Deer fat. That’s the wax-like substance that lines the roof of your mouth after eating poorly prepared, half-cold venison steak or roast. But deer fat posseses great attributes when on a live deer. It is especially valuable in the northern portions of the whitetail’s range.
Here, where winters are long, often brutally cold and the snow deep, deer need lots of calories to survive. Some of these calories they obtain from winter browse,
but most come from the deer’s fat reserves. The layer of fat on a deer’s body also has fine insulating qualities. Fat reserves start building up on deer
in the late summer months. The feeding done in spring and early summer is utilized for building up body losses resulting from the hardships of the previous winter.
The bearing does also need this feed for the birthing and feeding of their fawns. Nutritional demands of new-born fawns are great, yet the doe meeta them. Fawns of adult does who wintered well weigh about seven pounds at birth. If the doe is in good condition and on good feed, the fawn will double its weight in two weeks, and triple it within a month.
This rapid growth places much stress on the nursing doe and even if she is well-fed, she will lose weight through the first two months of summer. After that the
fawns, though still nursing to some degree, receive most of their nutrition from forage. Now the doe, if the feed is of good quality, will put on flesh and fat. Studies conducted in northern Michigan show that if fawns receive inadequate or marginal nutrition in autumn, growth stops or slows down.
Surprisingly, even these runty deer built up fat reserves. Growth gave way to the necessity of fat storage eseential for winter survival. Fat is concentrated energy. A small deer, with reserves of body fat, stands a good chance of living through a winter.
Adult bucks start losing weight during the rut. At this time, they drastically reduce their feeding. While their energy intake decreases, their activity and
energy expenditure greatly increase. Fat reserves are being utilized. This phenomena has been long recognized.
Diets of the English at the time of Henry IV (early 1400’s) were short of fats and, therefore, even venison fat was appreciated. They held their deer seasons
when deer were “in the grease” and closed it before the rutting season.
Energy demands of deer vary with age and sex. In the fall, fawns put a lot of energy into growth, and bucks into the rut. Does, with the fawns essentially
weaned, go into the winter with the best accumulation of fat and, therefore, the best chance to survive a severe northern winter.
In the winter months, snow and bitter cold force deer to seek the shelter of dense evergreen swamps or wind protecting hills. At this time, their basic metabolic rate decreases and their food intake drops sharply. In fawns, it may be a 50 percent reduction even though good quality browse is available. Browse eaten by deer is digested poorly. Studies show that utilization is only at the twenty-five to fifty percent level, while fifty to eighty percent of summer foods are digested.
As said previously, deer would not survive the winter months without the availability of browse, but the fat reserves provide most of their required calories. In severe weather, deer normally spend more time in their beds, relying on their lower metabolic rates, hollow, highly insulative hair, and their fat for their needs. Browsing at such times would expose more of the body to the cold and the energy gained would not equal that lost. Most of the browsing in harsh weather occurs during the warmer daylight hours.
Deer can, and sometimes do, lose twenty-five to thirty percent of their body weight in a severe winter and still survive. Thus, an adult doe with a live weight of 120 pounds in November could weigh less than ninety pounds in the spring. If she was pregnant, chances are that the fawn or fawns would be born alive but would die soon after birth.
Such losses are usually caused by the weakness and smallness of the fawns, the mother’s lack of milk, or by abandonment. Rarely are fawns aborted. Some of the does subjected to the rigors of a harsh winter will not become pregnant the following year. Fawns that enter a severe winter with little fat, yet live through it, may fail to ovulate or conceive the following fall or if they do conceive will have higher natal mortalities. Even older, hard-pressed does will have lower fawning rates.
It is the spring, summer and fall foods that allow deer to grow and accumulate fat reserves. Foods at these seasons of the year are largely succulents rather than woody browse. Grass, sedge, leaves of woody plants, agricultural crops, and ground cover forbs make up the diet. Diets change as the seasons progress and in the late summer and fall months grain, corn, fruits, acorns, beechnuts, and legumes become the choice deer foods. These, available in quantity, rich and easily digestable, build the fat which is so essential for the survival of wintering deer.
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