Hoary Bats Hibernate

First Direct Evidence of Migratory Bats Hibernating

hoary bat
Hoary bats are named for their fur, which is silver at the tips. Photo by Paul Cryan, USGS.

Hoary bats are wanderers – they sometimes migrate hundreds of miles and can be found in almost every state in the U.S. Researchers with the U.S. Forest Service’s Pacific Southwest Research Station documented hoary bats going into a state of torpor, or hibernation.

While it’s not unusual for some species of bat to migrate or other species to hibernate, it is unusual to find a species of bat that does both.

“It’s commonly assumed that species that migrate do so to reach areas that allow them to continue feeding and remain somewhat active throughout the winter,” said Forest Service ecologist Ted Weller. “But our findings surprised even our own research team by showing that hoary bats spend much of the winter in hibernation.”

In September 2014, Weller and his colleagues tagged several bats within Humboldt Redwoods State Park with GPS tracking devices and another group of bats with a device that monitored light levels, body temperatures and activity, which allowed them to understand how bats responded to varying weather conditions. Weller led the study, which was recently published in the journal Scientific Reports.

“While such tracking and monitoring technology has existed for a while, it hasn’t been until somewhat recently that these devices were small enough to be affixed to animals of this size,” Weller said.

Eventually, some of the GPS-equipped bats were recaptured. The data revealed that one of the bats stayed within a few miles of the initial capture site, while the other had travelled as far as 45 miles away. However, the third recaptured bat was the most intriguing.

For the month of October, Bat VHF5 flew more than 600 miles, making a loop into southern Oregon, then into interior California, then over to the Nevada-California border, and then back again into interior California.

“It’s hard to determine what led to such a journey,” Weller said. “Was he seeking favorable temperatures and humidity for roosting and foraging? Was he trying to intercept females to mate with as they migrated to their wintering grounds?”

hoary bat
Hoary bats can grow to 5 inches long, making them one of North America’s largest bats. USFS photo.

Results from the devices that monitored light levels also offered new insights. Two bats from that group were recaptured in spring, with one of the bat’s devices having captured 224 days of data. Based on lowered body temperatures and inactivity, that bat exhibited the signs of being in a torpor state from November 2014 through April 2015, including a 40-day stretch without flying.

Which again leads researchers to the question: Why would a species capable of migrating hibernate? The answer could lie within the bats’ roosting habits.

“Hoary bats roost outside in trees as opposed to inside caves,” Weller said. “It’s possible that hoary bats are evolved to hibernate, but would freeze if they did so in their northern summer territories.”

As with other migratory species, understanding the bats’ seasonal movements and wintering habits are essential for conservation efforts. And because most bat research is confined to summer when they are most active, these findings are especially useful.

“This research has provided us with a valuable look into the lives of hoary bats rarely before seen, and until now, never documented to this extent,” Weller said. “Knowing more about their lives outside of the summer months will help us better understand what steps might best promote their conservation year-round.”

Research partners included Wildlife Veterinary Consulting, the Swiss Ornithological Institute, Bat Conservation International, and the U.S. Geological Survey.

Read the full text of the article.

For more information, email Ted Weller at tweller@fs.fed.us.

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