Hibernation Versus Torpor: A Quandary for East Lansing Animals

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Monday, January 11, 2016, 12:30 am
Aron Sousa

Above: An East Lansing squirrel who will not engage in hibernation or torpor but who has recently chubbed-up for winter.

In the last couple of weeks, it has turned cold in East Lansing. For some animals, that means poofing out their coats like a squirrel and toughing it out, but some animals in East Lansing just go into a torpor.

Most people are familiar with hibernation. A hibernating animal, like a woodchuck, slows its heart beat to a few beats per minute and may let its body cool to near the temperature of its den or burrow. Animals that use this approach will be in hibernation for nearly all the winter and will consume fat saved up in their bodies to stay alive. Waking up from hibernation is a very slow process because the animal has to warm up before its brain will really wake up.

Only a few local animals, like chipmunks, woodchucks, and bats, truly hibernate. Many of our other local mammalian neighbors will occasionally enter a torpor during the winter. Animals in a torpor slow their heart rates some but not as severely as hibernating animals.

The most striking difference between torpor and hibernation is the change in the animals’ body temperatures: many hibernating animals might cool down by 20-30 degrees Fahrenheit (artic squirrels cool down to below freezing!) but an animal in a torpor will only cool by a few degrees.

In East Lansing, skunks, raccoons, mice, perhaps opossums, and many small rodents will enter a torpor for parts of the winter. These animals spend most of the winter actively looking for food, but when it gets really cold or food is scarce, they hunker down in a den, go to sleep, and drop their temperature a few degrees to conserve energy in a torpor.

A torpor is more than just a long nap or going to sleep, because the animal’s metabolic activity and temperature drop while in a torpor. Animals in a torpor have to warm up (usually through heat-generating brown fat cells) before they can really wake up. When you or a cat take a nap or sleep through the night, waking up is mostly about changing brain activity, not turning up your heat regulator.

Interestingly, torpor is not just for winter. Bats and some hummingbirds will enter a torpor to conserve energy during the summer when they are not feeding.

Here at ELi, we do not recommend capturing or handling bats, because they have a high rate of rabies in Michigan. Nonetheless, this author did capture a torporous bat in his house one summer, and it took the bat a good five minutes to come around and move at all. When it did come out of its torpor, it was a very grumpy and frightened bat.

Hibernation and torpor turn out to be very complicated phenomena. From species to species animals have varying and special systems for entering hibernation and torpor. Fish, amphibians, and reptiles have their own system, as already covered by ELi’s Nature reporter Paige Filice.

For a while, there was significant scientific discussion about whether bears hibernate or enter a torpor. In many ways, bears seem to hibernate, but their temperature drops only a few degrees. The current scientific consensus seems to be that bears hibernate but drop their metabolic rate with a mechanism independent of temperature, just as diving mammals can drop their metabolic rate without changing their temperature. Chipmunks appear to use both torpor and hibernation.

Of course, many animals and their grandparents have figured out that it is possible to migrate to warmer climates during winter. This is a complex set of instincts that is not of interest to the reporters for ELi on Earth, at least not this week.



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