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Abstract Social behavior, although rare, is a highly successful form of living that has reached its most extreme forms in eusocial insects. A tractable framework to understand social evolution is the study of major transitions in social behavior. This includes the transitions between solitary to social living, from species exhibiting intermediate degrees of sociality to species exhibiting true sociality, and from primitive to advanced eusocial species. The latter transition is characterized by the emergence of traits not previously found in primitive eusocial species, such as fixed morphological differences between castes and task specialization within the sterile caste. Such derived traits appear to exist in a binary fashion, present in advanced eusocial species, and absent or rare in primitive ones, and thus do not exist in a gradient that is easily tracked and compared between species. Thus, they may not be viewed as valuable to explore ultimate questions related to social evolution. Here, we argue that derived traits can provide useful insights on social evolution even if they are absent or rare in species with a lower social organization. This applies only if the mechanism underlying the trait, rather than the function it regulates for, can be traced back to the solitary ancestors. We discuss two examples of derived traits, morphological differences in female castes and primer pheromones regulating female reproduction, demonstrating how their underlying mechanisms can be used to understand major transitions in the evolution of social behavior and emphasize the importance of studying mechanistic, rather than functional continuity of traits. 

Abstract Social behavior has been predicted to select for increased neural investment (the social brain hypothesis) and also to select for decreased neural investment (the distributed cognition hypothesis). Here, we use two related bees, the social Augochlorella aurata (Smith) (Hymenoptera: Halictidae) and the related Augochlora pura (Say), which has lost social behavior, to test the contrasting predictions of these two hypotheses in these taxa. We measured the volumes of the mushroom body (MB) calyces, a brain area shown to be important for cognition in previous studies, as well as the optic lobes and antennal lobes. We compared females at the nest foundress stage when both species are solitary so that brain development would not be influenced by social interactions. We show that the loss of sociality was accompanied by a loss in relative neural investment in the MB calyces. This is consistent with the predictions of the social brain hypothesis. Ovary size did not correlate with MB calyx volume. This is the first study to demonstrate changes in mosaic brain evolution in response to the loss of sociality.