Ecology and energetics of partial migration and facultative hibernation of Mexican free-tailed bats

Temperate-zone animals must maintain long-term energy balance across seasonal periods of resource scarcity and inhospitable climate. To persist through environmentally-driven energetic challenges animals have evolved a suite of diverse coping strategies, including migration and hibernation. The costs and benefits of these energy saving strategies are dynamic and vary with individual and environmental conditions, often creating variation among individuals within a population. I worked in a partial migration system to investigate the characteristics and energetic strategies of individuals that forgo migration to understand both why and how non-migratory individuals persist through winter. I addressed these research questions in a system of subtropical mammals comprised of migrants and non-migrants. Mexican free-tailed bats (Tadarida brasiliensis mexicana) are generally considered long-distance migrants, but remnant populations forgo migration and overwinter near the northern extent of their range and near the southern terminus of hibernation in North America. I hypothesized that sex-specific differences, likely due to different reproductive investments, would affect migratory decisions and that bats would manipulate both energy intake (via foraging on warm winter nights) and expenditure (via reducing activity on colder nights and using torpor) to persist through winter. Despite the population at my study site being female biased in autumn and spring, 93% of winter captures were males, possibly indicating males may achieve a reproductive advantage by remaining at mating grounds. Male bats increased body mass by ~30% in preparation for winter and entered spring in good body condition, indicating these bats successfully maintain energy balance across seasons. While I expected bats to forage on warm winter nights, plasma triglyceride analysis provided no evidence of winter foraging. Bats used torpor daily, and extended torpor bouts over multiple days depending on environmental conditions. My research demonstrates the energetic flexibility of this sub-tropical mammal and advances our understanding of how animals balance the dynamic benefits and costs of migration and hibernation.