Fate of Fawns Depends on Spring Nutrition
By John J. Ozoga

Deer may seem more relaxed and healthy in the spring, but life for the whitetail is never easy.  Photo by R.G. Bernier

Among the 40-plus species of deer in the world, whitetails are considered to be the most prolific. They are also one of the most adaptable, being able to colonize a wide range of habitats and environmental conditions. Unlike many other species, they’ve literally thrived in the presence of modern man.

According to noted deer researcher Harry Jacobson, a primary reason for the whitetails’ success “is its reproductive strategies: The whitetail can breed at an early age, select the best sires in a herd, produce more than one offspring per year, arrange for the young to be born at a favorable time of year, and even alter the sex ratio within the herd to maintain a healthy population.”

Nonetheless, whitetail populations aren’t equally prolific or healthy. Doe health can vary greatly from one region to another, and even from one year to the next within the same region, depending primarily upon nutrition. Clearly, nutrition controls the growth, reproduction and survival of whitetails throughout their range.

Summer and autumn nutrition will determine the herd’s vitality prior to the breeding season. Hence, the level of nutrition during the summer-autumn period will determine the timing and conception rate among does and, therefore, the herd’s reproductive potential. And it will directly impact individual survival prospects during the critical winter season.

However, even if nutrition is ideal during other times of the year, herd productivity may fail completely if pregnant does suffer from poor quality habitat and inadequate nutrition during the spring period.

Therefore, the spring season is truly a decisive time for whitetails throughout their extensive range.

Setting The Stage

Fortunately, the whitetail’s reproductive rhythm is regulated by photoperiod and not by some other cue, such as temperature, which oscillates more wildly and follows a less predictable seasonal cycle. Nor is it determined by moon phases, as proposed by some, which would seriously advance the fawning season some years, while unwisely delaying it during others.

Instead, whitetails are short-day breeders, in that fewer hours of daylight during autumn cause certain hormonal changes responsible for their breeding behavior. In the North, the whitetail’s breeding window is relatively narrow and keyed to ensure that the vast majority of fawns are born on schedule in spring when weather and vegetative conditions are most favorable for newborn survival.

Understandably, the whitetail’s fawning season can be much more protracted in the South, where climatic factors are less stressful. But even in the South, early-born deer, particularly males, are superior at maturity to those born later in the season. The late-born never seem to “catch up.”

So, what transpires during spring – that precarious time between the vernal equinox and the summer solstice – will determine whether the next generation of whitetails flourishes or fails.

Prey Saturation

In order to survive, fawns must be born at the proper time in spring when climatic conditions are favorable and food and cover are abundant. Also, in the presence of effective predators – such as mountain lions, gray wolves, coyotes, bobcats and black bears – births should be reasonably synchronized; many fawn births within a relatively short period of time results in what ecologists refer to as “prey saturation.”

With most of the fawns being born within a few weeks, predators are literally overwhelmed with potential prey. As a result, a high percentage of fawns survive the vulnerable hiding stage of early development.

Even under the most ideal circumstances, some newborn fawns die due to natural causes. And in the presence of effective predators, a 20 to 30 percent loss can be expected, even among healthy 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 removes fawns with inferior physical and behavioral characteristics that would be harmful to the species if perpetuated.

Death in the Womb

Whitetails have a gestation period that is about 200 days long. However, it can vary from about 185 to 215 days in length, depending upon the mother’s physical condition. Pregnant does are in at least reasonably good health when bred. If they weren’t, they probably wouldn’t conceive. But the doe’s nutrition during pregnancy, and especially that during the final two months (in spring) will largely determine her reproductive success. If a doe is nutritionally stressed during pregnancy, to the extent that her fetuses do not grow properly, she sometimes has an extended pregnancy  greater than 200 days. The logic here being that extra time allows retarded fetuses to put on a little more growth.

On the other hand, if the pregnant doe is well nourished and her fetuses are large when full-term, she will likely give birth before 200 days.

Some shed ova are lost from unsuccessful fertilization. In other cases, very small embryos  those that die at a young age – are reabsorbed. However, studies conducted at the Cusino Wildlife Research Station in Upper Michigan indicate that such mortality is seldom greater than five percent, regardless of the doe’s nutritional status.

It is not unusual for fetuses to die during the latter part of gestation if the mother is severely stressed. Most such fetal mortality occurs during the final one-third of gestation. But, does rarely abort dead fetuses. Instead, dead fetuses are usually carried to full-term and expelled in atrophied or mummified form.

Nutritional shortage early in gestation tends to impact the growth of twin fetuses about equally. During the last third of development, however, serious dietary deficiency often results in physical dimorphism, wherein one littermate receives a greater share of the limited nutrients.

Consequently, does poorly nourished during late pregnancy are more likely to produce one dead fawn and one that is alive, but stunted. When this is the case, even the live fawn’s prospects for survival generally are not good.

Stillbirths

During the Cusino studies, 12 of 215 fawns (6 percent) died just before or during birth. These “stillborn” fawns had deep, red lungs, indicating they had not inhaled air. Although they were fully developed and haired, most were born to malnourished mothers and were smaller than normal.

Most of these stillbirths were born to penned does that received poor diets during winter and spring. Most were one member of twins or triplets. They were usually larger than mummified twins, but smaller than littermates born alive.

This indicates that limited nutrients are not distributed equally. Among twins, for example, one fetus is sacrificed for the potential benefit of the other.

Infant Mortality

Newborn fawns may die for a variety of reasons, including accidents, abandonment, predation, parasitism and/or disease. But the pregnant doe’s nutritional intake is by far the most crucial factor in determining the newborn’s fate.

Even if the mother’s diet is excellent and predators scarce or absent, about 10 percent of the newborn fawns can be expected to die from birth defects, abandonment and accidents. Such is generally the case in the rich Midwest farm belt, for example, where soils are rich, the climate is moderate and natural predators are scarce.

Studies conducted by Louis Verme rather clearly demonstrated the relationship between maternal nutrition and newborn fawn survival. Poor nutrition during winter contributed to increased newborn fawn mortality, but spring nutrition was even more important.

Experimental does fed moderate to good diets throughout pregnancy lost only five percent of their newborn fawns because of nutritive failure. Even does fed very poor diets in winter, but restored to high nutrition in spring, only lost 19 percent of their fawns.

By comparison, does on poor winter nutrition and poor spring nutrition lost a whopping 93 percent of their fawns at birth. In other words, does malnourished during winter were already predisposed to appreciably newborn fawn mortality. And that rate of loss increased in proportion to the deficiency level of the doe’s spring diet.

Many fawns born to malnourished does were so weak they couldn’t stand, despite the doe’s urging. However, some fawns weighing only two to four pounds scampered after the doe but failed to nurse. In either case, these fawns normally died within 48 hours.

The fawn’s size at birth hinged upon the mother’s nutritional status during the final trimester of pregnancy, and the fawn’s birth weight largely determined its prospects for surviving the first few days. 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, regardless of other circumstances.

Protein Effects

Verme did not determine the specific nutritional factors involved in fawn-rearing success, but believed his study merely reflected differences in total energy. At the time, he speculated the amount of protein in the pregnant doe’s diet might be even more important than the amount of digestible energy.

Several years later, researchers Dean Murphy and J. A. Coates reported the amount of protein in the diet of pregnant does greatly influenced newborn fawn survival. When fed a 13 percent crude protein ration, does didn’t lose any fawns after birth. By comparison, a group of does fed a 10.4 percent crude protein diet lost 25 percent of their fawns, and another group receiving 7.4 percent had 42 percent fawn mortality.

Because no dead fawns had milk in their stomachs, delayed or inadequate milk production was considered the primary cause of death.

Pregnancy typically increases the need for protein. If the diet is inadequate, the mother sacrifices her bones and body tissue to nourish her fetus. After prolonged malnutrition, however, the mother’s reserves are drained and her fetus suffers the consequences.

Maternal Rejection

Michigan researchers Ed Langenau and John Lerg also demonstrated that the pregnant doe’s maternal behavior, and ultimately her winter nutritional plane can influence the fate of her fawns. 

Langenau and Lerg assigned pen-raised pregnant does to one of three nutritional levels from January through March: unlimited feed, 75 percent of full feed and 50 percent of full feed.

During a two-year study, 11 of 89 fawns born alive died within 72 hours. All but one of these fawns were from does on poor-quality diets. The primary cause of death was abandonment, when does reject their fawns. Without formation of the critical doe-fawn bond, fawns never nursed and died from starvation. While well-fed mothers suffered only two percent fawn mortality, does malnourished during winter abandoned 27 percent of their fawns.

There’s a good physiological explanation for the maternal rejection phenomenon. That is, abnormal maternal behavior among malnourished does probably stems from insufficient production of prolactin. This hormone, which is produced by the pituitary gland, induces milk production and promotes the maternal instinct.

Maternal Defenses

Healthy whitetail does are extremely good mothers. They watch over their hiding fawns, tenaciously defend them and carefully move them from one bedding site to another throughout their home range. However, outright defense of her fawns against predators requires lots of energy, not to mention risking injury or death.

Healthy does can afford to defend healthy fawns. Such early action could be important in determining the fawn’s chances of survival, so in a sense, it’s “worth it.” Defending a small weak fawn is another matter entirely. Studies have shown that healthy does are more likely to aggressively defend large healthy fawns, but are more apt to abandon small weak ones because the risks outweigh the benefits.

The important thing to recognize here is the importance of nutrition. If spring nutrition is not adequate, fawn birth weights will be low and fawn mortality rates will be high.

Other Considerations

As mentioned earlier, on average, even well fed does living on excellent deer range lose about 10 percent of their fawns. The accidental death rate of newborn fawns will be highest, however, where deer density is high and surplus mothers must attempt to raise fawns in mediocre or outright dangerous fawn-rearing habitat.

Disease and parasitism seldom pose a serious threat to well nourished newborn fawns on northern range. In the South, however, bacterial diseases, screwworm flies, stomach worms, lung worms, ticks, and assorted other infections have periodically caused serious problems for whitetails. Regardless of the region or the environment, such maladies will be most prevalent among malnourished whitetails living in high-density populations.

Young whitetails are well endowed with inherited anti-predator strategies that help minimize their detection and increase their chances of surviving early life. Still, fawns are lost to various predators, but the extent to which predation is expedited by sickness obscures the number of deaths caused solely by predators. The effects of disease and parasitism are especially difficult to separate from those of predation because disease-stricken and parasite-loaded fawns are less likely to escape from predators once detected. Also, sick fawns frequently have diarrhea or produce mucous secretions, the odors of which may attract sharp-nosed predators.

An overabundance of whitetails and the resultant overutilization of their range can lead to a reduction in both nutritious foods and fawn-hiding cover, thereby improving predator hunting success. In addition, because fawn-rearing does are territorial, a spillover of surplus does into unsuitable fawn-rearing habitat when deer density is high yields newborn fawns exceptionally vulnerable to abandonment and predation.

Conclusions

Clearly, the newborn fawn’s birth weight and chances of survival are determined primarily by the mother’s winter and spring nutrition. A loss of 10 percent of the fawn crop can be expected even under ideal conditions. And in the presence of effective predators, a 20 to 30 percent newborn fawn mortality rate is not unusual. However, when poorly fed pregnant does lose more than 25 percent of their body weight in winter, more than one-third of their fawns will probably die from nutritive failure, even when spring nutrition is excellent. When nutrition is poor during winter and spring, nearly all of the offspring may die shortly after birth.