Brain Size Plasticity in Anamniote Vertebrates

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Brains vary enormously in size, cell density and connectivity. My long-term research goal is to understand how variation in brain structure and size influences organismic function to help identify the factors that drive evolution and plasticity of the nervous system. Fishes and amphibians (i.e. anamniote vertebrates) have a widespread and lifelong ability to produce new brain cells and thus are good models to study the factors determining brain size. Here, we will investigate the cellular mechanisms underlying brain size flexibility and its impacts on cognition using both lab- and field-based approaches. Compelling evidence of adult brain size flexibility in vertebrates is lacking, but our preliminary evidence acquired in the past year suggests that adult fish show changes in relative brain size across seasons and upon escape from captivity into the wild. Unlike birds and mammals, adult fish and amphibians have a widespread ability to produce new brain cells and thus are good candidates to display brain size flexibility. We will first investigate competing hypotheses explaining seasonal changes in brain size in adult anamniotes: the energy savings', sensory reorganization', and reproductive effort' hypotheses. To test the energy savings hypothesis, artificial hibernation of fire-bellied toads under laboratory conditions will be used to measure variation in relative brain size and energy expenditure during simulated seasonal change. The sensory reorganization and reproductive effort hypotheses will be investigated by assessing variation in the size of brain regions with known functions across seasons in wild fish differing in maturation status. Second, we will elucidate the cellular mechanisms responsible for change in brain size. Changes in brain size could involve modulation of the rates of birth of neural stem cells, differentiation of new neurons and glia from stem cells, and/or cell death. Markers for these processes will be measured by stereology analysis on brain sections of fish sampled across seasons and toads sampled across phases of artificial hibernation. Additionally, the number and density of neurons and non-neuronal cells will be obtained using cell counting methods. Finally, the relevance of changes in brain size to cognition and behaviour will be tested. Investment in brain tissue should increase cognitive performance, which will improve the performance of animals tested in behavioural tasks that engage cognitive abilities. We will use prey catching conditioning tasks with fire-bellied toads to study cognitive abilities related to learning and behavioural flexibility. The proposed work will tackle the controversial relationship between brain size, neuron number, and cognitive capacity. Additionally, it will support more applied concerns relevant to many stakeholders, such as helping to develop indicators of behavioural performance and energetic status in aquatic wildlife for environmental monitoring of Canadian watersheds.