Because whitetails are seasonal, short-day breeders, the timing of deer births is critically important for the species’ survival. Regardless of the environment, in the Northern hemisphere, whitetails are adapted to conceive at the proper time each autumn, so that fawns are born during spring (about 200 days later) when weather, food and cover conditions are favorable for maximum fawn survival.

Given the importance of this breeding/birthing schedule, reliable environmental cues are required to trigger the reproductive process in a timely fashion. Among whitetails, this cue is decreasing photoperiod — the ratio of daylight to darkness. Without fail, shortening periods of daylight in autumn cause physiological changes responsible for the whitetail’s breeding cycle.

If not for the infallible photoperiod cue, timing of the rut and fawn birth dates would fluctuate wildly from year to year and from one area to the next. Such a wide swing in the reproductive cycle would result in ridiculously early births some years and exceptionally late births during other years. In either case, newborn fawn survival would suffer.

Hence, natural selection has minimized poorly timed breeding/birthing schedules among whitetails. Breeding schedules that might hinge upon cues other than photoperiod, and contribute to untimely births, are soon lost from the gene pool because the resultant progeny die.

The Breeding Window

In the words of noted professor and deer researcher Karl V. Miller of the University of Georgia, “The whitetails’ breeding season can be viewed as a window of opportunity. Its opening and closing varies with latitude.” This window tends to be quite narrow in the North, but widens southward.

North of about 36 degrees latitude, most whitetails breed between mid-October and mid-December, with peak breeding occurring during November (hence, peak fawning occurs during late May and early June). If an adult doe does not become pregnant, she will probably recycle in 23 to 30 days. A Northern adult doe in top physical condition might even recycle a third time. However, the tight Northern breeding window is closely regulated by photoperiod, and promptly closes with the onset of cold weather, snow cover and reduced food availability.

By comparison, the Southern breeding window is potentially much wider. And I emphasize, potentially. The best managed deer herds exhibit relatively brief and intensive ruts, even in the South.

Between 28 and 36 degrees latitude, most whitetails breed between late September and late March. In some areas of the South, peak breeding occurs in November, but in other areas it does not occur until December or January.

In the South, an unbred doe might come into estrus as many as seven times in one season. This accounts for the South’s potentially long breeding season.

According to Miller, the late breeding in some regions of the Southeast might be self-perpetuating. “For instance, a fawn’s timing would have been affected by when its mother’s cues were set, and her mother’s, and so on,” he said.

Disruptive Forces

Many stressful factors might lead to a poorly timed or prolonged rut and affect a whitetail’s physical condition, reproductive performance and survival. High herd density, poor nutrition and social disorder, often the result of poor herd management (i.e., inadequate or excessive harvest of one or both sexes) are key factors that can disrupt and delay the whitetails’ breeding/birthing schedule.

Although the Northern rut tends to be brief and rigidly controlled by photoperiod, studies conducted in the North have shown that poor nutrition and high herd density can delay peak breeding by a week or two, which results in a number of consequences. Southern herds are equally affected by poor nutrition and herd density, but also seem to be more responsive to social imbalances.

For example, studies conducted by Professor David Guynn at Clemson University showed that unbalanced sex ratios and improper buck age structure can contribute to delayed and protracted breeding seasons in Southern deer herds. He concluded that the presence of mature bucks produced certain biostimulating effects on does that resulted in earlier and more synchronized estrous cycles. In his studies, proper deer harvesting to restore herd social balance resulted in less estrous recycling and earlier and shorter breeding seasons.

In contrast, we found no evidence of such biostimulating effects in our northern Michigan studies.

Depending upon the environment, deer might respond differently to various stresses. However, there is plenty of evidence that delayed and protracted ruts are detrimental to herd welfare. Invariably, proper herd management — aimed at improving nutrition and restoring natural social balance — will result in breeding/birthing schedules that favor deer’s physical development and maximum newborn fawn survival.

Social Order

Deer populations that are well-balanced — nutritionally and socially — exhibit a breeding order among does that helps maintain social order during the fawning season. Normally, older, maternally experienced does breed earlier than young, first-time breeders. This is important because it assures proper distribution of fawn-rearing does, orderly use of available fawning grounds and maintenance of cohesive kinship groups.

Because does aggressively defend fawn-rearing territories, competition for available space could be intense, and even chaotic, if not for orderly birthing. Ideally, older matriarchs should give birth first and reclaim their traditional fawning grounds. Younger related females should give birth a few days later and establish fawning territories nearby. Does fawning for their second time tend to disperse a short distance of a quarter-mile or so to establish new fawning territories and thereby expand the family’s ancestral range.

Buck age structure seems to be an important (and I might add, natural) factor in orderly mating of does during the rut. Although recent DNA studies indicate that all older bucks tend to do some mating, regardless of their dominance rank, a number of investigations suggest that does respond more favorably to mates closer to their own age. In the absence of mature bucks, as occurs in many areas of excessive buck harvesting, older does are more likely to shun the advances of younger sires and delay mating. On the other hand, yearling bucks might play a more instrumental role in the rut than formerly believed.

Miller postulates that primer pheromones deposited at rubs and scrapes by dominant bucks stimulate does and bring them into estrus early. Since younger bucks do minimal scent-marking, intense buck harvesting might contribute to less orderly mating and untimely birthing. In turn, this results in intense and chaotic competition among does for fawning space and leads to increased newborn fawn mortality.

Although rarely mentioned in the scientific literature, excessive doe harvesting might also cause social disruption by preventing the formation of cohesive kinship groups.

Because matriarch does are usually greater than 5 years old, intensive female harvesting and a shortage of these social leaders could cause a breakdown in natural grouping tendencies among related does. How this might impact breeding/birthing schedules is unknown. However, I’m willing to guess that such social disruption would be especially detrimental among migratory deer on Northern ranges.

Hiding Cover

Nursing does require ideal nutrition in order to nourish their rapidly growing fawns. However, because the young fawn’s chief defense against predation is hiding, dense, low-level vegetation is also critically important wherever whitetails and predators such as coyotes coexist.

Investigations conducted in South Texas rather clearly demonstrated how important fawn hiding cover can be. There, annual fawn mortality rates varied appreciably depending upon the amount of rainfall, which in turn determined the quality and availability of deer foods and fawn hiding cover during spring and summer.

Fawn losses ranged from 10 percent during years of heavy rainfall and good fawn hiding cover, to as much as 90 percent during years of drought and scant low-level vegetation. Predation of exposed fawns by coyotes was the primary factor causing high fawn mortality — as it would be in other populations if fawns were born before the flush of spring vegetation.

Prey Saturation

The importance of fawn birth dates was also demonstrated by University of Wyoming researchers Donald Whittaker and Frederick Lindzey. They studied the effect of coyote predation on the survival of 120 white-tailed and mule deer fawns in Colorado.

Tagged fawns born outside peak parturition, when density of newborns was low, had lower survival rates than those born during the peak.

According to Whittaker and Lindzey, “Of fawns surviving more than 30 days (11 whitetails and 49 mule deer) most (92 percent and 55 percent for mule and white-tailed deer, respectively) were born during or immediately after peak density of fawns … . Coyote predation accounted for 79 percent of early fawn mortality for both species. Date of parturition was the best predictor of early fawn survival. Because white-tailed deer fawns were born an average of eight to 10 days earlier, and mule deer outnumbered white-tailed deer 4:1, mule deer were afforded protection through predator swamping.”

Late-Born Southern Runts

If fawns are to survive their precarious early life, attain their maximum growth potential before the onset of winter and survive their first winter, they must be born at the proper time.

In the South, where whitetail breeding seasons are potentially long but winters are mild, even late-born fawns stand a good chance of surviving their first winter. However, late-born buck fawns, in particular, are at a distinct disadvantage. Compared to early born males, late-born individuals typically have a smaller body size and smaller antlers at yearling age. Noted scientist Harry Jacobson, from Mississippi State University, has shown that antler growth among yearlings in the South is closely related to fawn development during the first year of life.

“This becomes evident,” claims Jacobson, “when we look at the month a male fawn is born and his antler quality at 18 months. Antler data from deer raised in captivity show that date of birth is very important for antler quality. Regardless of birth date, all bucks cease antler growth for the year at about the same time. This takes place during September and October, when the antlers mineralize and velvet is shed. Thus, when fawns are born over a six-month period, as they are in some areas of the South, some have had only 11 months of growth to complete yearling antlers while others are as old as 16 months.”

In Jacobson’s studies, 89 percent of the yearling bucks born from September to November carried only spike antlers, whereas none of those born in June had spikes; 21 percent of the yearling bucks born in July and August grew spikes.

“Fortunately,” said Jacobson, “how a buck begins life may have little to do with antler qualities late in life. There is often little relationship between a buck’s antlers at age 5. A buck may get a bad start because of a later birth or poor nutrition but still can catch up in antler growth if adequate nutrition is available later.”

Unfortunately, however, late-born bucks often experience poor nutrition later in life. As a result, stunted, late-born yearling males quite often grow up to be sub-par bucks even at maturity.

Timing is Everything

Physical consequences of late birth are not limited to deer in the South. Even on Northern ranges, time for growth is extremely important, especially for bucks, because they have higher nutritional requirements for body growth compared to females. Even a week or two can make a big difference in a buck fawn’s body growth, should cold weather and snow cover restrict nutritious forage earlier than usual.

In fact, we found that birth date was even important for fawns provided with unlimited supplemental feed in our northern Michigan Cusino enclosure studies.

Quoting from an unpublished report: “Our data demonstrate the relative importance of birth dates between sexes of white-tailed deer. Males born earlier (i.e., in late May) had 10 percent greater mass the following spring at 9 months of age than late-born (after 6 June) males. Early born females also had greater mass the following spring than late-born individuals, but the difference between groups was less (3 percent). Although these data do not measure future fitness, they provide strong support to the hypothesis that birth date could differentially influence future fitness in male and female white-tailed deer.”

In the North, a fawn’s chances of surviving its first winter depend a great deal upon its body size going into winter. Small-bodied individuals are least likely to survive. Even those born during late June might be smaller and disadvantaged as compared to others born two or three weeks earlier. Hence, whether a Northern fawn survives its first winter might hinge heavily upon when it was conceived and when it was born — and even a couple of weeks might make a big difference.

Conclusions

The whitetail rut is primarily controlled by decreasing photoperiod, an adaptation that allows fawns to be born at the proper time in spring. This assures maximum newborn fawn survival and sufficient time for their physical development before winter.

Given the extreme importance of the timing of the rut — hence, birthing dates — delayed or protracted ruts are generally symptomatic of poor deer herd management. Inadequate nutrition, herd density stress and poor social structure are key factors responsible for an untimely rut.

Protracted ruts are a common malady in the South because of Southern whitetails’ potentially wide breeding window. However, even a week or two delay in peak rut can be detrimental to Northern deer.

Proper herd management — aimed at maintaining nutritional and social balance — will restore breeding/birthing schedules that assure maximum newborn fawn survival and favorable physical development.

— John Ozoga has been D&DH’s top research contributor for more than 30 years. He is a retired deer research biologist.