Successful Deer Management Starts with Large, Healthy Fawns

By John J. Ozoga

Any successful deer management program starts with large healthy fawns; stunted fawns reflect management failure. Small weak fawns are more likely to die soon after birth because of malnutrition or abandonment, are more likely to succumb to predators and disease, and are indicative of poor habitat and/or faulty deer population management. Those runts that survive seldom grow up to be large, productive adults.

Many factors can influence a fawn's prospects for survival, as well as general well-being and ultimate adult stature. However, the mother’s level of nutrition (hence fetal development) during her last trimester of pregnancy is the most important factor governing the newborn fawn's survival prospects. Malnourished does invariably give birth to small, weak fawns that die within a few days. Likewise, the young fawn's nutritional status during spring, summer and autumn will often determine whether or not it survives its first winter (especially on Northern range) and will determine its stature at maturity. Given favorable circumstances, some fawns (male and female) might even achieve early puberty and breed when less than a year old. Unfortunately, the exact nutritional requirements for favorable fawn growth are complex. Not only do the sexes differ in their dietary needs on a regional and seasonal basis, birth timing and social behavior factors can interact with nutrition to determine fawn growth and survival prospects. Understanding these relationships is essential to implement proper deer population and/or habitat management practices.   

Birth Timing

Fortunately, the whitetails’ reproductive cycle is geared to giving birth when conditions are best for newborn fawn survival. As one progresses northward, in the whitetails’ geographic range, a greater percentage of the fawns will be born during a relatively brief period in late May and early June. This is when new, lush vegetation provides excellent hiding cover for fawns and an abundant supply of nutritious forage, which is necessary if does are to produce their maximum amount of nourishing milk. Even fawns need nutritious vegetation to supplement their milk diet at an early age. In the North, natural selection has minimized poorly timed births. Those fawns born too early likely die from exposure. Those fawns born too late seldom achieve favorable physical size and fatness necessary to survive the hardships of their first winter. Late-born fawns are more prevalent in Southern states, where unbred adult does might re-cycle and come into estrus as often as seven times during one season. Further, many southern states have very long hunting seasons, starting in August or September. As is usually the case, bucks are preferentially harvested, which skews adult sex ratios heavily in favor of females before peak breeding, increasing the chances of late breeding/birthing. With unlimited nutrition, fawns born a few weeks late will catch up over time. However, in the wild, over-browsing, poor nutrition and density stress generally go together. As a result, late-born buck fawns, in particular, might never grow large bodies or large antlers, regardless of their genetics. In the North, few undersized fawns survive tough winters. In the South, researchers contend such trends are self-perpetuating, even with favorable nutrition, because it takes late-born deer longer to become sexually mature.

Birth to Weaning

Fawns born to malnourished does might weigh as little as two pounds at birth, whereas healthy individuals can weigh as much as 12 pounds. Generally, single fawns weigh more than those from twin or triplet litters. However, few fawns weighing less than five pounds survive more than a few days, because they are too weak to stand and nurse, are abandoned or their mothers produce no milk. At birth, buck fawns tend to be about one-half pound heavier than does among mixed-sex litters. Since this weight differential increases slightly as the mother's nutritional plane declines, researchers speculate that males are more resistant to prenatal malnutrition. Deer milk is higher in fat, protein, dry matter and energy content than milk produced by domestic ruminants. The doe’s diet has little or no influence upon the composition or quality of her milk, but poor nutrition might cause her to produce less milk than normal. Or, if the doe is seriously malnourished, she might produce no milk at all. Young fawns are very efficient in converting nutrients into skeletal and muscle growth, but they lay down minimal fat until they are weaned. The average fawn may double its birth weight within two weeks, about the time it starts to nibble some vegetation, but cannot survive without its mother’s milk. To grow properly, fawns require nourishing forage that has from 14 to 22 percent protein content, with males having higher requirements than females. Compared to females, buck fawns appear to be especially sensitive to nutritional shortage during the first four months. The capacity and function of the fawn's rumen-reticulum increases as the fawn gains weight. When fawns are five or six weeks of age and weigh about 25 pounds, their forage intake increases substantially. Fawns older than this can compensate somewhat for decreased milk intake by eating more vegetation. Although they can survive, when maternal nutrition is poor and the doe’s milk supply is limited, fawns undersized at birth will also be undersized at weaning age.

Autumn Requirements

Although dietary protein content may be more important than the amount of energy in the fawn’s summer diet, digestible energy needed for fattening becomes more important during autumn. Normally, fawns do not lay down appreciable fat until October, with peak fat reserves not accumulated until mid-December. Also, female fawns tend to be fatter that buck fawns in autumn. Our studies at the Cusino Wildlife Research Station in Michigan revealed that 10 weeks of inadequate autumn diet (October throughout mid-December) stunted fawn growth, but the sexes seemed to suffer equally. Autumn food-deprived fawns grew more slowly and stored less fat than well-fed ones. However, even those fawns on poor or marginal autumn range store appreciable fat. Therefore, it appears that deer inherently are compelled to store fat in autumn while sacrificing body growth if need be. In other words, well-fed fawns tend to be skeletally large as well as fat. By comparison, malnourished individuals might be fairly fat but stunted. Also, based on our studies at Cusino, when provided an unlimited supply of browse, stunted, lean fawns can survive winter by consuming more browse, feeding more efficiently and minimizing their activity to conserve energy. In my studies, conducted with an enclosed population of supplementally fed deer, does achieved about 88 percent of their skeletal growth (as determined by hind-foot length) by seven months of age, compared to 85 percent for bucks, but only 52 percent and 43 percent of their mature body weight, respectively. As yearlings (1-1/2 years old) bucks and does attained 98 percent of their skeletal growth and slightly more than 80 percent of their mature weights. It’s important to note that bucks accomplish most of their skeletal growth before they disperse from their natal range at yearling age. This indicates that habitat preferred by matriarchal groups for fawn-rearing might ultimately determine a buck's physical size at maturity — regardless of nutritional conditions on his newly established range.

Social Factors

Even with unlimited nutrition, social stress can impact deer welfare — just as readily as malnutrition. Since whitetail does exhibit territorial behavior, and aggressively defend their established fawning grounds, young subordinate does are relegated to the poorest habitat when deer density is high. When crowded, young does breed late and give birth later than adult does. Compared to adult does, stressed young mothers also experience higher newborn fawn mortality rates and produce inferior male offspring. Based on measurements of 264 9-month-old fawns born and raised with supplemental feeding in the Cusino enclosure, we saw only minor differences in the body size of doe fawns, regardless of their mother’s age. Only female fawns born to first-time mothers (2-year-olds) were smaller than average in winter. The difference in buck fawn weights with regard to the mother’s age (hence, social status) was more striking. On average, buck fawns raised by does four years old and older weighed about 89 pounds in March. This was roughly 10 percent heavier than those born to younger does. Poor nutrition results in uniformly smaller than average fawn weights at weaning age. In contrast, high deer density on fertile range (or supplemental feeding) will cause a wide spread in surviving fawn weights at weaned age. Sometimes, buck fawns raised by well nourished mature does will even be larger than yearling bucks raised by stressed young mothers, a sure sign of social stress. During a special three-year study, conducted in the same enclosure, we also examined the potential effects of disrupting the female whitetail’s social organization by removing all except one female from known family groups (“isolates”). We then compared their reproductive performance to other females (“socials”) that were members of intact family groups. For whatever reason, social does on average reared heavier and skeletally larger buck fawns to weaning age, compared to isolate mothers. In mixed litters, males from social does averaged 16 percent heavier than female litter mates, compared to a seven percent weight advantage for males reared by isolates mothers. There was no difference among doe fawns with respect to treatment. Apparently, young bucks somehow benefited nutritionally when associating with older related females — a situation scientists refer to as “social facilitation.” Because related does and their fawns commonly band together during autumn and winter, social male fawns probably had compatible associations with several does, had access to a relatively large ancestral range, and enjoyed superior nutrition.

Management Implications

The general health status of fawns during autumn can reveal a great deal concerning the nutritional and social well-being of a deer population. For example, the frequent occurrence of pregnant doe fawns and/or “infant” antlered buck fawns are good indicators of a nutritionally and socially well-balanced deer herd. Conversely, poor annual recruitment rates and small, lean fawns in autumn indicate a need for management change — but identifying the precise problems involved and determining corrective actions can be a daunting task. Remember, social factors can impact deer welfare just as readily as nutrition and that young male and female whitetails have different social and nutritional requirements. Hence, management practices favoring one sex might not necessarily benefit the other. Normally, social stress due to high deer density and nutritional shortage go together, resulting in increased newborn fawn mortality rates and small fawns destined to become poor quality adults. In such cases, the first priority would be to lower deer density to balance herd size with available food and cover resources. Identifying and correcting the adverse effects of poor population sex-age structure brought on by unfavorable harvest strategies, or improving seasonal nutrition due to poor range quality, may be more difficult. As I’ve discussed in other articles, good nutrition during the final one-third of gestation (late winter/early spring) is critically important. Invariably, offspring that survive poor maternal nutrition or grand maternal nutrition suffer lifelong consequences. The best advice is to create and maintain diversified habitat, especially on maternal range, that satisfies the contrasting seasonal food and cover requirements of young growing animals. From what is known about deer metabolism and forage value, lush summer herbaceous forage rich in protein and autumn foods high in digestible energy meet these needs. In short, to produce skeletally large fat fawns by winter, the benefit of nutrition must first flow through the pregnant doe. In areas of historically poor nutrition, removing negative maternal effects might take several generations — so be patient.