Long-term microclimate measurements add further evidence that there is no “optimal” temperature for bat hibernation

Abstract

Hibernation has long been known to be an energy bottleneck for temperate-zone bats. As such, considerable research effort has been expended to understand the physiological ecology of bat hibernation and the microclimates necessary for successful hibernation. Still, few long-term datasets of microclimate in bat hibernacula are available, and most descriptions of “optimal” hibernation sites are based on temporally or spatially limited datasets. Here, we summarize a long-term dataset of microclimate data from caves used by hibernating Indiana bats (Myotis sodalis) to draw conclusions about our understanding of microclimate selection of hibernating bats more generally. Ambient temperature varied among and within most hibernation microsites across the season of hibernation. Microsites near entrances were strongly and rapidly affected by external climatic conditions, while sites deeper in caves were more weakly related to external temperatures and show a greater lag time in response to those conditions. Comparison of microclimate and concurrent population counts suggests that bats select mid-winter microsites with a wider range of environmental conditions than is often stated; mid-winter survey counts increased between 1983 and 2011 in both cold and warm microsites. This extensive dataset provides some of the most exhaustive evidence yet that not all bats within a species choose (or likely require) microsites with the same microclimatic conditions. We argue that too often researchers and land managers have viewed microclimate selection through the lens of “optimal” conditions, and in doing so, often miss important variation that may actually be preferred by some bats.