Why Do Deer Starve Themselves in Winter?

A s a child, I liked to ride to the end of the road and observe the deer that gathered in the winter, flitting among the dark, overhanging hemlocks – - of their traditional deer yard along the creek banks.

To my young eyes, it seemed a cold and cheerless place; as I grew older and realized that each winter many deer actually starved to death in the yards, I wondered all the more why, year after year, they came back to a place where all the browse within reach had long ago been eaten. It seemed to show a sad lack of intelligence for the survivors of each winter’s ordeal to return again and again to the scene of such devastation.

When, a few months ago, I began some serious reading on the subject of behavioral ecology, I encountered the concept of “adaptive value,” or the way in which an animal’s behavior increases its chances of survival as a species.

Again I thought of the curious behavior of whitetailed deer in the wintertime. I wondered what could be the adaptive value of going into a winter yard where many deer were doomed to starve, and even the survivors were weakened and suffered terrible privations. There must be some overwhelming benefit from yarding to counterbalance the heavy death toll, but I did not know what that benefit might be.

Nothing I learned in my college years as a wildlife major, or in my five years of working for a state fish and wildlife agency, addressed the problem of yarding by whitetailed deer from a behavioral point of view or explored the mechanisms by which such an apparently stupid habit was perpetuated.

Intrigued, I turned to the accumulated literature of the last half a century in search of an answer to the question: if winter yarding behavior leads to starvation, what powerful selective pressures are at work to override the obviously non-adaptive starvation? In other words, what benefits do the deer obtain from going into yards that are so vital they are worth the cost in death and debilitated deer?

First, let’s look at what constitutes a deer yard. The classic deer yard, as described by writers from the 1930s through the 1950s, is the protected lowland of big conifers, like the one I recall from my childhood. C.W. Severinghaus and E.L. Cheatum describe such areas in “Life And Times Of The Whitetailed Deer,” a chapter in Walter P. Taylor’s The Deer of North America (1956). They also picture deer, after heavy snowstorms, lying in “dens” underneath the low-hanging evergreen branches where they are insulated from the cold and protected from the wind.

Contrary to my childish impressions of cold and gloom, such places offer deer a fair degree of comfort. The number of scientific papers describing concentrations of deer in coniferous lowlands or evergreen swamps attests to the fact that deer yards are common throughout the whitetail’s range in the northern United States and the forested parts of Canada. Not all deer winter in such areas, however, even in the northern states. Wildlife biologist Nathaniel R. Dickinson reports that in the southern tier of New York, where conifers are scarce, deer often gather in winter on steep, south facing slopes. In northern New York’s Adirondack Mountains they commonly yard in coniferous valleys or swamps, but sometimes use exposed south slopes, as do the deer of Vermont.

On the other hand, in northeastern Minnesota whitetails live in extensive forest, much of it wilderness, and winter in traditional deer yarding areas of coniferous lowland. Both types of wintering areas provide conditions that reduce the amount of energy a deer must uses now depths are less than in the surrounding summer habitat and the wintering areas are warmer.

Since deer winter in a variety of habitats, many biologists prefer to use the term “winter concentration area” rather than “yard.” “Deer yard” carries with it certain unfortunate connotations, among them well-defined boundaries, very restricted movement, and an ingrained response of deer to the changing seasons which brings them, waynilly, to the same spot each fall.

But many winter concentration areas do not fit this stereotype and deer are not automatons. Deer yards vary, and so does the animal’s behavior. Not all deerĀ  populations concentrate in winter. In fact, according to David Hirth’s Wildlife Monograph No. 53 (1977) on the relationship be tween deer habitat and social be havior, the deer in Texas may actually range over a larger area in winter than they do in the summer. Most do go into some sort of wintering area where their movements become more restricted than in the summer.

Yarding appears to be an adaptation that helps deer survive in harsh climates. As you go further north, to the limits of their distribution, the deer’s yarding behavior is more pronounced and the yards more closely resemble those described in the earlier literature than those in milder climates.

Also, severe winters tend to encourage yarding while open winters allow the deer to remain spread out over much of their summer range, or to concentrate in a number of small areas of winter habitat in or near their summer range, rather than to move to a large winter yard. Many biologists have studied the whitetail’s winter habitat and looked at the factors which cause deer to move into their winter yards. Severinghaus and Cheatum suggested that snow depth was the most important factor, and most research since that time confirms their findings. Wind chill may also be important.

However, deer often move into wintering areas before the arrival of deep snow, and may pass up patches of good winter cover close to their summer range to travel several miles and winter in a distant yard. In addition to the environmental factors, there is apparently an important be havioral component, whether inborn or learned, which makes them seek their traditional yards.

When discussing deer populations and management, biologists refer to the deer “herd,” but the whitetail is not a herd animal in the same sense as the red deer of Scotland, whose “life and times” are so ably chronicled by F. Fraser Darling in A Herd Of Red Deer (1937) and by T.H. Clutton-Brock and his associates in Red Deer: Behavior And Ecology Of Two Sexes (1982). Whitetails prefer woodland and brushy habitats, unlike the open moors favored by the gregarious red deer.

Skulking through the brush or scattered over a broad, mountainous forest, deer are hard to monitor; most observations have been done at feeding stations or along lakeshores and roads. Indirect observations, using track counts or browse sign, and more recent research with radio telemetry has revealed something of the whitetail’s behavior and social structure, but the big picture must be pieced together from research papers contributing facts like pieces of a jigsaw puzzle.

Whitetailed deer live solitary lives, or move about in small groups. In the summer an old doe, her yearling daughter, and her newborn fawns form a family group; some times two or three generations of does and their fawns remain together.

Bucks may be solitary or join small groups of other bucks in the non-breeding season. They are seldom part of a family group once they reach maturity. Bonds between the buck and doe are short-lived, lasting only five or six days at the time the doe is in heat, but bucks are found in mixed-sex groups at many times of the year outside the breeding season.

The only time of year when whitetails gather into what might be considered a herd is in the winter yard. Deer come into the yard singly or in small groups, often from widely scattered summer ranges, but within a few days a hierarchy is established among them so that they become not just a chance aggregation of idividuals
but a structured social group.

In some cases, several small “herds” exist within what appears to be a large, continuous deer yard. The details of this social hierarchy, while interesting, are outside the scope of this article. What is important is the fact that the structure exists, for social interaction of the individuals in a species is an important factor in the adaptation of the species to its environment.

We are concerned not with the survival of individuals, but with the survival of the species. Variation in behavior among individuals is necessary, for it is upon such variation that selection works in order to evolve the behavior pattern best adapted to the environment in which the species lives. If members of a species have nothing to do with one another, and act only as individuals, their experiences will not contribute to the collective knowledge or “culture” of the species. There will be no tradition.

Deer do show a capacity to learn, and fawns learn a great deal from their mothers, so learned behavior certainly influences their adaptation to a changing environment. Biologists have done innumerable studies of the whitetailed deer, its habitat, social structure and behavior.

But very few of these studies examine the adaptive significance of winter yarding behavior from an evolutionary standpoint, or attempt to explain how such behavior is perpetuated over the years.

Two of the most interesting studies on the subject are the doctoral research done by George Mattfeld at Huntington Forest in New York’s Adirondacks, and the work of Michael Nelson and L. David Mech in the wilderness of northeastern Minnesota. Mattfeld, in his 1974 doctoral dissertation, approaches the question of why deer move to winter yards from the perspective of energetics- yarding saves energy, thus increases the chance that deer who yard will survive to produce young the following spring.

Nelson and Mech, in their Wildlife Monograph No. 77 (1981), favor the hypothesis that predation by wolves is the selection factor which led to the development of traditional yards, and encourages deer to return each year. The idea that deer save energy by yarding is not original with George Mattfeld, but he builds upon the observations of earlier researchers such as Severinghaus and Cheatum, and D.F. Behrend who did research at Huntington Forest in the 1960s, to approach the subject quantitatively. His data support the idea that deer do not move into the wintering areas out of stupidity or slavish adherence to set behavior patterns; they go there because it is the place in which they have the best chance to survive the winter.

Mattfeld fitted a captive deer with a mask to measure the animal’s oxygen consumption, an indication of energy used when the deer was resting, walking, running, or driven through deep snow and over bare ground. He found that when a deer sinks into snow ten to sixteen inches deep, walking used much more energy than the same activity on bare ground. In very deep snow, more than forty inches deep, the deer changes to a gait and speed which conserve energy, moving one
plunge at a time, progressing slowly.

Given a choice, deer avoid deep snow. In deep snow they become vulnerable to predators such as wolves, mountain lions or bobcats, but Mattfeld feels that energetics is a greater factor than predation in determining the survival value of a deer’s winter behavior and habitat choice.

Deer appear to reduce their metabolic rates in winter, so that at rest they burn fewer calories. They survive not on the food which they find but on the fat reserves they built up prior to winter. Combatting the deep snow and the exposure to wind and cold in the open hardwoods, they are less likely to make it through the winter than they are lying among the protecting evergreens of the traditional lowland wintering areas.

Looked at from this point of view, it does not seem quite as stupid for them to leave areas in which food is plentiful and move to areas which may or may not contain much browse. The relative food supply in open hardwoods and winter yards is debatable anyway. Mattfeld’s analysis indicated that clumps of witchhobble, a favorite winter deer food in his study area, occur nine times as often along the deer trails in winter concentration areas of his study site as they do in the open hardwoods.

However, on their migration to large deer yards in which they congregate from as far as twenty-five miles away, many deer pass up small areas of thick evergreen cover which seem equally as good as that in the yard. Nelson and Mech present a very good case for wolf predation as the determining factor in the development of this tradition among deer.

Not only do deer create a trail system in the yard, which makes it easier for them to move about and escape an attacking wolf, but in a large group there are many eyes to look out for wolves and each deer can be less vigilant. Since deer scatter when chased, the group may serve to confuse the wolves somewhat so that each deer is less likely to be caught.

Perhaps even more significant is the relationship of the traditional deer yards to the wolf packs’ territories. Around a winter yard, two or more wolf packs may compete for the hunting rights to the deer herd. The dominant, or alpha, wolves of each pack may actually fight each other, sometimes to the death. This strife among wolves makes them less effective as predators upon deer, wasting their time and energy competing among themselves rather than spending it on catching and killing deer.

Also, many of the deer in the yard have come from miles away, moving out of the territories of other wolf packs which could have surrounded and killed them as small isolated bands or individuals.

In the 1960s, after a series of mild winters in Nelson and Mech’s study area, deer that might formerly have moved to their usual winter yard stayed on or near their summer range in small, scattered groups. Of those that stayed in such groups many, if not all, were killed by the wolves; only those that joined the larger herd in the traditional winter yard survived.

The two hypotheses, Mattfeld’s or Nelson and Mech’s, are not incompatible. In fact, it seems likely that both energetics and predation could be factors in developing and maintaining deer yarding behavior.

In the Minnesota wilderness, wolves are important. At one time wolves inhabited northern New York as well. Now, the wolves are gone, but the behavior that developed in response to the wolves is still favored by the energy savings that yarding represents.

In northern New York, deer behavior varies more than in northeastern Minnesota. In winters with little snow and few days when the temperature does not rise above zero degrees, deer in the Adirondacks can survive in many scattered locations, including more open hardwoods and mixed forests. There are no wolves
to contend with.

In really bad winters, the deer that remain isolated and in more open locations are less likely to survive because they use up their energy reserves battling deep snow and cold wind.

Deer yards are an adaptation against the rigors of severe winters, whether through predation or energetics, or a combination of the two, but the behavior is not necessarily coded genetically-the behavior of an individual or a group of deer is based to a large extent on learning. Fawns come to the yard with their mothers and thus learn where to go in the winter.

If there is a series of open winters, the deer will learn new places to go and the traditional yard may even be abandoned, or moved to a new location.

That deer respond to changing habitat conditions is illustrated in Mattfeld’s research. He compares deer in Huntington Forest, where logging created a variety of habitat types, to those on the neighboring Santanoni Preserve where the habitat is more uniform, perhaps similar to the wilderness described by Nelson and Mech in Minnesota but without the wolves.

On Huntington Forest, yarding behavior is more varied than it is on the Santanoni Preserve or in the Minnesota wilderness where the habitat is more uniform.

I did not find a clear-cut answer to my original question on the adaptive value of yarding to the whitetailed deer. The situation is too complex for simple explanations. Considering the deer yard from a sociobiological point of view sheds some light on the puzzling behavior of whitetails, but neither the energetics hypothesis put forth by Mattfeld nor the predation model outlined by Nelson and Mech has been proven.

It may be that neither is really subject to proof. Both hypotheses suggest likely ways in which what at first seems to be a rather stupid habit on the part of the deer might actually be a necessary adaptation for survival of the species, especially in the most inhospitable portions of its range.

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