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Inter and Intraspecific Differences in Neural Investment in Social and Solitary Sweat Bees Open Access Deposited

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The social brain hypothesis predicts that the cognitive demands that accompany the evolution of social behavior will be met with corresponding changes in neural investment. In Chapter 1, we use a bee species that has lost social behavior to test for an accompanying loss in neural investment in cognition. We compared relative brain investment in a social (Augochlorella aurata) and a closely related, derived solitary species (Augochlora pura). We compared females at the nest foundress stage so that brain development would not be influenced by social interactions. In support of the social brain hypothesis, our data show that the loss of sociality was accompanied by a 32.9% reduction in relative neural investment in the mushroom bodies, an area of the insect brain responsible for sensory integration and learning. However, the solitary species had a larger whole brain size relative to body size that was not explained by increases in other sensory neuropil. This is the first study to demonstrate differences in mosaic brain evolution between social and solitary species. At the intraspecific level, differences in neural investment have been observed between different castes of social bees, with MBs being larger in queens than workers. This suggests a strategy of increased cognitive investment in individuals who will need to exercise social dominance, consistent with the social brain hypothesis. However, larger MBs in queens may be a plastic response to social interactions in the nest. In Chapter 2, we show that nest foundresses—the reproductive females who will become queens but are solitary until their first workers are born—have MBs that are 15.8% larger and antennal lobes that were 28.3% larger relative to whole brain than workers in the primitively eusocial sweat bee Augochlorella aurata. Whole brain size, body size, and optic lobe size did not differ between the two groups, however. This suggests that increased cognitive investment is part of the broader queen phenotype, supporting the social brain hypothesis. Larger MBs among foundresses may reflect the increased larval nutrition provisioned to reproductive females to enable diapause and/or an absence of social aggression from a dominant queen upon adult emergence.

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