The whitetail’s fat cycle
During winter, most Northern whitetail deer must switch to eating poor quality woody browse. This results in a negative energy balance, because the amount of energy consumed in browse does not compensate for the amount used in body maintenance and loss due to cold exposure. This, in turn, leads to body weight loss, gradual physical deterioration and sometimes death.
But the Northern whitetail is a survivalist. It s well adapted — behaviorally and physiologically — for winter survival; it’s a master at conserving energy, while simultaneously avoiding hungry predators.
Even so, harsh winters can devastate Northern deer populations.
The Critical Level
Of Weight Loss
Some weight loss by 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 entering winter with maximum fat reserves can usually withstand a 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 can carry into the spring period, even after snowmelt, requiring reserve energy to compensate for periodic unexpected nutritional shortages. Surplus fat can always be “dumped,” or flushed from the system, when lush food becomes plentiful.
A Brushy Hillside
Bill Mautz, who has conducted extensive research on deer physiology at the University of New Hampshire, likens the annual fat cycle of deer to a sled ride down a brushy hillside. At the bottom of the hill is a sharp drop and potential death from starvation.
Deer must first climb the hill during summer and autumn, building fat along the way. The amount of fat stored prior to winter will determine how high the animal is able to climb on the hill, and hence, the length of its ride down; the longer the ride, the better the chance of surviving until spring.
Woody browse availability is depicted in the analogy as brush on the hillside. Food in the form of woody browse will not completely stop the deer’s downhill ride but will serve to slow the rate of descent slowing the rate of fat depletion and, thereby increasing the prospects of survival.
Keep in mind, deer cannot survive winter on body reserves alone. In fact, it is 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.
Each deer gets just one sled ride each winter. If it is lucky, and the hillside has sufficient browse, it will stop before reaching the bottom of the hill and falling off the end.
The Prolonged Winter
Healthy whitetails are quite capable of withstanding severe weather and food deprivation during January and February. This is when they exhibit sharply decreased movement activity, resulting in a lower metabolic rate, and voluntarily reduce their food consumption. 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.
Since young-of-the-year whitetails (fawns) do not achieve their maximum body size and fatness until mid-December, early onset of snow cover and cold temperatures curtails their development, rendering them especially vulnerable to winter stress. Cold temperatures also tend to drive deer into browsed-out conifer yarding cover earlier than normal, accelerating depletion of their vital fat stores.
The whitetail’s impressive physiological adaptations for winter survival diminish in value around mid-March. Thereafter, they become more active, their metabolism increases, young deer resume growing, and does carry rapidly enlarging fetuses that sap the mother’s remaining energy reserves. These steadily increasing food demands leave deer once again exceedingly sensitive to environmental stress. If restricted to yarding cover into April, many deer will perish.
During a prolonged winter in Northern Michigan, a hundred thousand, or more, whitetails may die — as occurred most recently in 1996, 1997, 2001, and 2008. But this represents only the tip of the iceberg; the number of newborn fawns that die following a tough winter will be staggering.
Fortunately, for Northern Michigan whitetails, we seem to be experiencing our fourth consecutive mild winter. As a result, however, this growing herd of naive whitetails living in poor quality winter habitat will undoubtedly suffer dire consequences during the next tough winter.