During the last week about two feet of snow fell around out house. Leonard and I always marvel, as we sit in our warm house with a full pantry and freezer, at how the birds and mammals can survive the harsh, bitter weather.
Mule deer (Odocoileus hemionus) have four winter survival strategies: 1) a warm winter coat, 2) stored body fat, 3) reduced metabolism and 4) bedding.
The summer coat of a mule deer is shed and replaced with a heavier winter coat. The winter coat has longer hair shafts that are hollow and a dense underfur. Mule deer also have special skin muscles that can adjust the angle of the hair shaft to maximize the insulation.
In the fall mule deer exhibit an “obligatory weight response”. The deer convert most of their food to fat in preparation for winter regardless of the quality and quantity of nutrition. Ideally about 25% to 30% of their body weight should be fat as winter approaches. In winter deer will also often turn to high calorie agricultural waste foods (gleaning cornfields or soybean fields) to supplement their diet. Even when high calorie foods are available, about 40% of a mule deer’s diet must still be woody browse (the tips or terminal buds of plants). This woody browse is necessary to help maintain the proper pH in their four-chambered stomach.
Mule deer metabolism and heart rate are reduced in the fall even if plenty of food is available. This reduction helps conserve their energy reserves. If there is a dearth of food, metabolism is reduced even more. In spring the metabolism and heart rate increase again.
“Bedding” or “yarding up” is also a survival strategy for mule deer. When the weather is harsh the deer congregate where they can avoid wind and deep snow (often in conifer cover). There they move very little and help keep each other warm.
Many deer still succumb to harsh winters, particularly fawns and bucks. Why bucks? The fall rut requires heavy caloric input and time. As a result bucks often cannot add enough fat to sustain themselves through bad winters.
These mule deer are eating the terminal buds and tips from some cottonwood branches that fell outside my study window during the most recent snowstorm. (near Lookout CA, Modoc County)