Just Five More Minutes: News On the Evolution of Hibernation

An international research team led by Senckenberg scientists Dr. Dimitrios-Georgios Kontopoulos and Prof. Dr. Michael Hiller has investigated the evolution of different types of torpor – the temporary sharp drop in metabolism and body temperature to conserve energy – in numerous mammals and birds. Their study, recently published in the scientific journal “Functional Ecology,” shows that the “energy-saving mode,” which is widespread in the animal kingdom, has emerged several times independently of each other in different groups of animals over the course of evolution. According to the study, the two main torpor types – the short-term “daily torpor” lasting up to 24 hours and the markedly more prolonged hibernation – represent an evolutionary continuum.

The ability to save energy by deliberately lowering the metabolic rate and, through this, the body temperature is an amazing survival mechanism that can be observed in many mammals and birds. Whether due to freezing temperatures, scarce food resources, or extreme environmental conditions, short-term daily torpor and longer-term hibernation allow animals to use energy and resources particularly efficiently. Hedgehogs, for example, spend about half the year in hibernation, reducing their heart rate from around 200 to less than ten beats per minute. The European dormouse hibernates for as long as eight months and can extend this to almost a year under extreme conditions, while hummingbirds and other avian species can go into a torpor for several hours, if necessary.

Until now, it was not clear how the impressive diversity of torpor and hibernation emerged over the course of evolution. In a new study, the research team led by Dr. Dimitrios-Georgios Kontopoulos and Prof. Dr. Michael Hiller from the Senckenberg Research Institute and Natural History Museum Frankfurt shows that daily torpor and hibernation represent an evolutionary continuum and that these fascinating mechanisms have evolved independently several times in different lineages.
“We used complex phylogenetic comparative models to analyze data from a total of 1,338 bird and mammal species. This allowed us to determine that short-term daily torpor and extended hibernation cannot be clearly delimited from each other in evolutionary history – there are frequent evolutionary transitions between the two,” explains first author Kontopoulos. “In the course of evolutionary history, the ability of short-term daily torpor has repeatedly developed into prolonged hibernation and vice versa – often within just a few million years.”

In addition, the research team investigated how various characteristics of the animals, their habitat, and the evolution of torpor are connected. Smaller body size, nocturnal activity, and living in regions with seasonal changes in temperatures and low food availability are apparently associated with the ability to switch to energy-saving mode. “This is in line with our expectations that torpor and hibernation are particularly advantageous in habitats with strongly fluctuating environmental conditions,” reports Hiller. “Small animals can ‘afford’ the energy-saving mode more easily, as they require less energy to bring their systems back up to the ‘normal level.’ For very large animals, this would be too energy-intensive and would outweigh the strategy’s benefits. Hibernating bears are one prominent exception. However, they only lower their body temperature to around 30 degrees Celsius – the body temperature of the hazel dormouse (Muscardinus avellanarius), on the other hand, can drop to as low as minus two degrees Celsius.”

An at least partially meat- or insect-based diet also appears to be linked to this ability. “This makes sense for insectivorous animals, in particular,” adds Kontopoulos. “In regions with strong seasonal temperature differences, the availability of insects also fluctuates significantly. Some bat species, for example, hibernate during the period with the lowest insect abundance. However, for each of these observed patterns, we simultaneously found numerous exceptions, which is an indication that torpor has evolved multiple times independently in different species groups and under different conditions.”

Because hibernation is such a widespread strategy in mammals, one of the study’s key questions is whether this ability was already present in the last common ancestor of all modern mammals and was later lost in many species – including humans – or whether it evolved several times later in evolutionary history.
“Because of the complexity of hibernation, it may seem unlikely at first glance that this survival strategy could have evolved multiple times,” says Kontopoulos. “It has also been hypothesized that a month-long hibernation could have been crucial for the survival of mammals after the mass extinction of many species at the Cretaceous-Paleogene boundary 66 million years ago. However, our results suggest that the last common ancestors of today’s mammals probably did not have the ability to hibernate – they must have survived in other ways. It is also not unique that such a complex ability as hibernation has emerged several times in the course of evolution – many other complex traits have also evolved several times independently of each other, including vision and flight.”

These findings on the remarkable diversity of hibernation and torpor immediately give rise to further research questions. “Our knowledge of the molecular basis of torpor in one species cannot necessarily be transferred to another species, even if they occupy similar ecological niches. We should find out more about the molecular, physiological, and ecological mechanisms of these strategies in different species – not least to better understand how hibernating animals will react to environmental changes such as climate change,” adds Hiller in summary.

Publication:
Kontopoulos, D.-G., Levesque, D. L., & Hiller, M. (2025). Numerous independent gains of daily torpor and hibernation across endotherms, linked with adaptation to diverse environments. Functional Ecology, 00, 1–16. https://doi.org/10.1111/1365-2435.14739