Trade-offs Influencing the Physiological Ecology of Hibernation in Temperate-Zone Bats

Seasonality of temperature and food availability can lead to trade-offs between the benefits of immediate reproduction and costs associated with mortality risk from starvation, inclement weather, or predation. Hibernating mammals exhibit an enormous seasonal shift in physiology and behavior and provide a useful system to examine the effect of this trade-off on key events in the annual cycle. Most of what we understand about the ecological energetics and phenology of hibernation comes from studies of rodent hibernators such as ground squirrels, chipmunks, and dormice. Temperate-zone, insectivorous bats, however, provide another useful model system to examine trade-offs influencing seasonal change within individuals. Here, I review recent studies from my laboratory on little brown bats (Myotis lucifugus) from central Canada to understand the interplay between capacity for energy storage, energy expenditure during hibernation, and the timing of key events in the annual cycle of hibernating mammals. These studies have relied on measurements of body condition to assess energetic status, biologging of skin temperature using temperature telemetry, and use of passive transponders (i.e., PIT tags) to quantify emergence timing. In general, these studies suggest that, in part due to constraints associated with flight, bats exhibit unique, or at least unusual, adaptations for extreme energy savings during winter. The results also support the optimization hypothesis that current energetic status and future energy requirements influence energy expenditure during hibernation and the timing of emergence from hibernation in spring. Taken together, this work provides insight into the influence of reproductive timing and energy availability on hibernation behavior and physiology. It also has implications for understanding responses of bat populations to anthropogenic impacts like climate change and white-nose syndrome.