Energy availability influences microclimate selection of hibernating bats

Abstract

SUMMARY Many species hibernate to conserve energy during periods of low food and water availability. It has long been assumed that the optimal hibernation strategy involves long, deep bouts of torpor that minimize energy expenditure. However, hibernation has ecological (e.g. decreased predator avoidance) and physiological (e.g. sleep deprivation) costs that must be balanced with energy savings; therefore, individuals possessing sufficient energy reserves may reduce their use of deep torpor. We tested the hypothesis that energy (fat)availability influences temperature selection of two fat-storing bat species during hibernation. We predicted that individuals with small energy reserves would select colder temperatures for hibernation in order to minimize energy expenditure, while individuals with larger energy reserves would choose warmer temperatures to minimize the costs of hibernation. Results from our field experiment indicate that little brown myotis (Myotis lucifugus)hibernating in warm microclimates were significantly heavier than individuals hibernating in cooler microclimates. To determine if energy availability was mediating this relationship, we limited fatty acid availability with mercaptoacetate (MA) and quantified its effect on torpid metabolic rate(TMR) and thermal preference of big brown bats (Eptesicus fuscus). Administration of MA caused a 43% drop in TMR at 10°C and caused bats to choose significantly colder temperatures for hibernation. Our results suggest that fat-storing bats minimize torpor expression using both physiological and behavioral mechanisms.

Energy conservation in hibernating endotherms: Why “suboptimal” temperatures are optimal

“…Our results directly contradict the notion that choosing microclimates where T a ≈ T min is the most energetically efficient expression of hibernation (Humphries et al, 2002;Dunbar and Tomasi, 2006), and add to a rapidly growing literature that takes an adaptive view of endothermic thermoregulation (Humphries et al, 2003;Boyles et al, 2007;Angilletta et al, forthcoming). These authors argue that a preference for microclimates warmer than T min should be driven by non-energetic costs of hibernation.…”

“…However, recent theoretical and experimental work suggests that the benefit of energy conservation is counteracted by physiological and ecological costs associated with maintaining a low T b , and that the optimal cost-benefit ratios are in reality attained by hibernating at T a s higher than T min (Humphries et al, 2003;Boyles et al, 2007;Angilletta et al, forthcoming). Admittedly, this view is based largely on assumptions, rather than empirical evidence, because the costs have only relatively recently received attention from researchers (Angilletta et al, forthcoming).…”

Bat Populations and Cave Microclimate Prior to and at the Outbreak of White-Nose Syndrome in New Brunswick

“…Hibernating at warm temperatures leads to increased energy expenditure but lessens the negative physiological aspects of hibernation (Boyles et al 2007). It is possible that, in areas with relatively long hibernation periods (like New Brunswick), conserving energy by hibernating at low temperatures is a more effective strategy than reducing the negative physiological aspects of hibernation by roosting at higher temperatures.…”

Body temperature and body mass of hibernating little brown bats Myotis lucifugus in hibernacula affected by white-nose syndrome

“…If WNS-affected bats are underweight during hibernation, this would suggest that body fat is being used too quickly throughout winter, not just immediately before death. Thus, we also compared energy (fat) reserves of affected and unaffected bats by comparing the mass of WNS-affected bats in New York State to unaffected bats from Ohio (Boyles et al, 2007). If WNS-affected bats are underweight, but survive hibernation, they may still be susceptible to death following spring emergence from hibernation.…”