As leaves turn brown and frigid winds start sweeping through the Northern hemisphere of the world, animals need to prepare for the winter months to come. This doesn’t look the same for all animals, though, as they’ve evolved several techniques to brave the cold and survive.
1. Animals Migrate
When the temperatures drop, many animals skip town and head south — chasing entirely warmer climates.
What Animals Migrate in Winter?
Some animals — like the chamois, elks, and ibexes — might descend a couple of miles from the mountaintop and spend the colder months at lower altitudes, where the weather is slightly warmer.
Migration is especially common in birds, who can fly long routes across the globe pursuing the sunshine. Geese, flying in V-shapes, are the first expert migratory travelers that usually come to mind, but even tiny hummingbirds migrate. The ruby-throated hummingbird, which lives in North America, can travel 500 miles across the Gulf of Mexico in less than a day.
Land mammals like caribou, and water mammals like whales also have long and winding migratory routes seeking warmth. Eastern monarch butterflies are famous for traveling 2500 miles from their nesting grounds in Canada all the way to Mexico, like hummingbirds.
2. Animals Go Dormant
Lacking the ability to adapt to the cold, or to travel far and wide to escape from it, some animals have evolved to literally sit winter out: they rest throughout the whole cold season until spring comes around. Dormancy doesn’t look the same for every animal though and can come in different steps.
What Is Torpor?
While bears are famous for going dormant during icy times, what most species do is called torpor and not hibernation.
Torpor is the term used to describe when animals involuntarily drop their body temperature and metabolic activity to a small fraction of what it normally would be: they slow down their heart rate and breathing rate as if they’ve passed out. Temperatures fall from about 38 degrees Celsius to 18 degrees Celsius, and metabolism plummets by 30 percent.
During wintertime, many reptiles and amphibians also go into a state of torpor, which reptile experts call brumation. This looks pretty much like torpor in warm-blooded animals: they lower their heart rates, lower their metabolism, and hide out for several weeks or even months. But since reptiles are cold-blooded animals, they cannot regulate their metabolism with food.
What Animal Can Go Dormant?
During winter, animals like bears enter periods of long rest behavior where they hang out in their dens to preserve their energies. For entire chunks of the day, they’ll fall into this state of suspended animation, and this is usually a daily and nighttime activity. Animals in torpor are still awake most of the time and sometimes leave their hideout to pee and poop, stretch out in the sunshine on a sunny day, and have a little snack. Female bears may even give birth.
“A routine drop in metabolic rate is much more efficient than what most mammals, like us, do — which is keep our body temperatures mostly high all the time,” says Elizabeth Dahlhoff, a biology professor at Santa Clara University studying the mechanisms by which animals respond to environmental change.
Read More: Could Humans Ever Hibernate in the Future?
3. Animals Hibernate
While torpor is usually a daily experience lasting a couple of hours, hibernation is more prolonged.
How Do Animals Hibernate?
Animals who hibernate slow down their heart rate, breathing rate, and metabolism, as well as decrease their body temperatures, and they stay this way for a long time — days, weeks, or even months.
“Hibernators have to do all sorts of tricks to ignore and trick the mechanisms that would make us non-hibernators stay at normal body temperature and metabolic rate,” says Dahlhoff. “There is also some recent evidence of how some hibernators suppress thirst. Imagine going months without a drink of water!”
During this time, most species are slowly burning through the fat energy reserves they acquired while gaining weight in the fall. They never wake up and never leave their den, from the outside, they are practically dead — you can pick them up and move them around. This mechanism is so powerful, it’s almost impossible to wake an animal up from hibernation.
What Animals Hibernate?
Hibernation is usually a feat for small mammals, who are also most at risk when the snow appears. Rodents are the best at hibernation. Groundhogs, for instance, go into a deep rest that qualifies as hibernation. Certain species of bats are also true hibernators.
“Until recently, we thought the largest mammal that hibernated was a marmot, but there is new evidence showing that some species of bears really do hibernate, rather than ‘just’ going into torpor,” says Dahlhoff.
Researchers think hibernation is triggered by changes in the environment — dropping temperatures and shorter days — as well as internal, hormonal, and biological triggers. The hypothalamus, a part of the animal brain, plays a big part in switching an animal’s metabolism rate.
“Coming out [of hibernation] is probably triggered by the animal running out of fat resources. Not everybody makes it through the winter,” says Dahlhoff.
4. Animals Burrow Underground
Many animals who stick around for winter months tend to spend their time hiding or resting in protected environments, like nests in trees or holes in the ground.
What Animals Live Underground in Winter?
Moles, who already burrow underground during warmer months, move their tunnels deeper underground to stay away from the freeze, and also store snacks for rainy days in special, secret storage tunnels.
Some creatures — like mice, shrews, and voles — spend most of their time under the snow, in subnivean tunnels in the layer between the ground and the snowpack. Here, it’s cozy as the outside world freezes over, and food is readily available in the form of plants, seeds, shrubs, and sticks left scattered around.
5. Animals Change Their Fur
Another trick to face the winter, that deer and foxes use, is to grow a thicker winter coat of fur to keep the body warm. Often, this winter layer isn’t just protection from the cold: it also helps as camouflage, changing from brown to more muted greys, blending in with the barren, frosty woods.
Why Do Animals Change Their Fur?
Snowshoe hares, arctic hares, and mountain hares, for example, shed their brown fur and turn white, to blend in with the fallen snow, avoid predators, and hunt prey. Weasels and collard lemmings do it, too. The ptarmigan is just one bird species that changes color in the winter, shifting from brown to white to blend in with the snow.
6. Animals Hoard Food
Adapting to the cold makes foraging and finding food difficult. So, some animals resort to stocking up on as much food as possible before the winter days come, to ensure they’re not left empty-handed.
What Animals Gather Food for Winter?
Many species of squirrels forage for extra acorns, nuts, seeds, and mushrooms during the fall, hide them in secret spots, and then go back to eat them throughout the cold months. These animals like to scatter their stashes, hiding their provisions in hundreds of hollowed-out nooks and crannies in trees or underground.
In some forests, squirrels hide so many nuts that they’re changing the ecosystem. This is no easy task, though. To remember where they’ve put their secret snack stash, chickadee birds actually grow bigger brains in the fall to store more location information ahead of winter.
Many animals also resort to ‘hiding’ provisions in their stomachs. They gorge on loads of food to gain weight (think Fat Bear Week) and have a lot of energy resources to keep them warm and healthy throughout the cold months of rest.