When a Bat Sees Its Shadow: How Winter Length Can Effect Bat Survival

By Catherine Haase

This year, Punxsutawney Phil, our favorite groundhog meteorologist, saw his shadow and gave us another six weeks of winter. Though relying on a rodent to determine how long winter will be is a silly concept, it is a good reflection of how variable winter length can be from year to year and across the country. For hibernating species such as bats, those extra weeks of winter can be really important to survival.

Mobile laboratory set-up at one of our field sites. Photo credit: Nate Fuller (Texas Tech University).

Most North American bat species rely on insects for food, and since insects aren’t active during the winter months, many bat species hibernate during the winter. During hibernation, bats alternate between the physiological states of torpor, where they drop their body temperatures and metabolism to about 5 percent of their normal values, and arousals, where they increase body temperature and metabolism back to normal levels.

Bats spend about 90-95 percent of the hibernation period in torpor, and thus can conserve energy during periods of food scarcity and cold temperatures. However, though bats don’t spend a lot of time during arousals, warming up from just a few degrees above freezing to normal body temperature uses a huge amount of energy. Therefore, the majority of winter energy use is driven by these relatively brief periodic arousals, and bats can easily use up all their fat stores if they arouse too frequently. How often bats arouse is dependent on the temperature of where they hibernate—called a hibernaculum—and other bat characteristics, such as their body size, body temperature, and metabolic rate.

Townsend’s big-eared bat (Corynorhinus townsendii). Photo credit: Nate Fuller (Texas Tech University).

If bats have fattened up enough before going into hibernation, then they will survive the winter and emerge to search for insects in the spring. Winter length is an important predictor of bat survival and population growth, as it dictates how long bats remain in hibernation.

In North America, hibernation can last anywhere from 3 to 9 months – imagine not eating any food for 9 months. Hibernation length is dependent on regional climate and latitude. For instance, bats in Montana may have to remain in hibernation for up to 6 months because of the high latitude and altitude of some hibernacula locations, while many bats in Tennessee can emerge after only 3 months of hibernation.

Setting up mist nets used to capture bats during swarming to measure fat stores during pre-hibernation. Photo credit: Catherine Haase (Montana State University).

Because bats are already challenged with budgeting their energy, they have to be careful about how they use up their fat, and any change to their behavior can be deadly. Within the last decade, a devastating disease called white-nose syndrome (WNS) has altered the hibernation behavior of many North American bat species and has resulted in the death of millions of bats. Currently, WNS has been recorded in eastern North America and is slowly spreading westward. WNS is caused by a fungus that is thought to have been introduced from Europe. The fungus grows on the wings and noses of bats and causes them to arouse more frequently.

This increase in the number of arousals reduces the amount of time bats spend in torpor during hibernation and causes bats infected with WNS to burn through their fat stores quicker than normal. To help manage these population declines, many bat biologists such as myself are trying to unravel this complicated relationship between WNS, hibernaculum environments, and bat survival.

Cave bat (Myotis velifer). Photo credit: Nate Fuller (Texas Tech University).

I currently work on a four-year research project funded by the Strategic Environmental Research and Development Program of the Department of Defense that aims to predict how this devastating disease will impact populations and species that have yet to be infected. We spend months each fall and winter collecting physiological data from multiple bat species across the western United States and measuring temperature and humidity of their hibernacula.

We are particularly interested in those species that have yet to be impacted by the disease and may potentially respond differently than their eastern counterparts, either due to differences in hibernaculum environments or physiological adaptations, or both.

Mist-nets are used to capture bats during swarming in order to measure fat stores during pre-hibernation.
Photo credit: Nate Fuller (Texas Tech University).

We are also using bioenergetic models in conjunction with our field data. These models can be used to estimate how much fat a bat will burn throughout winter hibernation, and allow us to incorporate the different physiological measurements for different species.

These models also permit the incorporation of the effects of WNS, hibernaculum conditions such as temperature and humidity, and winter length on survival. The model’s flexibility enables us to predict survival of multiple bat species across their entire range within North America, which in turn helps us understand how differences in bat physiology and winter length may impact survival from diseases such as WNS.

Holding bags containing captured bats waiting to be processed. Photo credit: Catherine Haase (Montana State University).

Punxsutawney Phil’s predictions could reflect detrimental effects on hibernating bat species in North America, particularly those infected with WNS. Hopefully next year Phil will not see his shadow and instead enjoy an early spring – the bats would certainly appreciate it!

Dr. Catherine Haase is a postdoctoral researcher at Montana State University, working with WCS (Wildlife Conservation Society).


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