Allometry, sociality, domestication and the evolution of the vertebrate brain.

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The brain differs greatly in size and shape across vertebrate species, but how and why this anatomical diversity exists is largely unknown. My research program addresses this fundamental issue by comparing brain anatomy within and across species using a suite of neuroscience, animal behaviour and evolutionary biology methods. Over this grant cycle, my research program will continue to work towards identifying what factors are responsible for generating anatomical diversity in the brain by pursuing 3 main objectives. First, I will challenge the assumption that the brain regions always expand through the addition of neurons. Using my large, comparative brain collection, I will test if larger sensory and motor regions evolve through the addition of neurons, expansion of neuron cell bodies or both. I predict that sensory regions expand primarily through the addition of neurons as a result of an increase in sensory receptors. In contrast, I predict that the expansion of motor regions in the brain arise primarily from increasing neuron sizes, rather than adding more neurons. These tests will provide new insights into the relative importance of neuron numbers to species differences in brain anatomy. Second, I will test if the evolution of social behaviour is associated with anatomical differences in the brain. The need to remember and use information about social interactions is thought to be a driving force for the expansion of the brain in primates and other animals. Empirical support for this theory is, however, lacking a well-designed test in which the size and composition of specific brain regions are quantified and confounding variables are controlled. I will conduct such a test by comparing the anatomy of two brain regions, the prefrontal cortex and hippocampus, vary across ground squirrel species that exhibit different degrees of social behaviour. This will represent the most thorough test of the relationship between sociality and brain anatomy to date. Third, I will compare brain anatomy across pigeon breeds to test if selection for specific behavioural or physical traits has consequences for brain anatomy. Domesticated species offer a unique means of testing for the effects of specific trait selection that cannot be achieved through species comparisons, by comparing breeds. I will compare the brain anatomy of homing pigeons relative to other breeds and the tactile and visual pathways of pigeon breeds that vary dramatically in beak size and shape. These inter-breed comparisons will provide a strong and effective test of whether selection for morphology alone is sufficient to drive major changes in brain anatomy or if brain anatomy only changes in response to selection for behaviour. This unique suite of studies within and across species will yield novel insights into the functional relationships between brain anatomy and behaviour and improve our understanding of why brain anatomy varies among and within species, including humans.