Bats and the Mystery of Hibernation

Hibernation is a well-known concept. It’s a simple enough idea — when resources are slim, some animals go into a sleep-like state to save energy. An effective strategy, it’s found in a wide range of mammals: rodents, marsupials and even primates.

How hibernation works and how it benefits animals is well understood by science, but some puzzles remain. In Colorado, one aspect is somewhat a mystery: Where do bats hibernate?

Bats use different roosts for different purposes. Females with pups prefer warm spaces for summer roosts. Attics, dead trees and shallow rock crevices are popular choices. High above ground, these spots give young bats learning to fly more time and space to fall and catch the air in their wings.

Hibernation roosts, or hibernacula, are a different story. 

“If you’re a species that hibernates, you want something that’s almost the exact opposite,” said bat scientist Dan Neubaum. “You want something that’s very cool and constant but doesn’t freeze.”

Neubaum is the Species Conservation Program Manager at Colorado Parks and Wildlife. A bat specialist, he’s focused much of his research on finding hibernacula.

Cool environments just above freezing (about 35–40 degrees) allow bats to maximize energy conservation but don’t damage their tissues. 

Equally important as a cool temperature is a stable one. Bats need a particular microclimate, a spot with temperature patterns distinct from the surrounding environment. Microclimates that keep a consistent, mild temperature like rock crevices are perfect hibernacula. 

Temperature spikes can cause hibernating bats to “wake up.” If bats wake too often, they burn through their fat reserves before there are enough insects to eat in spring. Consistent, cool temperatures keep bats on track, conserving energy until the end of winter.

Little brown bats, or little brown myotis, hibernate in snow-covered talus, spaces between broken rock fragments on mountain slopes. The humidity in these microclimates helps bats reduce water loss from respiration. Occasionally, bats may briefly wake up to drink snowmelt that drips through the talus openings. 

When we think of a bat roost, we often think of a big, spacious cave with countless bats hanging from the ceiling. Much research has focused on these large, colonial roosts because researchers can fit in them. Many Colorado bats, however, hibernate in less accessible spots.

While scientists understand many of the features of good hibernacula, pinpointing locations is tricky.

“For a little brown myotis, a bat that weighs 8 grams, you can only put a radio on it that weighs 0.4 of a gram,” said Neubaum. “They use watch or hearing aid batteries that are just tiny. In the early days, they lasted only two to three weeks; nowadays, if you’re lucky, they can last for maybe a couple months.”

Earlier in his career, Neubaum used radio technology on big brown bats. Because these bats are larger than little browns, they could be fitted with radios with bigger, longer-lasting batteries. These radios could emit a signal for long enough to show where bats settled for hibernation.

Battery life isn’t the only limitation on little brown bat research. 

When small radios were introduced, scientists learned a great deal about the summer activities of bats when they roost in above-ground spots. Things got challenging when researchers tried to follow bats into secluded hibernacula.

“You’re dealing with a weaker signal because the radio is smaller, and there’s nothing like rock to kill a signal,” said Neubaum.

Between summer roosts and hibernacula, bats use transition roosts. In these roosts, bats enter brief periods of torpor, saving fat in preparation for deep hibernation. This is when researchers catch and put radios on them to find winter roosts — when they’re still active and haven’t yet disappeared into hibernacula.

In a study at Colorado National Monument, Neubaum and his team attempted to track Yuma myotis to their hibernacula.

“When the first heavy frost set in, just like that, boom — the signals went off the air,” he said.

What caused the signals to vanish?

To see if the bats simply left the area to hibernate elsewhere, the team set up Motus towers, structures that pick up radio signals often used in bird research. The towers detected no signals.

One promising technique is acoustics. At Colorado National Monument, where a lack of snow allows for better sound detection below ground, winter activity has been detected. 

In this case, the acoustic detectors were run above ground and detected bats moving about during winter as the rock crevices the bats were using were not blocked by snow. Bats using talus with 12 feet of snow are locked in but have water, as mentioned previously.

But for the snow-covered taluses, what little brown bats prefer, the puzzle remains. 

Despite the challenges, research continues. “Between our studies in Colorado and several others across the West, we’re getting just enough data to show that many bats are crawling into rock crevices and tuning us out for the winter.”

The future is a matter of narrowing down the exact locations of hibernacula. Research tools and the efforts of scientists have produced knowledge on the movements, communication, mating and parenting behaviors of bats in Colorado. Perhaps further advances will reveal the details on where the winter retreats of Colorado’s bats hide.

John Anglin is a copywriter for Colorado Parks and Wildlife. Originally from Amityville, NY, he now lives in Lafayette, CO.