With the world heating up, many animals are reaching smaller sizes when fully grown. But for some reason, this doesn't seem to be the case for Bechstein's bats (Myotis bechsteinii), a small forest-dwelling species of bat that inhabits most of Europe. When the summers are hotter, the pups born during that time become larger adults than those born when the summers are relatively cool. Warmer summers often mean more insects for the bats to eat, but that doesn't appear to be what is making the juveniles turn into larger adults. This led Janis Wolf, Philipp Lehmann and Gerald Kerth of the University of Greifswald, Germany to ask whether Bechstein's bats were using the warmer temperatures – which are closer to their body temperature – to save the energy they would usually use to keep themselves warm.
In mid-June of 2022 and 2023, Wolf and colleagues headed into the forest for 5 weeks to measure the metabolic rates of these bats. This coincided with the time that the juveniles are still relying on their mothers for milk and staying with them during the day when they roost. First, the researchers measured how much oxygen and carbon dioxide the group of bats were breathing while huddled together in their roost boxes during the day. The following day, the team would heat up the box until the temperature inside was 30°C. At this temperature, the bats shouldn't need to spend any energy to warm themselves up or trying to cool themselves down. When the scientists compared the metabolic rates of the bats on days they heated or did not heat their roost boxes, they didn't see any differences.
Surprised by this finding, Wolf looked to see what happened to the metabolic rates of the bats when the boxes were hotter or cooler than the temperature the bats preferred. When the bats were cooler than they'd like, their metabolic rates were still rather low, but when the boxes were hotter, their metabolic rates were also high. This suggests that the bats have ways of dealing with the cooler weather that are rather energy-efficient, but they struggle when the weather gets too warm. The scientists already knew that the bats often huddle together when the weather is cold and this helps them stay warmer without expending as much energy, but could there be another way that the bats are able to stay warm?
To find out, the researchers looked at how the bat's metabolic rate changed over the course of the day. Interestingly, they found that their metabolic rates were highest in the morning. Since these bats spend much of the night hunting insects, this rise in their metabolic rate is probably caused by them digesting their meal. Luckily for the bats, they are also able to use the extra heat given off from digestion to help stave off the cooler mornings.
It seems that the bats have developed behaviour that allows them to deal with the cooler temperatures and helps them conserve energy at the same time, but warmer weather is more costly. When their roost temperatures are too hot, the bats have higher metabolic rates – presumably because they cool their bodies by panting and wetting their fur – and expend more energy, possibly making them dehydrated as well. Although the exact reason that Bechstein's bats grow larger when born in warmer summers isn't fully understood, Wolf and colleagues have brought us closer to the truth while showing that climate change is causing these bats just as much trouble as it is everything else.
