Biological Traits Help Animals To Survive In Winter

By Scott Shalaway
OVER THE LAST few weeks, temperatures have plunged into the single digits on more than a half dozen mornings. It’s enough to make a morning low of 15 degrees feel downright balmy.

I deal with extreme cold by dialing up the thermostat and staying indoors as much as possible. Animals, on the other hand, deal with cold winter blasts in a variety of ways. Many birds migrate south to warmer climates. Mammals grow denser coats of fur to reduce heat loss, some store food that gets them through the winter and some hibernate.

Chipmunks, for example, vanish between Thanksgiving and Christmas. They spend most of January and February underground in dens below the frost line where they sleep. Though their metabolism slows, they wake periodically and eat the food they had stored earlier in the year. On particularly warm winter days, they sometimes emerge from their burrows.

Black Bears hibernate for four to five months each year, though their body temperatures drop only a few degrees. Thus biologists can argue that bears are not true hibernators. Yet their dormancy is nothing short of amazing.

Late in the fall, as day length shortens, chemicals in bears’ blood trigger the urge to hibernate. One hormone, leptin, is secreted by fat cells. The brain’s response to leptin is appetite suppression. So in December, bears lose their appetite and yield to the urge to sleep deeply. They may retire to a hollow tree or crawl beneath a brush pile. Some simply curl up on the ground.

Biologists know that Black Bear body temperatures drop only a few degrees because they have actually visited dens and taken rectal temperatures. But because bears are easily aroused from their sleep, obtaining rectal temperatures could be, let’s say, risky. So bear biologists capture bears in the fall and equip them with radio transmitter collars. In mid-winter they find the bear, tranquilize it, and safely take its temperature and gather other biological data.

Hibernating bears are physiological wonders. They drink no water and do not urinate all winter long, yet urea (a compound in waste) does not accumulate in their bodies and poison them. Because bears survive by metabolizing body fat, they produce very little of it. And what is produced is converted into nontoxic creatine, and nitrogenous waste products are physiologically recycled back into protein.

So even though bears don’t exercise all winter long, they lose little muscle mass. Upon waking, bears are in remarkable physical condition, and they easily roam widely as they resume normal activities. It’s almost as if their type of hibernation is a death-like state in which a bear emerges in almost the same condition from which it entered.

Aquatic turtles and frogs hibernate under ice in an environment where oxygen is limited. When ice first freezes it is clear and sunlight passes through it. Submerged aquatic plants continue to produce oxygen, though at very low levels, via photosynthesis. But after a few snow falls, light can no longer penetrate ice and oxygen production ceases.

Yet reptiles and amphibians that hibernate under ice survive. In some cases oxygen is obtained through the skin. In others, changes in blood chemistry reduce the need for oxygen for the duration of winter.

And then there’s the hardy Wood Frog. It emerges in February, when any respectable frog should still be dormant. It hibernates in the forest leaf litter and can actually freeze solid (Gray Tree Frogs, Spring Peepers, and Chorus Frogs also tolerate freezing). Again, changes in blood chemistry permit Wood Frogs to endure temperatures that would kill most other creatures. Freezing temperature stimulate these frogs to produce extremely high blood glucose levels, which act as a form of biological antifreeze.

I could describe many more examples of animals that survive freezing temperatures in amazing ways. Honeybees maintain a near constant hive temperature all year long. Beavers and Muskrats survive in lodges which provide access to foods under the ice of frozen ponds. And in the southwestern United States, Poorwills, a relative of Whip-poor-wills, actually hibernate rather than migrate, a fact known by Hopi and Navajo Indians long before biologists even considered the possibility.–Pittsburgh Post Gazette