White-nose syndrome is a devastating fungal disease that is currently decimating the bat populations of North America. Specifically targeting hibernating bats, the disease appears to cause its victims to arouse from torpor more frequently, preventing them from conserving energy and leading to starvation. Craig Willis and Kristin Jonasson from the University of Winnipeg, Canada, explain that it is difficult to assess the energetic impact of this disease without realistic models of how hibernating bats use energy. So, the duo decided to monitor the skin temperatures of hibernating little brown bats to find out whether the hibernating animals optimise their use of torpor during hibernation and how well temperature measurements can be used to estimate the animals' energy consumption rates (p. 2141).
Fitting temperature-sensitive radio transmitters to 22 bats hibernating in a small cave in central Manitoba, Canada, the duo monitored the bats' body temperatures for 3 months. Recording the lengthy periods when the bats became torpid (13.1 days on average), depressing their body temperatures, and the brief arousals when the animals returned their body temperatures to normal, Willis and Jonasson found that the duration of torpor varied significantly from animal to animal. They also found that the bats were torpid for an impressive 99.6% of the hibernation period.
Next, the pair analysed the temperature recordings to find out whether the bats' age, sex or body condition affected their hibernation. They did not. However, when the scientists analysed the arousal periods, they noticed something unusual. Occasionally the bats dropped their body temperatures slightly during arousal and entered a period of shallow torpor. The duo suspects that the bats use these bouts – which they refer to as heterothermic arousal – to minimise the over all metabolic costs of hibernation. Finally, the team measured the masses of the tiny animals to estimate their energy use over the course of the hibernation, and found that their calculated predictions of energy use agreed well with the bats' mass loss, although the calculations slightly overestimated the energy consumption.
Having assessed the mammal's hibernation energetics, Willis says, ‘Our findings will be important for understanding the energetic and survival implications for bats suffering from white-nose syndrome.’