When winter weather arrives, most bats hibernate in caves, but a few species migrate to warmer areas. Warmer being relative, the migrating bats may still end up in places that are too cold for comfort, and sometimes hibernate under leaf litter for short periods of time.
Roger Perry, a wildlife biologist at the U.S. Forest Service’s Southern Research Station, studied these temporary hibernation sites to find out how much protection they offered bats, and how much energy bats expend to stay alive.
The leaf litter study took place in and around the Alum Creek Experimental Forest of the Ouachita National Forest in Arkansas, and focuses on eastern red bats, a migratory species that remains active through most of the winter. When winter temperatures are not too cold the bats roost in trees, but when temperatures plunge, the bats temporarily hibernate underneath leaf litter.
“Because freezing temperatures and predators kill a lot of eastern red bats each winter, the roost sites they select could be especially important to their survival,” Perry said.
To find out how much protection from cold the leaf litter provided, Perry measured and compared ambient air temperatures with temperatures underneath varying depths of leaf litter. He found that temperatures underneath the litter – especially during nights and when snow was on the ground – were much warmer.
“Similar to cave climates used by other bat species, leaf litter provides an important thermal refuge for bats during the winter,” Perry said. “The leaf litter buffers them from extreme fluctuations in temperature, and provides cold but above-freezing temperatures most of the time.”
Using previously published metabolic rates for bats hibernating at different temperatures, Perry also estimated bats’ energy expenditures when hibernating under a blanket of leaf litter or at ambient air temperatures.
“Predicted metabolic rates of eastern red bats were lowest under the deepest litter measured,” Perry said. “Depending on the leaf litter depth, energy savings from roosting under litter were two to three times greater than remaining in ambient air during periods of freezing weather, and around five and a half times greater when roosting under leaf litter with snow cover.”
Even a shallow layer of leaves should help eastern red bats save substantial amounts of energy when nighttime air temperatures fall below freezing. Moisture in the leaf layer may also help keep bats hydrated while hibernating.
“The only downside for bats is that hibernating on the ground instead of in the trees probably makes it easier for predators to find them,” Perry said. “There’s likely a trade-off between saving energy during cold weather and avoiding predators.”