Bats are awesome! There are over 1,400 known species of these flying wonders, each having fascinating adaptations which allow them to survive in diverse habitats on every continent on the planet except Antarctica. In the tropics, food is present year-round for these flying mammals to feast upon. Not so for our New York state bats, which are insectivorous and must cope with the seasonal lack of food when the temperatures plummet. To deal with this, some of these bats migrate south to places where flying insects are still active, while others slow their metabolism to incredibly low levels in order to conserve energy as they hibernate during the winter.
A simple fact - small insectivorous bats use huge amounts of energy just being awake. They have a very high metabolism. At rest, their heart rate averages between 250-450 beats per minute (bpm). By comparison, the resting heart rate of a healthy human is 60-80 bpm. A bat's heart rate shoots up even more while flying, and can reach 1000 bpm! Every night while bats are out hunting, their metabolic rate increases to 15 times higher than the resting rate. This increase generates more heat than they can readily dissipate. Body temperatures rise from 100˚F to 106˚F, which is the equivalent to having a pretty high fever… every single night.
Supporting a bat's high metabolism requires a lot of insects. As temperatures drop and insects become more difficult to find, this energy-intense activity level becomes very difficult to maintain. In order to compensate for the lack of energy coming in, bats are able to abandon control of their body temperature and enter a different physiological state in order to save energy: torpor. Bats can enter torpor daily when they are inactive, or can enter torpor for prolonged periods during hibernation. Entering torpor is not as easy as just falling asleep. In fact, when a bat is hibernating, it does not have the same brain-wave patterns as an animal that is asleep, and it may actually become sleep deficient. Sleep is only possible at normal body temperatures. Torpor is something different entirely.
Hibernation requires some preparation. Bodily systems that will not be used are shut down to decrease energy expenditures; this means that reproductive organs will atrophy and shrink down for the winter. All other metabolic activity will be significantly decreased. Immune function will be suppressed, activity in the digestive system will be greatly reduced, plus heart rate and breathing will slow considerably. Even after lowering the energy expenditures as much as possible, hibernating bats still must continue certain minimal life functions. A substantial accumulation of fat is necessary to provide the body enough energy to last until spring, so bats will need to eat as much as possible to increase fat stores in the fall. As fall turns to winter, they will get the signal that it is time to enter hibernation. The trigger is usually a sharp drop in the ambient temperature or a sudden decline in food supply. Then, the search will be on for the ideal place to spend the winter, or better yet, a return to a place that has worked well for them in the past.
In order to successfully hibernate, bats need to find a suitable environment with high humidity, few disturbances, and temperatures that are cold, but not below freezing. Different species of bat have slightly different requirements. Big brown bats (Eptesicus fuscus) are remarkably cold-tolerant and have been found in caves with temperatures ranging between 39˚F and 53˚F, but may sometimes be found hibernating in places that are close to freezing. The temperature in the hibernaculum is important because it will control the metabolic rate of each individual bat while in torpor.
Bats actively control their entrance into and their arousal from torpor. When bats are ready for hibernation, they actively slow down their breathing. This causes an accumulation of carbon dioxide and a cascade of events that decreases their metabolic rate and lowers their body temperature. Once bats start this chain of events, their body temperature will continue to drop until it closely matches the temperature of the air around them. As mentioned, there is an ideal temperature range that allows a bat to maintain torpor during hibernation. Within that range, the lower the air temperature, the lower the bat's metabolic rate will be. Temperatures above that range will cause the bat to burn too much energy, and can lead to the bat starving to death before spring arrives. Temperatures below that range can have the same effect by causing the bat to shiver and awaken, forcing them to move to a warmer area, which burns much needed fat in the process. Hibernation is an incredible way to conserve resources, but it is not without risks.
If all goes well, hibernation will be successful, and come spring and warm weather, the bat will wake up and fly off in search of food. As a bat rehabilitator, I get calls on many bats that wake up too early and are found by homeowners during the winter. Big brown bats are often found hibernating in attics, barns, or other human structures. The scientific name, Eptesicus fuscus, means "house flyer", and that is exactly how some of these bats are found! Young bats still learning how to survive are more likely to have difficulties. Much of their energy is going into growth, so they may not be able to accumulate enough fat for winter hibernation. They also may not know where to find the best hibernation spots. Many of these bats do not fare well.
With winter on the way, be aware that a bat found awake while it is cold outside needs help! Releasing the bat outside when there is no food available and without a safe place to hibernate is most likely a death sentence. Please help these bats having difficulty surviving the winter by finding a bat rehabilitator. You can search for a rehabilitator near you here:
Your kindness in finding help for a bat in need can go a long way in helping already declining bat populations and will be the difference between life and death for that individual bat.
I knew that some birds like black-capped chickadees enter torpor on some cold nights, but need food when they wake up which they can find in my yard. It must be so hard for young bats here that need to learn so much before winter. I guess like all young wildlife nature can be cruel. Thank you as I want to keep learning as much as I can and pass it along.. Michelle Banks