How Bats Hibernate

By Shannon Currie

Hibernation is a concept often misunderstood and poorly described for the general public. It is a special physiologic process that is present in a specific group of mammals and few birds. Unlike the common misconception that hibernation is simply the change in activity of an individual between seasons, hibernation or torpor can occur year round in specialised creatures such as bats, and is not a continuous process.

Animals that are small in size must balance the expenditure of energy, through movement, thermoregulation and other physiological processes, with their small energy gains. In particular, bats face the problem of a very energy expensive form of locomotion- flight. When temperatures are low and food is scarce, bats combat this energy loss by slowing down their bodily functions. By adjusting their thermoregulatory set point, the thermostat in their brains, bats can effectively stop producing body heat and in turn slow their metabolism, heart rate and breathing rate to extremely low levels. For example, at rest the heart rate of a bat sits at about 300-400 beats per minute (bpm) depending on the species. When in torpor a bat’s heart rate can drop as low at 10bpm, depending on the species and the external temperature.

In most microchiropteran species this occurs on a daily basis during spring, summer, and autumn for a few hours, depending on the weather conditions. When temperatures get too cold most microchiropteran species will head to sheltered areas such as caves or inside buildings and hibernate. Bouts of torpor during hibernation usually last from a few days up to a couple of weeks and are interjected by periods of arousal. Scientists are not sure of exactly why bats arouse from these torpor bouts. It could be to urinate or to remove toxins from the muscles by moving around, in some cases individuals even copulate.

The amazing thing about hibernators that sets them apart from other animals is that they are capable of rewarming their bodies from very low temperatures all by using internally created heat. Ectotherms, such as reptiles, have body temperatures that can reach low levels but they are incapable of rewarming themselves without external heat sources. Also, the hearts of hibernators can function at temperatures below that of any other mammal. Bats are a particularly good example of this. Non-hibernating mammals will die if the temperature of their hearts drops below 10°C, where as hibernators heart can function below this temperature, and bats hearts continue to work at temperatures approaching 0°C!

The history of research into hibernation has been primarily focussed on rodents because they are easier to catch and keep in captivity than some other mammal species. However their use of torpor is solely seasonal and they do not have the added energy pressures of costly locomotion. Bats exist at an extreme end of the spectrum. They are much smaller than many rodents and flight prohibits them from storing large quantities of body fat. They also undergo torpor daily and need to have perfected the process to survive. More recently scientists have investigated the patterns of torpor in a few bat species but there is still much information to be gathered. 

© 2012 Organization for Bat Conservation