Whitetail biology

Winters in the Upper Peninsula of Michigan control deer numbers, not deer hunters. Not only do thousands of deer die from malnutrition during harsh prolonged winters, the loss of fawns born to malnourished does can be enormous.

Most recently, the winter of 2008-09 was one of those whitetail-killers. On the other hand, the past two winters have in the mildest I’ve experienced here along Lake Superior in the past 45 years or so (Global warming, maybe?) As a result, in the past three years, we’ve probably experienced some of the lowest as well as the highest newborn fawn survival rates currently possible for this northern region.

Here’s How It Works

Most animals characteristically produce more young than can be expected to survive. White-tailed deer are no exception. Although the reasons vary from one region to another, a high proportion of whitetail fawns born each spring fail to survive their first few weeks.

Typically, fawns born in areas of high-quality habitat where deer numbers are maintained below carrying capacity of the range through adequate hunter harvest have the best chances of surviving early life. For example, more than 90 percent of the fawns born on rich farmlands in portions of the Midwest survive to weaning age. Conversely, infant mortality rates ranging from 50 to 90 percent often prevail in poor-quality habitat where food and cover are inadequate, in northern areas following particularly severe winters, or wherever deer are overly abundant.

Since deer on northern range commonly experience a negative energy balance during winter while subsisting on a diet of woody browse, pregnant does must catabolize (burn) fat deposits to make up for the energy deficit. If the doe’s diet is inadequate and especially low in protein, she will draw upon her bone and body tissues to nourish the fetuses she carries. Prolonged nutritional deprivation during harsh winters often contributes to poor fetal growth and sometimes results in the death of the unborn, if not that of the mother too.

The fawn’s size at birth will hinge upon the mother’s nutritional status during the final trimester of pregnancy, and the fawn’s birth weight will largely determine its prospects for surviving its first few days. Healthy whitetail fawns normally weigh from six to nine pounds at birth, but some may weigh as much as 12 pounds.

Investigations conducted by Lou Verme at the Cusino Wildlife Research Station revealed that over 90 percent of the offspring born to pen-confined does that were malnourished throughout pregnancy died shortly after birth. In contrast, roughly 95 percent of those born to well-fed mothers survived. Survivors, on average, weighed about eight pounds, whereas those that died weighed about four pounds at birth. Based on this research, fawns weighing less than five pounds at birth appear to have relatively slim chances of surviving more than a couple of days.

Verme’s investigations revealed that malnourished does rarely absorbed or aborted fetuses, but that still-births of full term fawns were quite common. In some cases even dead, mummified fetuses were carried to term. Most fawns were born alive but stunted in size and perished within a day or two. These fawns were either too weak to stand or too small to nurse, or were born to mothers that produced no milk.

Nutritional shortage early in gestation tends to impact the growth of twin fetuses about equally. During the last third of their development, however, serious diet deficiency often results in physical dimorphism, wherein one littermate receives a greater share of the limited sustenance that is available. Consequently, does poorly nourished during late pregnancy tend to produce fawns that differ greatly in body size. When that is the case, the smaller fawn’s prospects for survival, almost invariably, are not good.

Depending upon the magnitude of winter severity, total newborn fawn losses may be staggering. In all likelihood, serious fawning losses occur throughout northern deer range after hard winters. Records complied at the Cusino Wildlife Research Station during the late 1960s and 1970s revealed that newborn fawn losses in Upper Michigan varied from only 10 percent after mild winters to as much as 75 percent following a severe one.

Whitetail does normally make good mothers. Abandonment of newborn fawns is most commonly associated with malnutrition, especially during late pregnancy, and occurs most frequently among first-time mothers. Even when disturbed during critical period shortly after giving birth, maternally experienced does tend to prevail, imprint upon their fawns, and defend their young as best they can. Young does producing their first young, however, are more apt to abandon their young when threatened, in favor of saving themselves.

One of the most comprehensive studies of the effects of winter nutritional stress on maternal care traits of white-tailed deer was conducted by Ed Langenau and John Lerg at the Houghton Lake Wildlife Research Station, located in central Lower Michigan. In their study, 27 percent of the does on low quality winter diets abandoned their fawns, versus only two percent abandonment for well-nourished does. The most common form of newborn fawn mortality within a few days after birth resulted from a “maternal rejection syndrome.”

According to Langenau and Lerg, “Does which failed to nurse or care for their young displayed fear and aggressive postures toward fawns, birth fluids, and afterbirth. This lack of an initial mother-young bond caused the fawns to increase the amount of care-soliciting behaviors such as bleating, running at the doe, and time out of the bed. This response of the fawns probably reinforced the fear and aggressive behavior in the doe.” In Langenau’s and Lerg’s study, the lack of an initial doe-fawn bond was confirmed to be critical to fawn survival. Without the important bond, fawns never nursed, and died within 77 hours after birth. In the case of twins, both were usually abandoned.

This phenomenon of fawn rejection by malnourished mothers is similar to that resulting from cases of high deer density and crowding, wherein psychologically stressed first-time mothers abandoned otherwise healthy offspring because they cannot find solitude and fawn-rearing space. In either case, such abnormal behaviors likely arise because of an insufficient production of prolactin, a hormone produced by the pituitary gland and promotes the maternal instinct.

Even under the most ideal circumstances, some newborn fawns die due to natural causes. And in the presence of effective predators such as gray wolves, mountain lions, black bears, coyotes, or bobcats, a 20 to 30 percent loss can be expected, even among well-nourished deer living in optimal habitat. Healthy deer populations can easily absorb such loss. In fact, such a drain helps to dampen wild oscillations in deer population size, thereby protecting the habitat from overuse. Most importantly, the selective culling by predators functions to remove fawns with inferior physical and behavioral characteristics that would be harmful to the species if perpetuated.

On the other hand, most of the newborn fawns might die during some years in the areas where deer are overly abundant. Too many deer causes undue stress and opens the door for disease and parasites. Also, overuse of habitat by deer reduces the availability of preferred foods, devastates fawn-rearing habitat, and contributes to malnutrition. Obviously, then, in the absence of effective natural predators – without proper deer population control through regulated harvesting by human predators – the whitetail can become its own worst enemy.

It’s no secret deer numbers in the U.P. are as low as they’ve been since the late 1970s. Hopefully, given these past two easy winters, the population is on the up-swing. However, I’m not too optimistic, considering the sad condition of the deer wintering habitat. One winter of only moderate severity is likely to wipe out all recent gains.