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How Animals Survive Over Winter

By Shana McAlexander
Product Developer

Because humans thrive in such a small range of temperatures, students are impressed by the ability of hibernating animals to sustain life when their internal temperatures reach single digits. Bring life to the study of homeostasis by linking it to ecology and the behavior of animals during winter.


The winter environment

During winter months, in regions either far from the equator or at high altitudes, cold temperatures and limited light reduce vegetation and, therefore, the food supply at many levels of the food chain. To adapt to the environmental change, organisms change their behavior and physiology through migration, acclimation, or hibernation.


Homeostasis and thermoregulation

In order to survive, organisms must maintain homeostasis, a stable internal environment. Internally, cells perform an array of chemical reactions that must occur within specific temperature ranges. When temperatures are too low, these processes may be disrupted, which can lead to cellular damage and even death.

Homoeothermic endotherms, such as bears, bats, and birds, regulate their body temperature by generating heat internally. The balance between heat production and heat loss determines an organism's body temperature. At the cellular level, many chemical reactions within the body release heat as products.


Migration

When cold temperatures approach, some animals leave and move to warmer climates. The majority of North American bird species migrate between northern summer-breeding grounds and southern winter-nonbreeding grounds. These birds move because the migration is genetically ingrained in their behavior, but they also move from areas of decreasing resources to areas of increasing resources (food and nesting locations). Other birds, such as the hummingbird, could not withstand cold temperatures even if food remained plentiful. The study of wildlife migration has been enhanced by tracking technology. Recent studies have suggested that birds may navigate by the stars, polarized light, and magnetic fields.


Acclimatization

If endothermic animals elect to stay put for the winter, they may change their body composition and coverings. These changes are primarily caused by the changing photoperiod (ratio of light to dark). In mammals, acclimatization often means increasing the amount of insulation on their bodies. This is accomplished by consuming large amounts of food and storing the extra energy as fat.

Animals store fat as 2 different types: white adipose tissue (WAT) and brown adipose tissue (BAT). In mammals, WAT serves as a primary source of energy storage and helps animals retain heat, while BAT is abundant in newborn and hibernating mammals and contains lots of mitochondria. Animals use the fuel derived from BAT primarily to generate heat instead of ATP.


Hibernation

Dormancy is characterized by periods of reduced metabolic rate and lower body temperatures called torpor. True hibernators, such as bats and ground squirrels, are difficult to wake from torpor. Their body temperature can fall as low as –2.9° C and their metabolism can be reduced to 1% of normal rates.

Take a moment to compare this adaptation to human temperature requirements. The average human body temperature is 37° C (98.6° F), but this temperature may vary by about 1°. If an individual is unable to regulate his or her body temperature in a very cold environment, hypothermia will set in when body temperature falls to 35° C (95° F). Confusion, shallow breathing, a weak pulse, and eventually death will occur if body temperature does not return to the ideal range.

Torpor may be interspersed with arousal periods or may be continuous. During arousal periods, metabolic rate and temperature return briefly to normal states. During hibernation, dominant metabolic processes change to rely on stored energy sources as the animal eats little to no food for the winter.

In contrast, other mammals such as bears and raccoons are not true hibernators because they are easily awakened and occasionally wake and forage during the winter months. This type of torpor is characterized by slightly decreased metabolic rate and body temperature.


Cryopreservation and “antifreeze” proteins

Ectotherms, which are organisms that derive their heat from their external environment, can survive wintery conditions by a number of other mechanisms. Ectotherms include frogs, fishes, and many arthropods. Cellular changes in osmolarity allow cryoprotection for some frogs and toads. By increasing the solute concentration within the cell, the freezing point increases and, therefore, cells and tissues are protected at subzero temperatures. The animal is frozen and may seem dead, but as temperatures increase and metabolism resumes, the animal will recover from its frozen state, seemingly unharmed.

Certain fishes produce antifreeze proteins that prevent crystallization of solutions inside the organism, thus minimizing tissue damage and allowing the fish to function below the freezing point of water. It is also important to note the density property of water, which allows ice to float above liquid water while fish and other wildlife live below the ice.

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