Meeting Abstract
Migration evolves when the benefits of migrating outweigh the costs of remaining sedentary. However, migration is a characteristic of individuals and the costs and benefits of migration can vary among individuals. Such variation can result in partial migration where some individuals migrate and others forego migration. Previous investigations of partial migration have focused on homeothermic species where costs of foregoing migration are driven primarily by energy availability. However, heterothermic species can reduce energy expenditure in response to varying energy availability. We investigated the energetic strategies of Mexican free-tailed bats overwintering in Texas. We hypothesized overwintering bats would exhibit flexibility in maintaining energy balance by maximizing energy intake via foraging on warmer nights and reducing energy expenditure by decreasing activity and using torpor while inactive, with multi-day torpor bouts during longer periods of harsh weather. We regularly captured bats between September 2018-May 2019 and measured plasma triglyceride concentration of 137 individuals and deployed an acoustic monitor to continuously record bat activity. In February 2019 we attached temperature-sensitive radiotransmitters to 30 bats to continuously record their skin temperature. Bat activity was strongly affected by weather, with reduced activity on colder nights. Bats regularly used torpor during the day and were able to extend torpor bouts over multiple days during extended periods of harsh weather. Surprisingly, plasma triglyceride levels were extremely low during all winter capture events indicating bats rarely forage during winter. These sub-tropical mammals withstand winter via regular torpor use but despite being active do not capitalize on winter foraging opportunities.
