While walking through a Toronto ravine in all its fall glory recently, I got to wondering
how animals survive the harsh Canadian winter. Now, I knew the answer would be, "it's complicated" - I've seen my share of nature documentaries - but I still wasn't prepared for just how complicated.
First off, it depends on the distinction between endothermic (warm-blooded) and ectothermic (cold-blooded) animals, although even then some ectotherms are able to produce some heat thermogenically (by basking in the sun, for example).
Endotherms like mammals and birds employ a variety of different mechanisms to survive the sub-zero winter temperatures. Many birds simply migrate to warmer climes as the cold weather approaches, and even some mammals migrate - barren-ground caribou trek from the open arctic tundra to the more protected boreal forest; whales and some bats also migrate south; deer and elk move down the south-facing slopes of the mountains to warmer elevations.
Some animals are just so well-insulated and -adapted that they can survive even extreme temperatures. For example, the arctic fox can remain thermoneutral in temperatures as low as -80°C. Predators like polar bears and lynx are supremely well-adapted to the cold, to the extent that winter is their main hunting season. Even animals like deer and moose are surprisingly adept at foraging in the winter snow, and so do not need to hibernate.
Many animals and birds grow more and thicker fur or feathers to help them survive the winter, and some are able to fluff them up to increase the insulation. Grey wolves have two separate layers of fur to help them stay warm. Many mammals (and even some birds) huddle together to keep warm, and they may also curl up, draw in their legs, or cover their faces with their tails to help conserve heat. Many small animals, like squirrels, chipmunks, pikas, weasels, etc, store and hide food in the fall for the cold days ahead, as also do beavers and some birds like chickadees, blue jays and owls. All animals that have to survive the winter spend the fall eating extra food and storing it as body fat. Voles, mice and shrews take advantage of the warmer and more protected subnivean (under the snow) spaces, where food is relatively easily to obtain and the temperatures are more consistent and manageable. Grouse also sometimes dig themselves snow refuges for much the same reason.
Some animals, like deer for example, can reduce their metabolisms during cold spells so as to require less food, while others can voluntarily reduce blood flow to their skin and extremities. Some birds like chickadees lower their internal temperature and shiver durimg the cold of night to generate heat from their flight muscles, in a kind of controlled hypothermia. Some wading birds and gulls have complex vascular systems that operate like heat exchange systems, moving heat from warmer arteries to cooler veins (caribou also employ a similar vascular mechanism, as do the tails of beavers).
The best-known over-wintering technique is hibernation, in which the animal's metabolism is reduced down to 5% or less of normal, and respiration and heartbeats are reduced to very low levels. Groundhogs, for example, may breathe as little as 10 times per hour during hibernation, their heartbeats fall to 4-5 per minute, and their internal temperatures fall to just a few degrees above freezing. During full or true hibernation, an animal will probably not wake even for a loud noise or if moved. They use energy-rich brown fat for internal energy production during their dormant periods.
More common than full hibernation, though, is torpor or light hibernation. In this state, animals like bears, racoons, mice, shrews, chipmunks, ground squirrels, skunks, etc, sleep deeply, with a reduced heartbeat, but, unlike with true hibernation, their core temperatures remain quite high. These animals tend to wake quite often on milder days to search for food or change dens, etc. Their body temperatures may fall several degrees each night in order to conserve energy.
Reptiles Amphibians and Fish
The hibernation equivalent for cold-blooded animals like reptiles and amphibians is known as brumation. The condition can last for months during he cold weather, but the animals often wake up briefly to drink water or to shift position before returning to sleep.
Amphibians and reptiles typically lie dormant under decaying logs or in rock crevices, caves or the burrows of other animals. Snakes, many of which are freeze-tolerant (see below), gather in protected rock dens, often in multi-species groups. Toads burrow down into the soil, but some amphibians remain above ground, protected only by moss, ground litter and snow. Many amphibians, turtles and salamanders over-winter in the mud at the bottom of swamps, ponds and lakes (they can absorb oxygen from the water through their skins).
Some reptiles and amphibians are able to reduce their metabolisms enough to remain active in water throughout the winter. Many frogs are still capable of swimming in water that is almost freezing, provided it is deep enough to have enough oxygen. Some species such as wood frogs and spring peepers are freeze-tolerant, and produce extra glucose that acts as a kind of antifreeze in their extracellular spaces and prevents their cells from freezing. Once the weather warms and the ice melts, the frogs thaw and their hearts and lungs resume normal activity.
Freshwater fish are able to survive under the ice because their body fluids freeze at a lower temperature than water. Many of them hide under rocks, logs or fallen leaves. Saltwater fish typically just migrate to deeper waters, but some remain in shallower saltwater because they are able to produce chemicals with antifreeze properties.
Insects and Invertebrates
Most insects spend the winter in diapause, a hibernation-like state of dormancy when normal growth and development stops. Insects may over-winter in their egg, larva or pupa states, or in some cases even as adults, and they use a huge diversity of strategies to achieve this. A few insects like winter stone flies, crane flies and snow fleas are even able to remain active throughout the winter.
Some insects and invertebrates are "freeze-tolerant" (i.e. they can survive ice formation within their tissues, by a variety of different biological and biochemical means), while the others are "freeze-intolerant" or "freeze susceptible". Freeze-susceptible insects avoid freezing by "supercooling": they seek out dry sites for the winter, empty their guts, reduce their body water content, and pump a kind of antifreeze (like gycerol) through the anaerobic pathways of their bodies.
Freeze-tolerant insects, on the other hand, actively encourage ice crystals in their bodies, generating ice-nucleating proteins in their extra-cellular spaces (ice crystals within the cells would damage the cell walls and its organelles), and they limit total ice formation with antifreeze compounds. Many beetles, flies, wasps, moths and butterflies are freeze-tolerant, and they spend the winter in deep protected crevices in tree bark or plants. Others, along with many spiders, mites and ticks, survive in the relatively warm, protected and moist subnivean spaces. Some butterfies, like monarchs and painted ladies, are capable of long migrations to warmer regions.
Aquatic insects move from shallow pools prone to hard freezing to deeper pools or larger streams. Some over-winter out of the water completely, either as eggs or protected by cocoons like butterflies.
So, as I say, it's complicated - to put it mildly!
