Hibernation site dimensions influence bat abundance and diversity in the boreal zone

In the deep winter of the boreal forest, where temperatures plunge far below freezing and the ground is locked in ice, many small mammals face a choice: migrate to warmer climes or shut down their bodies to survive. For bats living near the northern edge of their range, migration is often impossible, so they rely on hibernation. This is a state of suspended animation where their heart rate slows to a fraction of its normal pace, and their body temperature drops to match the cold air around them. To survive this long sleep, they need a hibernaculum, a shelter that stays just cold enough to keep them in that dormant state but warm enough to prevent them from freezing to death. In the wild, these shelters are often caves or deep rock crevices, but in places like Finland, human history has provided a new kind of refuge: abandoned military bunkers. These concrete structures, left behind after conflicts, have become critical winter homes for local bat populations, offering a stable environment that the natural landscape sometimes cannot.

Scientists in Finland set out to understand how the physical size of these man-made bunkers influences which bats choose to live there and how many of them gather. They focused on the dimensions of the tunnels, specifically looking at how the length of the underground space correlated with the presence and numbers of different bat species. The researchers examined a variety of these abandoned structures, counting the bats inside and noting the specific conditions of each site. Their work revealed that the length of the bunker was the most important factor in determining whether bats would be found there at all. The longest tunnels, measuring roughly 25 to 26 meters, were the most likely to host a full community of hibernating bats, including the northern bat, the brown long-eared bat, and various species of mouse-eared bats.

While the longest bunkers were the most welcoming overall, different types of bats used these spaces in distinct ways. The northern bat, for instance, appeared to be a generalist, willing to sleep in bunkers of almost any length and in a wide range of temperatures. However, even though they were flexible about where they slept, they did not gather in large numbers in any single location. In contrast, the mouse-eared bats showed a strong preference for the longest tunnels, but only if the temperature remained stable throughout the space. These bats also seemed to care deeply about where they positioned themselves within the tunnel; in shorter bunkers, they huddled in the areas furthest from the entrance, likely seeking the most stable conditions, but in the longest tunnels, they spread out to occupy the entire length of the space.

The brown long-eared bat followed a different pattern, with its numbers increasing as the length of the bunker increased, yet this species remained quite rare in the studied sites regardless of the size. The study suggests that while these concrete bunkers are vital for the survival of bats in harsh northern winters, they are not all created equal. The physical scale of the shelter acts as a filter, determining which species can find a suitable spot and how many individuals can be supported. The findings highlight that for these animals, the simple measurement of a tunnel's length is a powerful predictor of life and death during the winter months, shaping the community of bats that can successfully hibernate in the boreal zone.