Climate change is making bats bigger

Climate change is making bats bigger

A Bechstein’s bat (Myotis bechsteinii) hangs in its roost. © Christian Giese

Climate change is affecting animals worldwide. For example, warmer temperatures ensure that young bats grow to be larger than average, as researchers have now discovered. Warmer summers mean that the young animals have to invest less energy to keep their own body temperature constant and can therefore grow continuously. However, larger animals also need more food and will starve more quickly if they cannot find enough to eat. This could decimate bat populations in the long term.

The Bechstein’s bat (Myotis bechsteinii), which is widespread in Germany, is somewhat reminiscent of a cute forest spirit with its large ears, black button eyes and gray-brown fur. The nocturnal hunter normally has a wingspan of 25 to 30 centimeters and a weight of between seven and twelve grams. However, it is repeatedly observed that their young develop into significantly larger adults in warm summers.

Bat cave with heating

Until now, however, it was unclear whether the warmer temperatures themselves were responsible for this growth spurt, or whether in warm summers there was simply more food available in the form of insects, which literally made the bat young “big and strong”. To clear up this mystery, researchers led by Carolin Mundinger from the University of Greifswald ensured different temperature conditions in the roosts of free-living Bechstein’s bats over four years and observed how these affected the size of the young.

“For the experiment, we developed mobile heating devices with which we were able to keep the temperature of the bat boxes constant at around 30 to 35 degrees Celsius over the first eight weeks after the birth of the young,” explains Mundinger. At these temperatures, unlike usual, the young animals hardly had to invest any energy to keep their own body temperature constant, which in theory allowed them to grow continuously.

Large bats live shorter lives

And indeed: On average, larger bats developed in the caves with radiant heaters than in the unheated quarters, as the researchers report. “Female bats of this species are usually around five percent larger than males, but in our experiment the heated males reached a body size similar to that of unheated females,” explains Mundinger’s colleague Gerald Kerth, also from the University of Greifswald. The researchers therefore assume that it is the warmer temperatures that cause the growth spurt, and not the richer food supply of warmer than average summers.

Although at first glance the size of bats seems irrelevant to their future lives, it actually has a significant influence on them. For example, larger females reproduce earlier than smaller ones, which leads to an increase in annual bat offspring, as the researchers report. But given the global decline in insects, there is probably not always enough food for everyone. Especially if the offspring continue to grow so large and therefore need more insects than smaller animals. These circumstances could cause the larger Bechstein’s bats to die prematurely in the future. And not just them: Mundinger and her colleagues assume that climate change and its warmer summers will decimate the entire bat population in the long term.

Source: University of Greifswald; Specialist article: Current Biology, doi: 10.1016/j.cub.2023.08.004

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