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The ambient temperature has a profound impact on the physiology and behavior of most species. In regions where individuals rely on low temperatures to hibernate effectively, global warming is likely to significantly affect their survival.

A team of scientists has studied how ambient temperatures shape the energy expenditure of common noctule bats and built a model to predict at which latitudes they could survive hibernation.

This model also predicts how the hibernation areas of these bats could change over time. It accurately tracks the northward range shift of this species over the past 50 years and shows a further northeast expansion of up to 14% of its current range by 2100—driven by shorter and warmer winters in Europe.

The study was carried out at the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) by a team of scientists from the Departments of Evolutionary Ecology and Evolutionary Genetics. First author Dr. Kseniia Kravchenko is now a postdoc at the University of Luxembourg and senior author Dr. Shannon Currie is now a lecturer at the University of Melbourne. The paper is in the journal Ecology Letters.

Energy expenditure is closely linked to ambient temperature. When conditions become unfavorable, many mammals, such as bats, hibernate to save energy. "Hibernators tend to be overlooked in biophysical models because they switch between two physiological states during hibernation, making modeling more difficult," explains Dr. Currie. "So, it's still unclear how climate change will impact these species."

To investigate how this essential life-history trait affects their survival in warmer winters, Dr. Kravchenko and her colleagues conducted two experiments. "We assessed how much time common noctules, which are bats weighing around 30 grams, spent in torpor—the physiological state animals enter during hibernation—at different ambient temperatures. To detect torpor, we measured the skin temperature because individuals lower their body temperature to save energy," Kravchenko explains.

In a second experiment, the scientists measured CO₂ production as a proxy of the bats' energy expenditure under different ambient temperatures.

Model accurately reproduces historical shift of hibernation areas

The results were combined with daily temperature forecasts produced by the Potsdam Institute for Climate Impact Research, under different climate change scenarios. This way, the scientists could calculate the energy budget required to survive winter for more than 12,000 locations spread throughout the whole of Europe.

They compared energy budgets using historical data (1901–2019) as well as under future projections (2019–2100) of four different scenarios of climate change. "Our computations for current temperature data produced a hibernation area which closely matches the actual wintering distribution. This was reassuring given that the model turned to be accurate based on ambient temperature and physiological parameters only.

"We were also happy because, after all the experimental work and the programming efforts we put in, it showed that our approach actually worked," says Dr. Alexandre Courtiol, scientist and modeling expert at the Leibniz-IZW. "Further computations showed that the hibernation area shifted towards the northeast of Europe between 1901 and 2018, thereby expanding by 6.3% in its original size."

Hibernation areas are expected to shift and expand further north and eastward

Feeding the model with different projections of future climate scenarios reveals that both the southern and the northern limits of the potential hibernation area shift further northwards—the southern limit even more so than the northern limit. Since 1901, the suitable wintering grounds have already moved about 260 kilometers northward.

"The current spread towards the northeast is predicted to continue by about 80 kilometers averaged across models, increasing the potential hibernation area by 5.8 to 14.2% between 2019 and 2099, depending on the climate change scenario," the authors conclude.

Under the most severe climate change scenario—where emissions are expected to increase, winter temperatures to rise by 2.35°C and average hibernation seasons to shorten by 41 days—this northward shift is predicted to extend to about 730 km, yielding a predicted total northward shift of about 990 km over two centuries.

Common noctules are capable of range shifts of several hundred kilometers in only a few decades, as previous studies by Kravchenko and colleagues have shown, so it is possible that, as temperatures keep rising, this species will keep tracking changes in the potential hibernation area by continuously expanding its hibernation range toward the northeast of Europe.

Yet this could lead to challenges when other requirements for hibernation—such as appropriate hibernation sites and food availability before the start of the winter—are not available in the new areas where temperature becomes suitable.

The scientific team found that the hibernation niche of the common noctule bat is adequately explained and accurately approximated by only two straightforward statistics: mean daily ambient temperature during the hibernation season and duration of the hibernation season.

"This means we could potentially map the hibernation niche of other species using the same metrics. Yet we still need to closely investigate and monitor the effects of climate change on wildlife physiology without forgetting that the environment is more than just ambient temperature," Prof Dr. Christian Voigt, head of the Leibniz-IZW Department of Evolutionary Ecology, sums up.

This ecophysiology research is crucial to tailoring conservation interventions and wildlife protection measures in times of environmental change.