Winter Kill

When the temperatures dip below -30° C, it doesn't take a rocket scientist to realize that the cold temperatures are more difficult for those local residents without the option of staying indoors until the mercury begins to rise. Winter mortality is one of the most important selective forces for many species of animals and is usually the result of a wide variety of factors.

The most common causes of winter kill are a combination of malnutrition, starvation, disease and predation. Years with very deep snows make it difficult for many animals to move about. Moose for instance, tend to sink deeply into the snow making travel almost impossible. If the snows persist late into the spring, when the moose are at their weakest, mortality rates will increase. Also, breaking through hard crusts can cause deep wounds on the legs leading to infection and potentially the introduction of parasites and disease.

Some animals, like deer and bighorn sheep, deal with the problem of heavy snows by "yarding up", or living in large groups. This communal living tends to pack all the snow down in a particular area making travel easier. The movement also tends to uncover more plants for feeding. Far from a perfect solution, the competition for food created by yarding can cause intense food shortages. It also provides the ideal pool for the transmission of disease and other parasites. In bighorn sheep it can lead to increased transmission of pinkeye and lungworm.

For many animals, the autumn rut leaves the males completely spent at a time when they need to be in prime condition. It's not uncommon for a large elk to be so weakened after the rut that he cannot survive the demands of a harsh winter. It's ironic that his strength, which put him in a dominant mating position, inadvertently led to his winter mortality. Moose have another unusual factor causing winterkill. Most people find themselves cringing at the thought of finding a single tick along their hairline. Moose on the other hand can have upwards of 50,000 ticks on them at a given time. These moose ticks (they are specific to moose and don't bother people) can cause the moose to aggressively try to rub them off, leaving them with a thinner coat of fur and leading to death by exposure.

Man is another important factor in the winter kill scenario. Domestic animals often harass and sometimes even kill winter weakened animals. Fence lines and other human barriers can prevent animals from following traditional migration routes and leaving them in areas not suitable to wintering. Animals can also become entangled in barbwire fences and die.

Highways are another very serious cause of animal winter kill. The nicely packed roads form a convenient route on which to travel, and the black pavement often leads to melting along the road margins earlier than other areas. More food is available and so the animals tend to congregate along the roads. As a result, it's even more important to drive carefully in the winter than in summer.