Edible Dormouse: Hibernation for 34 Million Years

Rodent fossils suggest that hibernation already existed as a survival strategy 34 million years ago.

Until now, it was assumed that the ability to hibernate only developed around 2.6 million years ago, at the beginning of the Quaternary Ice Age. However, this behavior may have appeared in certain mammals considerably earlier. This is the conclusion of a study supported by the Swiss National Science Foundation (SNSF)study.

Together with his team, Olivier Maridet, curator at the Jurassica Museum in Porrentruy and palaeontologist at the University of Fribourg, has examined the evolutionary history of the family of dormice: small rodents with bushy tails, which include the edible dormouse, the garden dormouse, and the hazel dormouse. When the researcher compared the number of species in this family over time with the development of the climate, he noticed that diversification occurred three times, and each of these episodes coincided with an ice age. “These rodents must have possessed an advantage that allowed them to thrive so well during these periods. We believe it was hibernation.»

An Archaic Deficit Becomes an Advantage

It was already known from fossils of certain rodents that hibernation existed 2.6 million years ago — their incisors show seasonal growth pauses. Accordingly, this ability developed and spread in response to the harsh survival conditions of that era at the beginning of the Quaternary Ice Age — the most recent period in Earth’s history. In their study, however, Olivier Maridet and his team now conclude that dormice may have spent winters sleeping as far back as 34 million years ago.

This could therefore be an archaic trait that was preserved in certain species to this day while being lost in others, because they chose different survival strategies than hibernation.

Olivier Maridet

The research team studied dormice as part of its research into the transition between the Eocene and Oligocene epochs, which in Europe is also known as the “Grande Coupure” (great cut) due to its far-reaching consequences. At that time, a significant cooling occurred that led to the formation of the polar ice cap in Antarctica and a drop in sea levels, which facilitated the migration of animal species between continents.

Mass extinction and survival artists

In Europe, mammals underwent major upheaval at this time. Many of them, such as the Adapoidea (extinct primates), palaeotheres (distant cousins of modern horses), or certain rodent groups native to Europe, disappeared. Younger species originating from Asia settled in the region, including hamsters, beavers, rhinoceroses, pigs, and tapirs. Dormice managed to survive this crisis — and even diversified in the process. «After the mass extinction at the Cretaceous–Paleogene boundary 65 million years ago, which claimed the dinosaurs among its victims, this transition is one of the events with the most far-reaching impacts on ecosystems», says Olivier Maridet.

Teeth as witnesses of time

The researchers arrived at these findings by analysing over 500 fossil teeth from dormice found in France and Spain. They analyzed the size and shape according to 124 different criteria, thereby identifying the degree of kinship between species. They also scanned teeth using X-ray tomography to extract information from their interiors. The result is impressive: for the first time, the evolution of the dormouse family could be mapped in its entirety — including fossil species. In addition, the researchers were able to confirm the close relationships between currently known species using molecular methods. The detailed analysis of dormouse dental anatomy also showed that it was time to dust off the family tree of this group.