Winter can be a hard season to get through. We’ve had some bitterly cold days in Massachusetts this week, with lows in the single digits Fahrenheit. As we humans brew warm beverages, put on our cozy sweaters, and fret about our heating bills, the wildlife outdoors have their own methods of coping with extreme cold. This week, we’re taking a look at some of the fascinating ways that animals survive below-freezing temperatures year after year.
Stock Up on Food
For endothermic (“warm-blooded”) animals like mammals and birds, food is a daily necessity, and this is even more true during wintertime. By converting food calories into heat, animals generate the warmth they need to prevent their bodies from freezing. Food with high fat content – like nuts and seeds – is especially valuable, so many animals put some food away for the winter when foraging is difficult. We’ve all seen squirrels burying and stockpiling acorns for the winter, but they’re far from the only animals that do so. Many birds, including blue jays and chickadees, hide seeds and nuts in specific spots to come back to later. This is called “scatter-hoarding,” and remembering where all those seeds are hidden requires a lot of cognitive power. One black-capped chickadee can store a thousand seeds a day, all in different places, and while getting ready for the winter may store tens of thousands in total. Even more amazingly, the birds’ brains change in real time to accommodate this task: a chickadee’s hippocampus (which plays an important role in spatial memory) actually grows every fall and shrinks every spring!
Dream Away the Season
Many animals drastically slow down their metabolism in winter to cope with food shortages – getting by with less energy rather than planning to expend more. Many people think of bears hibernating as a classic example – in fact, bears are not true hibernators. While they do go into torpor (a general term for states of reduced metabolic activity) over the winter, bears are capable of awakening and foraging for food on mild days during the winter, which true hibernators can’t do. Many rodents and some bats are true hibernators, literally sleeping in a deep state of reduced metabolism for several months straight. Most reptiles and amphibians take this approach, too, but ectotherms (“cold-blooded” animals) undergo a different process called brumation.
Brumation is an extension of the natural tendency of ectothermic bodies to slow their metabolism as their ambient temperature (and therefore, their body temperature) drops, but it’s more extreme than the day-to-day variations in activity level that these animals experience and far beyond anything mammals can manage. Hibernating marmots (like groundhogs) can reduce their heartbeat to as slow as five beats per minute, whereas a turtle brumating under a frozen pond can go several minutes on a single beat! Even birds use torpor to conserve energy on cold nights – small birds like hummingbirds and kinglets can go into torpor almost every night.
Dress For Success
Animals that stay active all through the winter typically have a coat of fur or feathers to keep them warm. These coats are grown especially for winter; mammals grow a fluffy warm “undercoat” as well as an “overcoat” of specially-adapted guard hairs. The guard hairs of moose, for example, are hollow, and they channel the warmth of sunlight down towards the moose’s dark skin, where it is absorbed to keep the animal warm. Its fluffy undercoat then traps that heat next to the moose’s body. Otters, on the other hand, have tiny interlocking scales on their fur, which link together and form a waterproof shell when the otters enter the water. Their fur is also incredibly thick, with more than 400,000 hairs per square inch!
Birds, too, grow thicker coats (of feathers) during their fall molt to help insulate them through the winter. You may see birds looking “fluffed up” in the winter – this posture increases the amount of air trapped between their feathers and their skin, which insulates them more effectively. You may also see birds standing on one leg, or sleeping with their bill tucked under a wing. Bird bills and legs don’t have any feathers, so they lose heat quickly, but birds can pull these exposed parts inside their feathers to warm them up for a little while.
Have you seen other wildlife behaviors or adaptations to help them beat the cold? Let us know in the comments!