The Whitetail Fat Cycle… A Matter of Life or Death

By John J. Ozoga

Most northern whitetails must switch to eating poor quality woody browse during winter. 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 master survivalist. It is well adapted – behaviorally and physiologically – for winter survival.

During winter, whitetails shift into low gear, metabolically speaking. They seek shelter from the cold and snow, become less active and eat less, all due to the lowering of their metabolic rate.

But certain critically important adaptations, such as coat molt and fattening, are initiated weeks, if not months, before the onset of winter weather. The proper timing of these changes is highly dependent upon favorable nutrition during the late-summer and autumn period.

Because fat reserves can be metabolized more readily than protein for energy needs when nutritious forage is scarce, storing fat in autumn is a mechanism that enhances deer survival during the bleak winter months. Energy-rich foods high in carbohydrates, such as acorns, beechnuts and other starchy mast crops as well as apples, cherries, grapes and a host of wild growing and cultivated crops are choice foods because they promote fattening. When available, a deer will eat about 1.5 pounds of acorns per 100 pounds of body weight daily.

Imperial Whitetail food plots planted specifically to meet the whitetail’s high energy needs during late summer and autumn also can provide huge amounts of vital forage that promote fattening.

The Physiology of it All

In the Northern Hemisphere, the whitetail’s seasonally cyclic rhythms in physiology, metabolism, coat molt, reproduction and general behavior are closely regulated by, or cued to, the changing amount of daylight or “photoperiod.” The shortening day length in autumn triggers a complex chain of physiological events that also lead to the whitetail’s growth of a thick winter coat and the accumulation of heavy fat deposits.

In other words, fattening up is an obligatory process. It is hormonally controlled, meaning that all deer are inclined to become fat in autumn.

The pineal gland, tucked deep within the mid portion of the deer’s brain, serves as the whitetail’s nerve and hormone transducer. That is, in response to changes in day length, the tiny gland translates light signals into chemical signals that cause hormone changes responsible for setting the whitetail’s seasonal rhythms.

According to George Bubenik, who has done extensive deer research at the University of Guelph in Ontario, the whitetail’s pineal gland produces a hormone called melatonin, which at dusk is increasingly released into the bloodstream.

“The fluctuations of melatonin levels modify the secretion of the pituitary hormone prolactin. It is the level of prolactin – rising in spring, peaking at the summer solstice in June, and falling under the dark night of the winter solstice – that ultimately triggers the coat molt,” Bubenik said.

The Coat Molt Fattening Connection

Bubenik is quick to caution, however, that other hormones may also affect the timing of the fall molt. An animal’s sex, age, reproductive status, level of nutrition and probably numerous other factors will also interact to set the precise timing of an individual deer’s coat change.

We often observe, for example, that nursing does keep their red coats longer than does that fail to raise fawns. One reason for this difference is chemistry. Prolactin, the hormone that signals the body to produce a winter coat, is also the hormone that regulates milk production.

“The high level of prolactin associated with milk production is at odds with the low level associated with hair growth,” claims Bubenik.

“The other reason is energy,” he notes. “Both processes drain the doe’s energy reserves, and she cannot accomplish both at the same time.”

Molting, a process that averages about three weeks in duration, is metabolically expensive. The four to five pounds of hair produced by the average adult deer each season require a diet especially high in protein, and, according to Bubenik, “The drain on energy and protein reserves deer experience during the molt explains why animals in good physical condition molt first – before weak bucks and late-born fawns as well as before lactating does. In fact, a late onset of the development of wooly fur is a better (and easier-to-read) indicator of undernourishment than the estimation of fat.”

Obviously, molting and accumulating heavy fat reserves go hand-in-hand. Both occur at about the same time of the year and are hormonally driven. Both events also require an excellent diet high in protein and digestible energy.

Come autumn, deer still wearing red – mostly nursing does and fawns – probably don’t have much body fat. By comparison, the ones already molted into their brown/gray winter coats – more likely adult bucks and does that didn’t raise fawns – probably already carry heavy fat deposits around their intestines and kidneys.

The Rutting Buck

Adult bucks usually commence fattening earlier than other deer — generally during late July. They are also the first to molt into their winter coat, usually in early September, about the time they shed antler velvet. Given good nutrition, prime-age bucks will be “hog fat” by early October.

In many ways, the buck’s annual fat cycle seems maladapted. A buck prepares himself almost totally for mating and siring future offspring, but holds very little in reserve for winter survival.

The strenuous autumn rut drains the buck of his fat stores. He’ll probably lose 20 to 25 percent of his peak autumn body weight during the course of the breeding season, leaving him quite lean and seemingly vulnerable to the climatic stress and food shortage that winter is sure to bring. In that regard, one must wonder if the mature buck does not possess other special, poorly understood capabilities that more than offset such an otherwise seemingly suicidal tendency.

For that reason, energy-rich food, such as that provided by late autumn food plots of Imperial Whitetail Clover, Alfa-Rack Plus, Extreme and No-Plow, along with whatever mast the forests may provide, can be critical in preparing the rut-stressed buck for the forthcoming winter.

Raising Fawns Is Expensive

As implied previously, adult does within the same herd may vary considerably in their degree of fatness when winter starts, depending largely upon their reproductive history. Given good range conditions, favorable weather, and ample forage, mature does that have not been burdened with nursing fawns the previous season tend to replenish their body reserves well in advance of the rut. More than likely, they will conceive twins (or triplets) and enter winter hog-fat and well prepared.

On the other hand, those does that have raised fawns may not be quite so well-off at the start of winter, especially if they gave birth to fawns late in the season, were burdened with nursing twins or triplets, or for some reason experienced poor nutrition during the autumn period. For them, an abundance of highly digestible forage during autumn can mean the difference between life and death come winter.

Vulnerable Fawns

Of all age-classes of deer, fawns find themselves in greatest jeopardy because they must consume enough nutrients to maximize their growth prior to winter and simultaneously lay away adequate fat stores for reserve energy. If they fail, their prospects for survival are poor.

As a result, fawns are particularly sensitive to the adverse effects of deer overpopulation, poor soil fertility, drought or early snow cover that may bring about a food shortage and prevent them from achieving their potential skeletal size and degree of fatness prior to winter.

The importance of digestible energy versus protein content in the autumn diet of fawns was demonstrated in our studies at the Cusino Wildlife Research Station, located in Upper Michigan. During a 10-week period, fawns provided diets high in energy (3,000 kcal per kilogram of pelletized feed) exhibited better body growth and fatness, as compared to those fed low (2,700 kcal) energy diets, regardless of feed protein content (16.2 percent or 6.6 percent).

As a result, we concluded that level of protein in the autumn diet of fawns had minimal impact upon their well being, whereas even minor reductions in the amount of digestible energy slowed their growth rate and decreased their level of fatness.

Surprisingly, however, even fawns on restricted autumn rations accumulated some fat, at the expense of additional skeletal growth. In other words, healthy fawns tend to be skeletally large and fat, whereas malnourished fawns may be fairly fat, but stunted.

Sledding Down 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 eating, 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.

Prolonged Winters Are Deer Killers

Healthy whitetails are quite capable of withstanding severe weather and food deprivation during January and February. This is when they exhibit decreased metabolism and movement activity and voluntarily reduce their food consumption. It is the 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 fawns do not achieve their maximum body size and fatness until mid-December, they are especially sensitive to nutritional stress prior to winter. For them, drought, early snow cover and overpopulation, as well as many other factors, can curtail their development, rendering them especially vulnerable to winter stress. Cold temperatures also cause deer to congregate in sheltered, browsed-out cover earlier than normal, accelerating the depletion of their vital fat stores.

The whitetail’s impressive physiological adaptations for winter survival diminish in value around mid-March. Thereafter, their metabolism rises, they become more active, 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 deer are restricted to wintering cover into April, many deer will probably die. However, that loss will only represent the tip of the iceberg, as heavy newborn fawn mortality will undoubtedly follow.

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.

Death from malnutrition is a pathetically slow process. Fat depletion and physical weakening progress with nearly undetectable signs until it’s too late for recovery. Deer so weakened become easy prey for predators – 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 food is available. A physically stressed animal in particular incurs irreversible damage to its rumen lining, and its rumen microflora lose the ability to digest cellulose. At some point in time, rumen attrition and adrenal gland exhaustion become irreversible. Thereafter, a deer so stressed could no longer handle the metabolic stress of feeding.

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, 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.

Conclusions

From what is known about deer metabolism and forage value, one can argue that lush forbs and succulent grasses, as well as hard mast (acorns, beechnuts, pecans, etc.), ought to be favored in autumn deer habitat management because they are foods that tend to be high in protein as well as digestible energy and promote fattening. Intensively managed food plots – those that are fertilized, limed and planted specifically for deer – can be especially beneficial for deer when mast production is low.

The importance of digestible energy in a whitetail’s late summer and autumn diet cannot be over emphasized. Apparently, a protein-rich diet is not really vital, whereas even a relatively small shortage of energy can slow the growth rate of young deer and leave deer lean and highly vulnerable to winter stress.

Generally, if deer are able to meet their dietary energy needs, they will probably satisfy their needs for other nutrients as well. Forages that are high in digestible energy are also usually high in protein, minerals and other essential nutrients but relatively low in fiber.

While food plots and supplements enhance the whitetail’s diet, they should not be expected to completely replace natural forage. Also, keep in mind that no deer can survive a prolonged winter on fat reserves alone, no matter how fat it was going into winter. Therefore, the goal of deer habitat management should be to not only plant food plots and use mineral/vitamin supplements, but also increase plant diversity to insure that deer have favorable food and cover during all seasons.