Donnell Sean , Rosenberg Gary 
Published: March 2022

Chapter 1: Cognitive evolution research relies heavily on phylogenetic comparisons of cognitive traits between groups. However, there is no universal brain to body scaling relationship, and scaling relationships can differ significantly between even closely related groups. Thus, comparative research must be paired with investigations of scaling within the subject taxonomic group, in order to detect differences in cognitive traits that are indicative of differences in function. This study investigates allometric scaling relationships between cephalothorax volume, total central nervous system investment, and investment in specific brain regions in Anelosimus spiders. Brain scaling relationships in spiders have been previously understudied, and previous research has predominantly focused on miniaturized spiders. We found that in Anelosimus, total central nervous system size scaled positively with cephalothorax size, and that relative central nervous system investment decreased with increasing cephalothorax size, following Haller's rule in pattern of brain investment. Brain regions differed in scaling relationships; there was a negative relationship between relative arcuate body investment and supraesophageal ganglion size, while relative investment in the protocerebrum was not dependent on total central nervous system volume. Further, we identified sex-specific allometric relationships between relative arcuate body investment and supraesophageal ganglion investment, with females having relatively increased arcuate body investment compared to males. In total, these results lay the groundwork for future comparative work in Anelosimus by establishing scaling relationships. Chapter 2:The evolution of social systems can place novel selective forces on investment in expensive neural tissue by changing cognitive demands. Previous hypotheses about the impact of sociality on neural investment have received equivocal support when tested across diverse taxonomic groups and social structures. We suggest previous models for social behavior-brain relationships have overlooked important variation in social groups. Social groups vary significantly in structure and function, and the specific attributes of a social group may be more relevant to setting cognitive demands than sociality in general. We focus on three attributes of social groups that could influence cognitive demands in different ways: division of labor, including worker specialization and task redundancy, intragroup communication, including information sharing and behavioral coordination, and intragroup conflict, including competition and dominance hierarchy. We outline how variation in these attributes can affect selection for individual cognition, resulting in the possibility of selection for either increased or decreased cognitive investment with transitions to sociality in different taxa. Finally, we test some of the predictions generated using this framework in a phylogenetic comparison of neural tissue investment in Anelosimus social spiders. The social system of Anelosimus is distinct from previously tested social vertebrates and invertebrates because social spiders engage in task sharing and cooperative brood care but lack behavioral and reproductive caste. This decoupling allowed a direct assessment of the cognitive impact of task sharing and cooperative brood care. We hypothesized that in social spider species, these characteristics would reduce selection for cognitive ability relative to subsocial species. We found that social species had significantly decreased investment in the arcuate body, the higher-order cognitive center of the spider brain, supporting our predictions. Future comparative tests of brain evolution in social species should account for the special behavioral characteristics that accompany social groups in the subject taxa.