HOW WELL DO BUCKS WEATHER WINTER?        

By John J. Ozoga

In more ways than one, the buck’s annual fat cycle seems suicidal. Mature bucks prepare themselves almost totally for siring offspring, squandering their energy reserves during the rut. As a result, they enter the stressful winter season lean and seemingly ill-prepared for the hardships that are sure to come. Meanwhile, healthy does and fawns enter winter hog-fat, with energy to burn when the going gets tough.

It’s not unusual for a rutting buck to lose 20 to 25 percent of his peak body weight during the coarse of the breeding season. Typically, this rut-related weight lose occurs from about mid-October to mid-December, leaving little time to recover before winter sets in.

In some areas, bucks can feast on energy-rich acorns and lush herbaceous forage, or even standing corn, immediately post-rut. In the process, they can restore some of their spent body reserves before the onset of harsh winter weather. However, in many northern areas deer must subsist upon woody browse starting in late autumn. For them, this means a negative energy balance and little in the way of physiological recovery until spring green-up.

Most deer can handle no more than a 30 percent weight loss during winter before they die from malnutrition. Since northern bucks enter the perilous winter season with scant fat stores, they have a slim margin of safety.

Intuitively, one would expect rut-stressed bucks to be seriously handicapped during winter. However, I question such logic. Instead, I’m convinced big northern bucks, in particular, employ special adaptive strategies that improve their survival prospects when faced with snow cover and cold weather.

The Inefficiency of South Texas Trophy Management

In South Texas, where trophy buck management is the primary goal, Charles DeYoung and Mickey Hellickson documented high post-rut buck mortality.

Remember, severe drought during summer and autumn periodically contributes to impoverished range conditions and sets the stage for heavy winter deer mortality due to malnutrition in this southern region, despite mild winters.

Where trophy whitetail management is practiced in south Texas, bucks are protected from harvest through the ages of 3-1/2 or 4-1/2 years so they grow large antlers. As a result, many  bucks live to extreme old age. Unfortunately, studies conducted by DeYoung demonstrated that this is an inefficient practice because bucks die at a rate of 25 to 29 percent annually before reaching maturity.

Hellickson found that natural mortality was especially high for male fawns, yearling bucks, and bucks 7.5 years of age and older. About 77 percent of the deaths occurred during the post-rut from January to March, with most deaths occurring in January. He speculated that many bucks lost so much weight during the rut, they could not recover and died of malnutrition, disease or were killed by coyotes. 

Even under the droughty conditions of South Texas, however, Hellickson found natural mortality of middle-aged bucks to be relatively low. Unfortunately, he provided no information regarding mortality rates of female deer.

Pennsylvania Studies    

Studies conducted in a Pennsylvania enclosure by Alan Woolf and John Harder also revealed little (less than 7 percent) natural mortality of prime-age bucks. Examinations of animals found dead disclosed gunshot wounds, either from harvest efforts or malicious shooting through the fence, as the leading mortality factor for bucks, accounting for 27 percent of the mortality.

A disease, diagnosed as enterotoxemia and caused by excessive acorn ingestion in the fall, accounted for 12.3 percent of the mortality, and was the single most important disease in the study. That particular problem was later resolved, by including corn and oats in the supplemental ration. Like acorns, corn and oats are high in carbohydrates, which seemed to condition deer to the seasonally abundant and heavily consumed oak mast.

Of the total 780 deer found dead, there were 404 males and 376 females. Female fawns had a higher mortality than male fawns and males 1-1/2 years and older had a higher mortality than females. However, the adult sex mortality difference largely stemmed from wounding and the fact that enterotoxemia claimed more adult males than females, not because of malnutrition per se.

Furthermore, the Pennsylvania study was conducted in a seriously overbrowsed enclosure, where the high-density deer herd was in poor physical condition, despite a supplemental feeding program. Under such conditions, mature bucks are more aggressive and dominant at feeding sites, which may account for their getting a lion’s share of the artificial ration.

Farmland Bucks

Studies conducted by Charles Nixon and his associates on intensively farmed land in east-central Illinois also revealed low mortality of deer due to natural causes. In fact, deer mortality was considered “rare” from January through September.

Adult buck mortality was largely due to hunting, wounding, poaching and automobiles. For some reason, radio-marked adult males died at a higher rate compared with other marked adult males on the same study area – a fact the investigators were unable to explain.

In the Illinois study, yearling bucks suffered the highest annual mortality rates, whereas fawn males and adult females had the lowest. Yearling males, in particular, were more likely to die from human induced causes when they dispersed from unhunted refuges.

In this particular area, forest (hiding) cover for deer is seriously limited, and a major factor influencing deer behavior and survival. Adult male deer are particularly vulnerable during the rut because of their greater movements, likeliness to leave refuge areas and more often die from hunting, poaching or collisions with automobiles.

Nonetheless, with the relatively mild winters and superb nutrition, which is characteristic of this region, adult bucks in east-central Illinois rarely suffer serious malnutrition as the result of rut-related stress.

Buck Mortality in Oklahoma

Stephen Ditchkoff and his co-workers at the McAlester Army Ammunition Plant in southeastern Oklahoma conducted one of the more recent studies of buck mortality. The area is managed according to Quality Deer Management and hunting is limited to either-sex bow-hunts. As a result, the doe-to-buck ratio is about 2-to-l, and more than 50 percent of the bucks are 3-1/2 years of age or older.

The researchers tracked 80 radio-collared bucks to determine mortality factors. During the study, 39 bucks died. The greatest single mortality cause was hunting, 18 deaths, followed by vehicle collisions, poaching and predation – three deaths each. Natural causes, including a leg infection, thoracic puncture from fighting, brain abscess and exhaustion from rut related activity killed four bucks. Causes of death could not be determined for eight bucks.

Most buck mortalities – 72 percent – occurred during or immediately following the breeding season. Interestingly, the researchers did not find bucks from any age class more likely to die. That is, young bucks were just as likely to die as older bucks. However, the reasons for mortality differed among age classes.

For example, yearlings and 2-1/2 year olds were nearly five times as likely to die from human-induced causes than natural mortality. In contrast, bucks 3-1/2 years old and older were more susceptible to natural mortality following the breeding season. Although elderly bucks were more likely to die from natural causes, the Oklahoma researchers found no evidence of age-induced physiological deterioration among bucks.

This study demonstrated that a structured buck population suppresses the rut-related activities of younger bucks. As a result, this mate competition decreases a young buck's energy expenditure, improves its growth rate and decreased the likelihood it will die from natural causes during the post-rut.

Females More Vulnerable in the Adirondacks    

An extensive study conducted by William Severinghaus in the central Adirondacks of New York, from 1930 to 1951, revealed that female deer more likely die from starvation than males.

Among 1,822 winter-killed deer found in some 50 wintering areas, nearly two-thirds (61.2 percent) were fawns, about 4 percent were yearlings and 35 percent were older deer. Among adults, the proportions for prime-age deer (2-1/2 to 6-1/2 years old) were about 2 to 3 percent by age class. Frequencies of older age groups were somewhat higher.

The chief cause of overwinter mortality was starvation. Consequently, the percentage of adult deer found dead was higher during severe winters.    

Interestingly, winter losses were heavier among females than males for deer of all ages. In the case of fawns, for example, the sex ratio among winter-killed fawns averaged 84 males per 100 females, compared with an average fall ratio of 106.2 males per 100 females.  

Severinghaus emphasized, “...there was little differential in the fall sex ratio among adults, probably about 80 to 90 males per 100 females, while the ratio among winter-killed adults averaged only 50 males per 100 females. That an almost even sex ratio among adults in the fall was maintained in spite of the annual removal of antlered bucks by legal hunting seems largely attributable to the proportionately greater loss of adult females from starvation together with some illegal kill.”

The Importance of Body Size  

The buck’s larger body size compensates somewhat for his lack of fat stores when faced with cold weather and deep snow. Not only do bucks have longer legs, enabling them to plow through greater snow depths, but the larger-bodied bucks retain body heat much more efficiently. This means they can make do with poorer-quality shelter in order to occupy habitat that has better food sources.

Adult bucks normally weigh 30 to 50 percent more than adult females and have a lower whole-body metabolic requirement per unit gut capacity. This difference allows males to subsist on

lower quality foods when nutritious ones become scarce. Females and fawns, however, seem unable to meet their energetic needs by filling up on less-nutritious foods, even when those foods are plentiful.

The Importance of Good Nutrition

I certainly have not reviewed all the scientific articles that consider sex-age differences in whitetail winter mortality. And I do not refute the evidence that adult bucks in south Texas, and possibly elsewhere, sometimes suffer unusually high winter mortality associated with rut-related stress. However, in most such cases, heavy winter mortality of bucks can be traced to poor range conditions, often associated with an overabundance of deer.

Since young male whitetails have higher nutritional requirements for growth than females, the impact of too many deer, or inherently poor range, can have a double-barreled effect on buck overwinter survival. 

For one thing, malnourished males never achieve their maximum body size – a serious disadvantage, given the metabolic and behavioral benefits associated with large body mass.

Small-bodied fawns, in particular, have certain physical laws working against them when winter-stressed. Not only do they have shorter legs, which hinder travel through snow, but also more heat is lost from a square meter of surface of a small deer than from similar surface area of a large deer.

Aaron Moen calculates the critical body weight for fawns at somewhere between 77 pounds and 88 pounds. Animals weighing less than those weights lose considerably more body heat to cold exposure and are more likely to die during prolonged cold winters.

Normally, healthy male fawns weigh about 10 to 15 percent more than female fawns at onset of winter, which gives males a decided advantage. When nutritionally stressed, however, the weight differential narrows. On very impoverished range, average weights of male and female fawns may be nearly equal. And, in some areas, little difference may exist between the weight of male yearlings versus female yearlings. Even at maturity, nutritionally-stressed males are likely to be smaller than normal and suffer the consequences.

Secondly, high deer density contributes to overbrowsing of peripheral yarding areas, which bucks prefer as wintering cover. This drives hungry bucks to occupy food shortage areas within core yarding areas, where bucks are likely to experience greater than normal mortality due to starvation and predation.

Conclusions

Given the available evidence, deer seem to have different winter survival strategies, depending upon their sex, age, and physical condition, and resource availability. On northern range, for example, females and their young benefit from the safety of yarding, but must burn more fat because of impoverished food conditions normally associated with traditional deer wintering habitat. Large-bodied, comparatively lean adult bucks, on the other hand, survive better in peripheral yarding areas with less shelter, provided browse is more abundant there.

Where the habitat and deer populations are well managed, even rut-stressed whitetail bucks appear quite capable of surviving winter – certainly better than fawns and, in some instances, even better than adult females. Consequently, heavy winter mortality of adult bucks can generally be linked to poor nutrition, often associated with deer overabundance and overbrowsed range.