July 01, 2014Some deer are likely to die from malnutrition on northern range each winter. However, at times the annual deer die-off has been staggering. The winter of 2013-14 will be one of those real deer-killers throughout Northern Michigan.
Michigan isn't the only state that has experienced excessive overwinter deer losses. Every state in the Upper Great Lakes region and in the Northeast, as well as the southern provinces of Canada, has recorded massive deer die-offs during certain winters. Sometimes as much as 20 percent to 40 percent of the herd may perish. The magnitude of the loss will hinge entirely upon the severity of the winter and quality of the deer wintering range.
Each deer has a certain starvation threshold beyond which it can no longer survive even though feed is available. Jordan Browne photo
The reasons deer die in winter aren't always clear. Obviously, not all of them starve to death. Some are shot illegally; some die from hunting season wounds or naturally incurred injuries; others die in accidents or from disease, and still more (especially malnourished ones) fall victim to predators.
Nonetheless, the highest losses always occur during severe winters, and most deer die, either directly or indirectly, due to malnutrition. The evidence rests in their fat-depleted carcasses, where even the last energy reserves have been drawn from the cores of their bones, leaving a red gelatinous material where the marrow was once white with fat.
For does and fawns, the amount of body fat they accumulate prior to winter, and the amount of digestible browse available to them during winter, will determine whether or not they survive. Clearly, various scenarios may account for excessive winter-kill of whitetails. However, given the environmental constraints they are likely to face in northern forested areas, does and fawns entering winter in poor physical condition are doomed.
Healthy, fat does and fawns, on the other hand, are reasonably well equipped to withstand several months of severe winter weather, even when faced with moderate food deprivation. In fact, the adult doe is unbelievably tough.
Body fat is especially important because it serves as reserve energy. If deer are on a negative energy balance, which is normally the case for northern deer during winter, the body tissue is utilized to help meet their basic energy needs. Breaking down of body protein occurs simultaneously with the burning of fat; the greater the dietary energy shortage, the more rapid the degradation of body protein. However, deer cannot survive winter on body reserves alone. In fact, it's doubtful if a healthy adult doe could live for more than 45 to 60 days without food in winter, regardless of how fat she was at the start.
Some weight loss by northern deer during winter is normal. Even well-nourished adult does and fawns are likely to lose 10 to 15 percent of their body weight. Those that enter winter with maximum fat reserves can usually withstand upwards of 30 percent weight loss without dying.
This is not to imply that complete depletion of fat stores during winter is normal, necessary, or a wise strategy. Nutritional stress may very well carry into the spring period, even after snowmelt, requiring reserve energy to compensate for periodic unexpected nutritional shortage. Besides, surplus fat can always be "dumped," or flushed from the system, when lush food is plentiful.
Adult bucks deviate from the highly adaptive fat cycle exhibited by does and fawns. Bucks exhaust most of their energy reserves during the autumn rut, when they might lose 20 to 25 percent of their body weight. This leaves them very lean for a four- to five-month stretch encompassing all of the winter season.
Certainly, it's not unexpected that some bucks, battle scarred and injured while fighting during autumn, occasionally fall easy prey to alert predators or succumb later to their injuries and malnutrition. In fact, it's more of a mystery how so many bucks in seemingly wretched physical condition can survive the stressful winter season.
Although seemingly handicapped in winter because of their scant fat stores, the buck's large body size contributes to greater metabolic efficiency and ability to withstand greater cold-stress. The size of the rumen in relation to body size will determine the quality of forage that can be digested; the larger the body size, the lower the basal metabolic rate per unit of body weight.
Adult bucks normally weigh about 30 to 40 percent more than females of similar age (but the spread narrows when summer nutrition is poor) and have a lower whole-body metabolic requirement per unit of gut capacity. This difference permits bucks to subsist on lower-quality foods when nutritious ones are scarce, a real benefit during winter. Also, since largebodied bucks are better equipped to withstand cold-stress, they can occupy winter habitat separate from does and fawns where the shelter may be comparatively poor but the supply of browse is better.
Healthy whitetails are quite capable of withstanding severe weather and food deprivation during January and February, when they're geared-down physiologically and adjusted behaviorally to endure great adversity. It is prolonged winters -- ones that start in November or early December, extend into April, and overlap periods of high energy demand -- that can be so devastating to deer. The whitetail's impressive adaptations for winter survival diminish in value around mid-March.
Thereafter, steadily increasing food demands render deer once again exceedingly sensitive to environmental stress factors.
Death from malnutrition is an insidious, pathetically slow process. Fat depletion and physical weakening progress with nearly undetectable outward signs, until it's too late for recovery. In the final stages, however, a deer's coat roughens, its hip bones show, hollows appear in its flanks, and fawns, especially, become fuzzy faced. Even then, it may be weeks before the animal dies.
The starving animal spends most of its time bedded down, in a curled head-to-tail position to minimize body heat loss. It adopts a lethargic uncaring attitude, no longer bounding away, flag waving, as danger nears. Small deer stand hump-backed, their front legs spread slightly, back legs close together, and hold their head up at a 45 degree angle. Deer so weakened become easy prey for wolves, coyotes, and bobcats -- a sudden and merciful fate compared to a lingering death from starvation.
Each deer has a certain starvation threshold beyond which it can no longer survive even though feed is available. Physically stressed animals, in particular, incur irreversible damage to their rumen lining, and their rumen microflora lose their ability to digest cellulose. At some unknown time during starvation, rumen attrition and adrenal exhaustion become irreversible. Thereafter, a deer so stressed can no longer digest its food.
Harassment and physical exertion tend to speed-up the adverse effects of malnutrition and lower a deer's starvation threshold. A sudden heavy energy demand, for whatever reason, may kill a deer even though its energy reserves are not completely exhausted. Harassment by dogs (or snowmobilers), unusually cold weather, or exceptionally strenuous travel conditions during late winter, for example, could have especially devastating effects on malnourished deer and lead to the death of some that might otherwise have survived.
Although some small deer may succumb to malnutrition early in winter, most deer starve during March and April. Fawns lose body weight faster than adults and usually are the first to die.
At this writing (mid-March), heavy deer losses can be expected this year during the late winter/early spring period across Northern Michigan. Fawns and aged does will probably comprise 80 to 90 percent of overwinter deaths, whereas relatively few adult bucks or prime-age does will be among those that succumb to winter effects.
Ironically, since fawns represent such a large proportion of winter deer losses, there may be relatively few deer carcasses visible in the deeryards this spring -- last year, 70 to 80 percent of the fawns died at birth or soon after because their mothers' were extremely malnourished during late stages of pregnancy.
Remember, in 2013 snow cover lingered into late April and even early May in some areas. That's the nutritionally critical time for deer fetal development. As a result, many of those stunted and weak newborn fawns died soon after birth.
Given all this, coupled with expected high loss of newborn fawns again in 2014, deer hunting in Upper Michigan looks rather bleak, at least for the next couple of years.
Meanwhile, hunters are squabbling about trivial matters such as buck hunting regulations. This is like fretting about a sliver in your finger when you're near death from cancer. Wake up guys! The deer wintering habitat in Upper Michigan is getting worse each year and the deer population is shrinking accordingly.