Bears do it. Bats do it. Even turtles do it. Now, scientists from the University of Veterinary Medicine in Vienna have identified a species of dormouse that can spend almost an entire year doing it.
Hibernating, that is.
The edible dormouse — a native of Austria, and so named for its appearance on Ancient Roman dinner tables — holds the title for the longest recorded mammal hibernation in the wild. Normally, the edible dormouse spends the food-scarce winter holed up, hibernating to save energy. But some seem to be spending their summers underground, too. To understand why, Dr. Franz Hölzl and Dr. Claudia Bieber, with their colleagues in the Department of Integrative Biology and Evolution at UVMV, studied the mammal’s hibernation habits. They implanted tiny electronic devices to measure their body temperature, then let them go. The scientists wanted to see when and for how long the mice were hibernating.
The answers were surprising, even for the sleepy dormouse.
It was generally believed that hibernation was avoided whenever possible; that only bad environmental conditions triggered the behavior. “Interestingly,” says Dr. Bieber, “dormice choose to hibernate even if the food supply is sufficient and the climate mild.”
Indeed, eight of the 17 dormice tracked in the study began hibernating in the height of summer. Five went down for 11 months or more. The scientists believe extended periods of hibernation protect the dormice from predators, or that the dormice snooze through lean years to skip reproducing in vain.
The big sleep? Not exactly.
Before we consider the potential implications for human health, let’s clear one thing up. It’s not quite correct to say the dormice are asleep. Hibernating animals aren’t sleeping; for the most part, they’re in a phase called torpor, “a state of hypo-metabolism, which includes a lowered body temperature, a lower breathing rate and pulse rate,” according to Dr. Hölzl.
Unlike sleep, torpor is not restorative. “These phases are not relaxing at all,” Dr. Hölzl says. “It appears more as a very stressful period, but it saves a lot of energy.”
Hibernating animals, including the edible dormouse, actually show little brain activity during torpor. In contrast, a sleeping brain is quite active. As Van Winkle’s has previously reported, brain activity during REM sleep is very similar to brain activity when awake.
However, like sleep, hibernation is not a single steady state. Animals experience periods of arousal every three to six weeks. During these phases, called euthermia, animals increase their metabolism and heat up their body to its normal temperature for an average of four to six hours before reentering a new phase of torpor.
It’s believed these periods are meant to keep the body running properly. “Hibernation can cause several severe consequences for the animals, such as brain damage, cardiovascular damage, memory loss,” Dr. Hölzl says. “One explanation for the inter-bout arousals could be that the animals have to ‘wake up’ to keep the body going.”
It’s also possible, he says, that animals have to arouse themselves briefly from hibernation to keep the immune system working; they may also be restoring parts of their memory.
Why can’t humans hibernate?
Hibernators are described as heterothermic, which means they can change their body temperature in response to environmental signals. Humans, on the other hand, are homeothermic animals — we must maintain a constant body temperature.
“A human will die by hypothermia if the core body temperature drops below 28°C,” says Dr. Bieber. “The heart tissue can’t work below this temperature, and the heart stops beating.” The hearts of hibernating animals, meanwhile, continue pumping blood even as their body temperature drops.
Protection from tissue damage caused by hypothermia seems like a useful trait for all of us living north of the Sun Belt. So, why can’t humans shut down for the winter?
The answer, says Dr. Hölzl, is evolution.
Hibernation allows animals to save energy by lowering their metabolism, but it comes at a cost. When other options are available — e.g., there is enough food and water available, or the animal can move to a more favorable environment — hibernation is generally avoided.
“Humans evolved in tropical Africa, so it was simply not necessary to evolve [with a] mechanism to save large amounts of energy,” says Dr. Hölzl. “That is the reason why humans are homeothermic.”
Human hibernation may be out of reach, at least for now, but research on the subject has led to several biomedical advances. For example, “therapeutic hypothermia” (that is, intentionally lowering the body’s core temperature) can help prevent certain types of neurological injury. It may also lessen damages caused by heart attacks, according to a 2011 paper by Dr. Elizabeth M. Moore from the School of Public Health and Preventive Medicine, Monash University, Australia.
Scientists at Carleton University are studying the biochemical mechanisms that regulate hibernation, hoping to learn how hibernating animals stay healthy during torpor. They hope the answers will lead to new, better methods to preserve organs for transplant and limit muscle wasting during long periods of skeletal muscle inactivity.
Additionally, at least one aerospace company is researching the potential for therapeutic hypothermia as a strategy for safe, long-distance space travel.
But, as Dr. Beiber cautions, there is yet no identified mechanism to induce human hibernation. When it comes to the safe, long-term slumbers, the tiny edible dormouse has us all beat. For now.