A Glut of Does Harms Bucks - by John Ozoga

In white-tailed deer, the adult sexes live separately during much of the year, just as they do in mule deer, red deer, elk, moose, and many other ungulates. Scientists refer to this social and geographical separation as “sexual segregation" or "niche separation" of the sexes.

Related does live in close-knit matriarchal societies, composed of mothers, daughters, grandmothers, great-grandmothers, and so forth. Bucks, on the other hand, form fraternal or bachelor groups generally composed of unrelated males. In either case, social group size and the degree of social complexity varies depending upon many different factors that influence deer herd sex-age composition and density.

While the female whitetail might spend her entire life on a relatively small ancestral range, the male generally disperses to a new range. At some point in time, the young male must leave one societal unit to join another if he is to become a successful breeder.

Because adult bucks and does differ so much in their behavior, they can almost be treated as though they were different species. Researchers around the world still debate the adaptive advantages of the totally different lifestyles demonstrated by the whitetail sexes. Unfortunately, their seasonal differences in food, cover, and social requirements are seldom considered in deer management plans.

Given the potential significance, some deer researchers question whether current deer habitat and herd management strategies adequately serve both sexes. We are often bound to a system that overexploits the male segment of the population and under-harvest the females. The goal, it seems, is to produce an abundance of whitetails, regardless of their condition, to satisfy steadily increasing recreational demands on this species.

In many parts of the country, antlered bucks are currently being harvested so intensively that mature bucks and true buck groups are virtually nonexistent.

From the standpoint of whitetail social evolution, groups of yearling bucks, in the absence of older bucks, do not represent true fraternal groups. Groups of yearling deer may also include females, and tend to occur only during late spring and summer while does are rearing fawns. Thus, these groups of young deer differ greatly from exclusive, age-structured groups of older bucks.

In a natural population, only relatively few dominant bucks do most of the breeding. A young male can only hope to become a dominant male by engaging in competition with other bucks over a long period. The achievement of dominant status by young males requires outliving older, stronger males and dominating males of similar age. It is not enough to simply maintain a rank. To succeed, the young male must continually strive to move up in dominance. This requires that he associate not only with animals of lower rank, but also with those of higher rank.

In an age-structured male society, a high rank is not easily attained, it requires experience and the learning of competitive skills. Success, as in any contest of strength, skill, and endurance, comes from long and diligent training. A young male choosing not to join all-male groups would not be able to obtain the necessary skills to compete successfully.

Considering that the whitetail buck's chances of breeding are largely determined by his rank in the male dominance hierarchy, the young buck has little choice but to associate with other males. Although he is tightly linked to the mother-young system early in life, he must eventually break those bonds. When sexually mature, the buck must seek out and interact with older males, achieve male group membership, and rise in dominance rank. If he does not, but instead remains with female relatives, it is my opinion that he would become a "psychological castrate," never achieving respected breeder-status.

Even casual observers may note that, as with any behavioral trait, the whitetail's social organization is an ada
ptation. It evolved in response to numerous environmental stresses, including predators, diseases, climate, habitat conditions, and hunting by Native Americans. And, as with any adaptation, the whitetail's social organization is genetically linked, inherited, and essential for the species' healthful existence—it promotes social order, genetic selection and physical fitness, and improves the prospects for survival.

Since the whitetail sexes differ in many aspects of physiology, behavior, and anatomy, it is likely that they evolved differently. For example, bucks and does differ in size, shape, growth rate, metabolic rate, life span, food and cover requirements, and in many aspects of physiology and biochemistry.

Based upon intensive behavioral investigations conducted in Southern Michigan's George Reserve, investigator Dale McCullough concluded that, due to sex differences in use of space, food, and cover, whitetail bucks and does do not compete equally for the necessities of life on a year-round basis.

Therefore, if bucks and does evolved differently, then they must also respond differently to environmental change. As a result, habitat management practices that benefit does may not necessarily benefit bucks equally, or may even be detrimental to bucks.

Also, harvest management strategies that inflict unnatural patterns of mortality (such as buck-only harvesting) and create deer herds with abnormal sex and age composition could impact the welfare of one sex, either favorably or otherwise, more than the other.

Many hypotheses have been advanced to explain the evolution of sex segregation among ungulates. However, two Oregon researchers, Martin Main and Bruce Coblentz, propose that females select habitat that is best suited for rearing offspring. Normally, that means diversified food and cover arrangements with ample hiding cover for both mother and young, as a predator defense, during the critical stage of early fawn-rearing.

By comparison, when available, males tend to select areas where nutrition is superb, which allows for maximal body growth necessary for the attainment of high dominance rank and improved breeding success.

Generally speaking, both male and female whitetails in northern latitudes subsist upon relatively poor quality forage during winter and leave their winter ranges in depleted, poor physical condition.

Forage and environmental conditions normally become much better in spring and summer, allowing for fairly rapid replenishing of energy reserves. Ungulates from northern latitudes have evolved to give birth during this period, when conditions are most favorable for rearing young. This is when the reproductive patterns of the adult sexes differ the most and they show the greatest niche separation.

The Oregon researchers emphasize that body size, physical strength, and general body condition influence a male's mating success. Therefore, the “replenishment of energy reserves should coincide with major growing seasons, and optimization of forage resources by males should be most evident during these periods as they prepare for the rut. Optimal foraging by males may require avoidance of heavily grazed areas or adoption of foraging patterns that exploit temporal resources of high quality. The importance of maximizing body condition for males apparently exceeds even increased risk of predation."

When a deer population is socially balanced, my observations indicate that adult bucks intensively scent mark their favored summer habitat as soon as they return to it in spring, probably as a means of reclaiming range that had been vacated during winter. This marking, which is done primarily on overhead branches, serves to intimidate other deer, including pregnant females that require solitude for fawn rearing. As a result, buck scent marking helps to segregate the adult sexes and distribute the herd more evenly during the nonbreeding period when does are rearing fawns and bucks are growing antlers.

In McCullough's words
, "Resource partitioning between the sexes in white-tailed deer adds a new dimension to the role of social behavior as it relates the animal to its environment." Indeed, if bucks differ from does in their use or space, food, and cover resources, on a seasonal basis, then deer herd and habitat management considerations take on an entirely new level of complexity.

McCullough suggests that this issue should be carefully considered especially when it comes to determining deer harvest management strategies. As he points out, "unbalancing populations toward females intuitively would be expected to increase productivity, but in practice seldom does in moderate- to high-density populations."

Clearly, poor growth rates among young deer and reproductive failure among adult does, when associated with food competition and malnutrition, invariably is the result of too many female deer, not because of too many bucks.

Martin Main was probably correct in criticizing traditional deer management practices which promote and expand female groups—especially predator control efforts and bucks-only harvesting—run counter to how the white-tailed deer's social system evolved.

Wherever fawn-rearing females are overly abundant, over-browsing and range forage deletion is likely, thereby excluding buck use of such habitat. Conversely, female groups may be prevented from becoming established in areas used by males, because predation or other factors reduce fawn-rearing success.

In other words, when deer density is high, whitetail bucks often occupy certain habitats strictly by "default." They browse depleted areas where no does live or where does live in very low numbers.

Today, young bucks in Michigan probably have great difficulty finding suitable habitat during spring and summer, in an environment saturated with too many antlerless deer. As a consequence, many bucks are forced to occupy nutritionally poor areas, where they grow poorly, sport undersized antlers, and may even suffer greater than normal over-winter mortality.

John Ozoga is a former Wildlife Research Biologist for the Michigan Department of Natural Resources where he spent more than 30 years conducting deer research at Upper Michigan's Cusino Wildlife Research Station. He now devotes much of his time to consulting and popular writing and is a panel member for QDMA's Whitetail Wisdom column.

Article provided by the Quality Deer Management Association. www.qdma.com

Back to the Articles Listing