Ultimate Quick Guide to How Animals Survive Winter
For life that must survive low temperatures and harsh weather in the Northern Hemisphere, there are three major routes to success. Each comes with its own advantages and disadvantages; a balance must be found between calorie intake and calorie expenditure. All groups of living things seem to use a good mix of each survival tactic, bringing their own special adaptations to the table. Read on for a quick look at how the Northern Hemisphere survives winter!
1. Migration (Getting out of Dodge)
For migration, the advantage is that an animal is physically getting away from the inclement weather. The disadvantage, however, is that they have to burn a lot of fuel to make the trek. Some animals make a relatively short journey, like many elk and deer species, which may stay within the same geographic location but adjust altitude by migrating down a mountain. They still face cold weather, but use physiological and behavioral traits to survive the less-harsh weather of the lower altitude.
Other animals, like the Rufous Hummingbird (Selasphorus rufus), make an astounding migration of nearly 4000 miles twice a year – once to escape northern winters, and again to return in spring. It’s a death-defying journey, but the hummingbirds have no choice; deer can manage on bitter shrubs during the winter, but there are no blooming flowers and abundant nectar for the hummingbird. Its body is so tiny and its metabolism so fast that it needs to eat as often as physically possible to stay alive for the journey, which may include crossing large bodies of water with nowhere to land for rest. The hummingbird can conserve energy at night by going into torpor (a state of reduced activity, like a mini-hibernation), but it’s still a tenuous journey.
Migration is a key survival strategy for many animals, but size and mobility are factors. Birds are the most common migrators because they can fly. The Monarch Butterfly also migrates, but the difference is that the adult insect makes one leg of the journey and perishes, and its young make the return the following year. Large fauna like elk would burn too much energy if they tried to move south every year from the northern US; likewise, small terrestrial animals like mice or snakes cannot make long distance migration.
2. Adapting to the Cold (Toughing it Out)
If you’re going to just be out and about like a boss during winter, you’re going to need some nifty tools to survive: there are physiological adaptations and behavioral adaptations.
Physiological ones include a change in digestive enzymes and insulation, such as thick stores of fat and a thicker coat of hair that keeps an animal’s own heat trapped close to its body. White-tailed deer develop a delicate cocktail of digestive enzymes for their winter woody diet (which is why feeding them corn in the winter could kill them, as explained by Scott Shalaway at FarmandDairy.com). For some animals, a winter coat may come in a different color to help camouflage them from predators while they’re active during winter (think snowshoe hare). Brown fat is a type of fat different than white adipose (the kind of fat we’re mostly familiar with). It has a unique ability to generate heat in an animal without the animal having to shiver, and is a critical factor in hibernation as well.
Behavioral adaptations include storing food, denning together (even for some species that are typically loners), and shelter construction that partially aids in protection from extreme weather. Squirrels and jays cache food in the snow and underground, but big predators like cougars and fox may also store fresh meat in the snow for a later meal. Shrews, which must eat constantly and cannot hibernate, hoard invertebrates.
3. Hibernation (Peace-ing Out Til it’s Over)
Hibernation is a dormancy in which an organism’s metabolic rate and temperature decrease. In many critters, it looks like sleeping, but it’s much deeper than that: the body temperatures of some living things can fall to near-freezing. Hibernation is also typically characterized by intervals of waking when the hibernator rouses briefly to consume, excrete waste, and possibly mate, before returning to the resting state. Hibernation can last several months and these waking intervals may happen approximately every two or three weeks. Hibernation seems to be divided into two categories: “true” hibernators (whose body temperatures take a drastic dive), and everything else that goes into a period of dormancy, with particular characteristics for each taxa that uses it.
The larger the animal, the higher the cost of lower body temperatures. An animal the size of a bear cannot lower its body temperature to “true hibernation” rates because it would require more burnable fat than it can make to survive. Smaller animals, especially rodents, are small enough to survive these low internal temperatures, particularly when they can rise every now and then to consume more food. The largest animals that truly hibernate – featuring extremely low body temps and periodic arousal for consumption – are the marmots, such as Marmota monax, better known as the groundhog.
Winter Lethargy, or What Bears Do
We’ve all been raised to think of bears as true hibernators, but they’re not. While hibernation is indeed a state of seasonal dormancy and bears go seasonally dormant, the term “winter lethargy” has come to better define what bears do in the winter. Bears and true hibernators differ in a few key ways: 1) though bears do experience a drop in body temperature (10 degrees or less), it’s not as drastic as hibernating rodents like the groundhog; 2) bears can go more than three months without stirring for food or defecation, while hibernators get up regularly to do so; 3) while true hibernators may take quite a while to rouse from hibernation, bears can snap out of it relatively quickly, and they have a reputation for doing so with hikers that are too bold for their own good!
When water drops below the freezing point, ice crystals form around “nucleators,” particles like dust, pollen, or another ice crystal. (This is how snowflakes are formed!) What happens if there are no nucleators? There are no ice crystals! This is what happens in the body of the Arctic Ground Squirrel (Spermophilus parryii). Although the squirrel’s body temperature can drop as low as -2.9° C (26.7° F), its blood remains liquid because ice crystals do not form. Every two to three weeks, the squirrel will shiver its body back to its normal temperature of 36.4° C (97.5° F), but only for maybe half a day or so before re-cooling. It seems like these periods of warming are critical for the squirrel’s brain survival, since such low temperatures cause serious brain atrophy. When they rouse, their brains appear to compensate almost immediately for the number of synapses lost during hibernation. These critters are seriously amazing!
Brumation, or What Reptiles Do
Apparently, there’s quite a bit of controversy over this word. The word appeared in the ’60s to separate ectothermic (cold-blooded animals) and mammalian hibernation, and has been argued over since. When winter arrives, reptiles and amphibians mostly do much like hibernating mammals: they experience a drop in body temperature and metabolic activity, go into a state of torpor, and may regularly rouse to drink water. “Brumation” is essentially hibernation, and seems like a word used more in the herp trade than in scientific publications.
Nevertheless, we shan’t ignore our reptilian and amphibian brethren’s winter survival tactics. Some aquatic turtles (like the Painted Turtle, Chrysemys picta) dig themselves into the mud and glean only a little oxygen from the surrounding water. Terrestrial turtles (like the Eastern Box Turtle, Terrapene carolina carolina) hunker down in loose dirt, mud, and leaf litter. Some species of snake, like Garter Snakes and many species of rattlesnake, seek out an underground hibernaculum (a refuge to rest) and hibernate together as a group.
Aquatic frogs tend to hibernate beneath the water, wherever the water is rich in oxygen, as Painted Turtles do. Terrestrial frogs and toads that can burrow into the soil may stay safe below the frost line, but others, like the Spring Peeper (Pseudacris crucifer) and the Wood Frog (Rana sylvatica), actually freeze. Their heart rates drop – or even stop – and they produce no signs of life. Proteins known as nucleating proteins cause the frogs’ blood to freeze, but the cells are protected from ice thanks to a concentrated sugar “antifreeze” sent out by the liver. When the temperature rises, the frogs thaw and become animated again. This is a useful adaptation for frogs that live in areas where freeze/thaw cycles happen regularly, like the Northeastern USA.
Diapause is the state of dormancy that gets some insects through the winter. These insects come in two styles. They may be “freeze-susceptible,” in which the likelihood of freezing is reduced due to fancy natural antifreeze (like glycerol or sorbitol) which supercools their tissues and fluids, or they may be be freeze-tolerant, whereby the insects go dormant when the fluid around their living cells freezes. In this way, excess water is squeezed from the cells, and this dehydration prohibits ice crystals of a damaging size from forming in the tissues. Freeze-tolerant insects aren’t necessarily more tolerant of extremely low temperatures than freeze-susceptible insects; these are just two different methods by which diapause works. Prompted by hormone changes and environmental cues (like shorter day length), diapause causes a reduction in oxygen consumption, metabolism, and activity.
Diapause is the same process by which insects exposed to extremely hot, dry climates can survive as well. In fact, the insect that can tolerate the lowest temperatures tested is found in Africa, where it survives the drying out of seasonal rain basins through diapause. Polypedilum vanderplanki, the Sleeping Chiromonid, has survived being frozen in liquid helium at -270° Celsius, thanks to the sugar antifreeze trehalose.
So, out of the three major types of winter survival strategies, which one would you most like to employ? As a proud member of the USA states that got hit by the polar vortex this year, I’m really liking the idea of hibernation, particularly bear-style, especially now that Sherlock is over. Or I’d love to take a trip south, but that whole wing thing is an issue. What about you? Where would you go? Tell us in the comments!
For more information on hibernation and bears, visit the National Park Service’s page on denning and hibernation, this article on bears from eNature, this Synapse article, this Arctic Centre article, and this article from the University of Calgary on hibernation.
Read Scientific American here for more about the amazing Arctic Ground Squirrel.
For more on freezing frogs, here’s a National Geographic article
Posted on February 8, 2014, in Biology/Ecology, Fauna and tagged adaptations, amphibians, bears, bears hibernate, cold, diapause, extreme, hibernaculum, hibernation, how do animals survive, in winter, insects, migration, northern hemisphere, reptiles, rodents, survival, weather, what do animals do, winter, winter lethargy. Bookmark the permalink. Comments Off on Ultimate Quick Guide to How Animals Survive Winter.