DNR Division of Fish and Wildlife - Spring 1997
Whitetails weather winter's worst
New DNR research illuminates adaptations by deer to cold, snow, and lack of food
The wind howled, the mercury plummeted, and snow flooded the woods and fields across Minnesota. Yet most white-tailed deer survived this recent winter, having adapted over thousands of years to withstand even these harsh conditions. An ongoing DNR research project looking into the adaptations of deer to winter verifies that these animals are indeed well suited to surviving Minnesota's cold months.
"Northern white-tailed deer have been adapting to this climate for a million years," says Glenn Del Giudice, a Grand Rapids? based scientist who leads the research project. "These evolutionary adaptations help them to survive the challenging conditions of winter."
The key to the deer's survival, according to Del Giudice, is their ability to maintain a sufficient balance of incoming energy (through food) and out-going energy (used to move and to maintain body temperatures). As long as they maintain ample energy levels, says Del Giudice, the whitetails can survive.
Thick coat, slow movement
The combination of scarce food, deep snow, and cold temperatures requires deer to change their physical makeup and behavior to slow the loss of fat and protein energy reserves. The primary physical change is the long, thick winter coat, which deer begin to grow in late fall. Just as humans dig out their down parkas when the cold winds begin to blow, deer grow 1-inch-long guard hairs and thick underfur, trapping warm air around the animal's body and allowing deer to retain body heat. Late fall is also when deer attain peak body weight. These reserves of fat and protein help deer through the lean months when food is scarce.
Because highly nutritious food is so scarce in winter, deer can't take in much energy. As a result, they maintain the balance of energy needed to survive by decreasing the amount they expend. One method is migrating to dense conifer stands, where thick groves of pine, cedar, fir, and spruce act as gigantic down comforters that block cold winds and retain infrared radiation between the ground and tree canopy.
"Greater infrared radiation within conifer stands contributes to lower losses of radiant energy from deer," says Del Giudice.
In addition, snow is shallower within the conifer stands, requiring less energy for deer to move about. "We know that wherever snow cover reaches depths of 20 inches, deer have to expend critical amounts of energy to travel," says Del Giudice. "Conifer groves typically contain snow depths of less than 16 inches, so deer can conserve energy there.
" Though deer do travel in winter to find food, they tend to move less during this season than in other months. "This slowdown, along with the growth of the winter coat, is triggered by the shortage of daylight, which is sensed by the deer's pineal gland, located in the brain," explains Del Giudice. In addition, the decrease in food intake slows the deer's metabolism and thyroid activity, helping them conserve fat reserves.
Del Giudice's insight into winter deer survival comes from an ongoing, long-term study of how a projected increase in conifer harvest could affect Minnesota's deer populations. A growing demand for wood products points to the strong possibility that supplies of aspen and other hardwood stocks could diminish and cause loggers to harvest more balsam fir and other conifers.
This possibility concerns deer managers, who realize that they don't know just how critical these conifers are to northern deer survival.
"We need better information to determine how much conifer cover can be cut without affecting deer survival," says Del Giudice, who began the 12-year study in 1990 to gather such information. Now in its seventh year, the study has led to important information about how deer use conifers in winter, how logging affects deer's use of conifers, and how deer are equipped to handle the brutal winter months.
"Some of the most important data we're getting has to do with how well deer survive from winter to winter," says Del Giudice.
Though some deer die every winter, most survive even the worst conditions. Fawns, which are smaller and have lower fat reserves, are most vulnerable. The next most susceptible are bucks, which often cannot build up large fat reserves in fall because they expend too much energy during the rut. Least vulnerable are does (the reproductive members of the species), which enter the winter in peak health.
Even after the most devastating winters, deer have one more adaptation to rebuild their numbers: reproduction. By protecting does with hunting restrictions, as it did in the Arrowhead Region during the 1996 hunting season, the DNR can help deer rebuild populations diminished by severe winters. Pregnancy rates of does at least one year old can reach over 90 percent. As a result, biologists know that even a weakened deer herd can rebound within a few years.
"People should try to understand that what may seem like a cruel season is just another natural event that deer will continue to survive as they have for eons," says Del Giudice.