Keywords Zoology

Cold-blooded Animals
Behavioural and complex biochemical responses to deepening cold and other environmental factors allow poikilothermic animals to survive winter in the Canadian temperate zone. Most poikilotherms become dormant but others remain active and a few combine both strategies.

Insects and Other Invertebrates
A period of arrested growth and development (diapause) characterizes the life cycle of most INSECTS that winter in egg, larval, pupal or, uncommonly, adult stages. Generally, overwintering insects are either freeze-susceptible (freeze-intolerant) and cannot survive ice formation in their tissues, or they are freeze-tolerant.

Freeze-susceptible insects avoid freezing by supercooling. They empty the gut, reduce body water content, produce antifreeze (cryoprotectant) substances, for example glycerol, through anaerobic pathways and seek dry sites for the winter.

Freeze-tolerant insects encourage ice formation with ice-nucleating proteins a few degrees below 0° C but restrict ice to extracellular spaces and limit total ice formation with antifreeze compounds. They may survive temperatures to -50° C or lower. No animal is known to survive complete freezing of tissue water because intracellular ice (ice crystal formation within cells) damages the organelles.

Aquatic species overwintering as adults often move to terrestrial sites or from shallow ponds to larger streams and deep pools while others overwinter as eggs above or near water. Chironomid (nonbiting MIDGES) larvae construct firm winter cocoons and CADDISFLIES seal the summer case for winter. STONEFLIES, caddisflies and MAYFLIES, although believed to be freeze-tolerant, avoid freezing temperatures. Some marine intertidal animals such as MOLLUSCS are also freeze-tolerant.

Terrestrial species wintering in plants and tree bark enter deep crevices or fissures. Others winter in ground litter, soil, rock piles, caves or animal burrows. MOTHS, BUTTERFLIES and other insects overwinter in special cocoons or hibernacula which may resist ice inoculation and desiccation. Many BEETLES, FLIES, WASPS, moths and butterflies are freeze-tolerant. Species in vegetation enter microsites protected by plant material and snow. A snow pack deeper than 20-50 cm stabilizes soil surface temperatures between 0° C and -10° C. Snow also maintains humidity, protecting insects from drying out.

Insects active above the snow, including snow fleas, moths and scorpionflies, raise body temperatures by basking in the sun. Most winter-active terrestrial insects, mainly adults but also larvae, live at the soil surface below the snow (subnivean) together with SPIDERS, MITES and TICKS.

Lower Vertebrates
Freshwater fish, either semidormant (EELS) or active through an elevated metabolism, are protected under the ice by their body fluids, which freeze at a lower temperature than fresh water. Marine fish migrate to deeper water or, like COD, SCULPIN and flounder, produce antifreeze compounds (peptides or glycopeptides) for resistance to freezing in ice-laden shallow salt water.

AMPHIBIANS and REPTILES survive winter lying dormant under decaying logs, in rock crevices, caves or burrows of other animals. Many amphibians shelter in streambanks or near springs, or burrow in mud of swamps, ponds and lake bottoms where TURTLES and SALAMANDERS also overwinter. Like many poikilotherms, red-spotted NEWTS can adjust metabolism over time to become acclimatized to cold and remain active in water. Blue-spotted salamanders, likely freeze-tolerant animals, may migrate over snow and ice in temperatures of 1° to 3° C.

Stream and lake-bottom hibernators such as green and leopard FROGS, although torpid in water temperatures of 0.5° to 2° C, are capable of swimming. Bullfrogs survive in deep ponds where adequate oxygen is available. TOADS burrow down into the soil to overwinter. A few amphibians and reptiles remain above ground in winter, protected only by moss, ground litter and snow.

Freeze-tolerant species such as wood frogs and spring peepers convert liver glycogen to glucose, which acts as an antifreeze to prevent cells from freezing but allows extracellular ice. With an ice content of 60-65%, the heart stops, breathing ceases and the frog survives through anaerobic metabolism. If temperatures drop below -7° C, the frog will die; thus snow cover is important for its survival.

SNAKES, sometimes in mixed species groups, gather in rock dens. Garter snakes, which are freeze-tolerant, may travel several kilometres to overwinter in the same site each year.

Warm-blooded Animals
BIRDS and MAMMALS regulate body temperatures within narrow ranges, but some can lower metabolic rates and either become torpid or enter hibernation in winter.

Torpor
Some winter-active mammals, and birds such as CHICKADEES, undergo nightly torpor by dropping their body temperatures a few degrees to reduce heat loss. BLACK BEARS, which accumulate fat in autumn, are deep sleepers because their temperatures remain nearly normal although respiration and heart rates decrease sharply. They may lose up to 40% of their pre-dormancy weight over winter. Bears, RACCOONS, SKUNKS and CHIPMUNKS awaken on mild, midwinter days to seek food or change dens.

Hibernation
Hibernation, during which metabolism may be only 1-5% of normal, is marked by body temperatures 20° C or more below normal (hypothermia). Respiration and heart rates drop to low levels and become irregular.

Woodchucks may breathe 10 times per hour, their hearts slow to 4-5 beats per minute and internal temperatures drop to a few degrees above freezing. Hibernators such as jumping MICE, nonmigratory BATS and ground SQUIRRELS (including woodchucks and MARMOTS) accumulate body fat for insulation and energy to survive long dormant periods. Energy-rich brown fat is used by hibernants for internal heat production (nonshivering thermogenesis) during frequent periods of arousal.

Other Strategies
Mammals rely on heavy, efficient insulating coats and thick fat deposits to meet the stresses of winter. They respond to cold by fluffing out fur (piloerection) to capture air between the outer coat surface and the skin. Air trapped in the fur of BEAVER, MUSKRAT, MINK and river OTTER increases tolerance to cold water. Red FOX and PORCUPINE have increased thicknesses of underfur in winter. Red-backed VOLES add brown fat in autumn for heat production in winter. Red and grey squirrels, although active throughout the winter, retire to well-insulated nests during stormy weather.

Arctic Fox
Arctic foxes are small, with rounded ears and heavy white winter fur and dark brownish summer fur (Corel Professional Photos).

Physiological reactions of homeotherms to chilling include raising or lowering heat production through metabolic processes, producing heat by muscle tremors (shivering thermogenesis) and reducing blood flow to the skin (vasoconstriction). DEER can slow their metabolism during cold periods and so require less food. Birds shiver to maintain body temperatures when not flying, an activity that generates heat from the flight muscles.

Some animals, including wading birds, GULLS and CARIBOU, have complex vascular networks in their limbs so that heat moves from warmer arteries to cooler veins for return to deep body tissues. These countercurrent heat exchange systems, also present in the tails of beaver, greatly reduce heat losses in winter.

Snow
Snow is used by animals as a travel surface and insulating blanket. Thickly feathered feet of ptarmigan and wide, densely furred feet of snowshoe hare, LYNX and caribou are adaptations for moving over the snow. Fringed toes of GROUSE enable them to grip ice and snow-covered branches firmly. Grouse, ptarmigan and some small birds burrow into snow for a few hours up to 2-3 days where they may reduce heat losses by up to 45%.

Lynx
The lynx is distinguished from the North American bobcat by its tufted ears, large feet, long legs and lack of a white patch below the tail tip (Corel Professional Photos).

Much winter activity of SHREWS and small rodents, including breeding, occurs under the snow, which offers a stable, windless microclimate with less extreme temperatures than those prevailing above. Runways and tunnels, trampled through ground litter and granular snow, connect food caches with nests of dead grasses where animals curl up, alone or together, to conserve heat. Few small mammals could survive winter above the snow since the energy costs of doing so are 15-50% greater than remaining below, where they are also less vulnerable to predators.

Author P.J. AUSTIN-SMITH

Suggested Reading
Peter J. Marchand, Life in the Cold (1991).

Links to Other Sites
Canadian Biodiversity Website
A great information source for all budding biologists. Learn about biodiversity theory, natural history, and conservation issues. From McGill’s Redpath Museum.

Wildlife
Browse this extensive "Images Canada" website for historical images depicting human interaction with various species of Canadian wildlife.

Fur Institute of Canada
This Fur Industry of Canada website focuses on such industry issues as animal welfare, humane animal capture devices and wildlife conservation. Also includes educational resources about fur bearing animals and sustainable use practices.

Canadian Amphibian and Reptile Conservation Network
The CARCNET website features an illustrated biological key to North American amphibian and reptile species, details about related conservation issues, and much more.