Is a hibernating bat safe?

Underground bat hibernation sites, called hibernacula, can attract predators. Finding signs of predation among the bats overwintering in twelve World War II bunkers in Zuid-Holland in The Netherlands, researchers set up trail cameras to identify the culprits.

Wood mice!

In the bats’ hibernacula, winter predation by wood mice (Apodemus sylvaticus) was found to be a background constant. A severe winter or a poor crop of acorns the previous autumn caused the number of fatalities to spike, with up to 9% of a hibernating population of bats succumbing to what the research paper called, rather quaintly, mouse-related mortality. A mild winter or a fruitful autumn caused the numbers to drop but never to cease.

Fatalities rose towards the end of each winter during the study period. This could be because the wood mice’s other food resources were dwindling or because the bats, already known to move closer to the entrance of their hibernacula as the winter ends, were more easily discovered.

Wood mice are usually considered to be scavengers, opportunists on the lookout for anything edible, but the research group’s trail cameras seemed to show the mice actively hunting for bats inside the hibernacula, looking into the crevices and cracks where bats hide away to hibernate.

Bats reproduce slowly: Daubenton’s bats, one of the species found in the hibernacula in this study, give birth to a single pup each year after a pregnancy of almost two months. Population numbers can be affected quite dramatically by the pressure of predation, particularly if that number is small to begin with.

In the reserve, we have identified thirteen species of bats, some of them endangered, some of them very rare and all of them protected by national and international legislation. We know some of our bats hibernate in holes in our trees, others retire to caves in the Special Area of Conservation near Bradford on Avon but some prefer to hibernate underground.

We also have wood mice, cute woodland creatures – or so we thought….

Header image: Daubenton’s bat by Rauno Kalda, (CC BY-SA 3.0) via Wikimedia Commons