Energy conservation has long been a focal point in hibernation research. A long-standing assumption is that ambient temperature (Ta) largely defines the rate of energy expenditure because of well-known relationships between Ta, metabolic rate and frequency of arousal from torpor. Body condition and humidity also affect energy expenditure but are usually considered secondary factors. We held tricolored bats (Perimyotis subflavus) in captivity under multiple environmental conditions to directly compare the importance of Ta, fat mass and humidity for hibernation energy expenditure. Fat mass was the best predictor of female mass loss, followed by Ta and humidity. However, males had less fat and adopted a more energetically conservative hibernation strategy. Our results demonstrate that understanding the evolution of behavior, physiology and ecology of hibernation requires disentangling the relative contributions of multiple drivers of hibernation energetics, and that Ta is not always the most important factor driving energy expenditure.

Insectivorous bats regularly experience dramatic and sometimes rapid changes in nutrient stores, yet our ability to study these changes has been limited by available techniques. Plasma metabolite analysis has proven effective for studying individual rates of mass change in birds but has not been validated for other taxa. We tested the effectiveness of plasma metabolite analysis by conducting a study with captive big brown bats (Eptesicus fuscus) and little brown bats (Myotis lucifugus) in the field. In the lab, we varied food availability to induce various rates of mass change. As predicted, individual rate of mass change was positively correlated with plasma triglyceride concentration, but there was no relationship with plasma beta-hydroxybutyrate concentration, whereas such a relationship has been found in birds. In the field, we collected blood samples from postlactating females as they emerged in the evening (fasted) and when they returned from feeding in the morning. Plasma triglyceride concentration was greater in fed bats than fasted bats, and the increase was less when rain limited foraging. Contrary to predictions, beta-hydroxybutyrate concentration was also greater in fed bats than fasted bats. Analysis of plasma triglyceride concentration provides a technique for assessing individual feeding state and rate of mass change of bats and will facilitate further study of bat nutritional ecology and energetics.

In this contribution to Dr. Brock Fenton's Festschrift, we briefly reflect on Dr. Fenton's seminal works examining bat swarming behaviour in Ontario and use these reflections as a launch pad to conduct a global review on autumn swarming in bats, and underlying hypotheses to explain this behaviour. Our review frames the swarming period as a time of critical transitions, during which bats must balance multiple life history trade-offs, and we consider how various intrinsic and extrinsic factors may contribute to inter- and intraspecific differences in autumn behaviour. We discuss the transition away from summer residency, including maternity colony breakup, day roosting, and migration during autumn. We review key life history elements of swarming, including mating behaviours and associated reproductive condition, genetic exchange during swarming, and variation among sexes, ages, and species. Finally, we discuss the behaviours and physiological states of bats transitioning from the swarming period to hibernation. Throughout, we identify common patterns and also exceptions. Over 50 years of research has yielded many insights into autumn swarming, but knowledge gaps remain. Future research focus on a greater diversity of species will reveal general principles underlying the transition from summer active season, through the swarming period, and into winter hibernation.