Wild Colorado: Hibernation in the High Country | SummitDaily.com

Wild Colorado: Hibernation in the High Country

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Special to the Daily/Kim Fenske

SUMMIT COUNTY – With snow on the way in the High Country, much of wildlife is hitting the hay.

It’s that time of year when even some humans want to bury themselves under the covers and hide out until the first green plant tendrils creep out of the frozen ground next spring. Still others delight in the snowfall, staying active all season.

Many animals have packed on the last bits of fat, loaded the last shelves of the pantry, and picked that perfect place to go to sleep for several months. They’ve gotten the hint from the changing seasons to go hide and not come back until long, warm days return.

These animals are entering hibernation, the time of year when air temperatures drop and food availability becomes virtually nonexistent. The opposite is estivation, when animals hide away to escape extreme heat, such as in the Mojave Desert.

To survive during winter hibernation, animals reduce their body temperature and metabolism to stay alive. That physiological response is known as torpor. It’s akin to a human’s ability to survive eight hours of sleep without eating – but animals can do it for up to six months at a time.

In a general sense, in winter, everything simply slows down – whether it’s the animal’s heartbeat and breathing, putting them into deep sleep – or it’s the ecological practices of hunting, gathering, resting, and interacting among species.

Animals in the High Country have a wide variety of behaviors which, with the help of Colorado State University biology professor Gregory Florant, we’ll examine in this week’s Wild Colorado. Florant specializes in animals’ mechanisms of adaptation, with recent research on hibernation and ways animals regulate energy stores. Florant owns a home in Frisco and does much of his research in the High Country.

Large rodents, such as ground squirrels and marmots, delve into their burrows in winter and let their body temperature drop close to freezing. They arouse, or awake, periodically throughout winter. This adaptative mechanism is known as a torpor bout.

They prepare for hibernation by eating large amounts of polyunsaturated fats – such as seeds, nuts and plants like dandelions and cow parsnip – enough to grow to two or three times their body mass. It’s what their bodies use to survive after going underground until spring.

“They burn fat to survive, but they burn it slowly,” Florant said.

Similar to a human’s sleep cycle, but much extended, torpor for these animals means dropping their metabolic rate to about 1/50 to 1/100 of normal operation. The rodent’s heartbeat slows from about 150 beats per minute to six. Many breathe once every two minutes. Body temperature is just slightly above the burrow’s ambient temperature, which is about 6 degree Celsius. They stay this way for about two weeks.

A hibernating animal is usually curled into a ball and looks dead – barely breathing with a very slow heartbeat and cold to the touch, Florant said.

But then they wake up!

“They spontaneously arouse – wake up – and bring their temperature back to 37 degrees (Celsius) for a few hours,” Florant said. After a bit of moving about in the burrow, they go back into torpor and the body temperature drops again to just above freezing.

“We don’t know why they go through these bouts of arousal,” Florant said. “But they’re not just going to sleep and staying asleep all winter long. We call it the mystery of arousal.”

Many small rodents, such as chipmunks, can’t build enough body mass to survive a full winter’s sleep, so these creatures stockpile food to eat when they awake from each torpor cycle. They remain in the burrow, but each time they awake, they pull something out of the pantry to munch.

Though pikas don’t hibernate, they do stay in a burrow all winter. They, too, collect food for the winter pantry. Then, they live normally – though underground and insulated from the elements – and eat when necessary.

It’s unwise to enter a bear cave, even during hibernation, Florant said.

That’s because bears don’t go into as deep a sleep as other creatures. Their core body temperature drops about 6 to 8 degrees, as opposed to the 30 degrees in large rodents. Florant said that’s because it would consume too much energy to rewarm a bear’s mass to such an extent multiple times. Bears wake up and move around fairly often during winter, but they don’t eat or defecate in winter.

Another twist in a bear’s situation is that they give birth during hibernation, Division of Wildlife spokesman Randy Hampton said. They breed in the fall, but the embryo doesn’t implant until late winter, if the mother bear is in good physical condition. At that time, several embryos (possibly from different males) could implant simultaneously or they may not implant at all. Once born, cubs drink their mother’s milk until it’s time to come out of the cave in spring.

Some animals, such as birds, enter what’s known as daily torpor, or dropping their body temperature at night and awaking in the morning.

Other animals – such as badgers, weasels, skunks and martens – enter torpor as necessary. Sometimes it’s daily, and sometimes its less or more often.

Mice, tree squirrels and other small rodents that can’t store enough body mass to survive a full winter’s sleep remain out and about, rummaging for food between the snow and the ground and hiding from their carnivorous brethren. Rabbits and hares stay active, doing the same thing.

As such, part of the food chain stays active, but it’s extremely slowed. The animals eat less regularly, but they still must continue to feed.

SDN reporter Janice Kurbjun can be contacted at (970) 668-4630 or at jkurbjun@summitdaily.com.

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