Collaborative Research: Social brains and solitary bees: A phylogenetic test of the effect of social behavior on brain evolution across multiple gains and losses of sociality

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How do social interactions drive brain size? Living in a social group with other animals may require a bigger brain in order to deal with the cognitive challenge of maintaining social relationships. Alternatively, animals in social groups may have smaller brains because members of social groups can work together, thus reducing the cognitive challenge faced by each individual. This research will measure brain size in closely related species of social and solitary bees. Social cooperation has evolved many times in bees, and has also been lost many times: some solitary bees evolved from social ancestors. This project will also measure levels of neurotransmitters and hormones in bee brains that are associated with aggression and reproduction. Scientists still do not have a clear understanding of how even relatively simple brain differences are associated with the evolution of complex behavioral traits. This research will use the small brains of bees and the multiple comparisons between many different social and solitary species to investigate how living in a cooperative social group affects the brain. The scientists involved in the project will mentor students through internships in their labs, and will write versions of their research publications aimed at middle- and high-school aged students that will be made freely available on the web for education and outreach. This project tests the hypothesis that the evolution of social cooperation leads to increased brain size. This research focuses on measuring changes in the volume of the mushroom body (an area of the insect brain), and biogenic amines of the brains of social and solitary bees. The researchers focus on the sweat bees (Halictidae) because this group is ancestrally solitary with three evolutionary origins of sociality and multiple evolutionary losses from within the social clades. Also, many species are socially polymorphic, exhibiting both social and solitary behavior. This allows for multiple phylogenetically independent social-solitary comparisons. The researchers focus on mushroom body volume because this area of the brain is used for learning, memory, and sensory integration. They measure biogenic brain amines because these chemicals are involved in regulating aggressive, social, and reproductive behavior, and thus likely involved in the evolution of social cooperation. Lastly, they test whether changes in levels of brain amines are mechanistically linked to changes in brain size, and also measure changes in amine receptor density associated with social behavior. This work uses a phylogenetic comparative approach, confocal microscopy measurements of brain volume, high performance liquid chromatography (HPLC) measurements of amine titers, and antibody staining of amine-reactive neurons to test how evolutionary gains and losses of social cooperation affect the brain. This work addresses issues of broad public appeal - brain evolution and cooperation. As part of the broader impacts of this project, researchers will work with professional editors to create open-access adaptations of their studies for use in classrooms or other educational or outreach purposes. They will also train graduate and undergraduate students across five different institutions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.