Whitetail Breeding Strategies – How They Vary from North to South

By John J. Ozoga

With few exceptions, deer-hunting seasons occur shortly before, during or shortly after the whitetail's breeding season – a hectic period we hunters call the rut, when whitetail bucks become recklessly active.

The timing of the breeding season is set by shortening days in autumn; in other words, changing photoperiod. However, many environmental, biological and behavioral factors might interact to determine when or if an individual doe will breed. Therefore, knowing how certain factors influence breeding conditions in an area – hence the vulnerability of bucks – could be critical to hunting success.

Hunters can apply some basic principles to rutting behavior across the whitetail's vast range. But they'll also see inherent differences. How a Northern whitetail responds to environmental pressures, social factors and nutritional changes sometimes mimic the responses of its Southern cousin. Then again, sometimes they don’t. Research shows some factors that control the rut's timing in Southern deer have little or no effect o rut behavior in Northern deer and, probably, vice versa.

Basics

Regardless of the environment, the timing of deer births is critical to the species survival. Breeding must occur so fawns are born when their survival chances are best. Natural selection has minimized poorly timed births. Unusual traits that contribute to untimely breeding are soon lost from the gene pool because the resultant offspring generally die.

The pineal gland – a small, pea-sized gland deep within the brain – measures the amount of daylight and responds to changing photoperiod. In darkness, this “third eye” secretes a hormone called melatonin, which influences the release of sex hormones from the pituitary gland. It is this response of the pineal gland to shortening day length in autumn that ensures timely breeding so fawns are born on schedule in spring.

Sex Segregation

Whitetails, mule deer, moose, elk, and other ungulates commonly display separation of the adult sexes outside of the mating season. Clearly, bucks and does live separately –  spatially and socially – except during the rut. Does live in matrilineal groups composed of older does, several generations of female offspring and young bucks. Adult bucks band together in all-male groups. Yearling bucks generally disperse from their respective family groups just prior to the rut.

Because the adult sexes live separately most of the year, important social changes take place during the pre-rut period. This is when bucks expand their range, interact with does, compete for dominance and advertise their established social rank. This requires an elaborate system of communication, involving scent marking in the form of rubs and scrapes, as well as other poorly understood forms of display and advertising.

The Narrow Northern Window

It's important to recognize that environmental pressures vary greatly by region. That means the timing and duration of breeding season can vary regionally. The whitetail's breeding season can be viewed as a “window of opportunity.” Its opening and closing varies by latitude. The whitetail’s breeding window is generally narrow in the North, where it's tightly regulated by photoperiod. North of 36 degrees latitude, most whitetails breed between mid-October and mid-December. Peak breeding usually occurs in mid-November.

A doe will accept a male only during peak estrus, which lasts 24 to 36 hours. If she is not bred or does not become pregnant, her cycle might recur in 23 to 30 days. If a Northern doe remains in peak physical condition but does not become pregnant, she might recycle a third time. In the North, however, a “negative energy balance” closes the breeding window once cold weather increases body heat-loss and snow cover reduces food availability. This doesn’t mean Northern does don’t sometimes breed unusually late. They do, often for unknown reasons.

Normally, we expect young does breeding for the first time to breed later than experienced does. Doe fawns (when 6 to 7 months old) in the North usually breed only under the most ideal conditions, usually around mid-December. But even yearling does (1 1/2 years old) tend to breed later in the rut.

The Wide Southern Window

By comparison, the Southern breeding window is potentially wide. A multitude of factors might combine to time breeding. Between 28 and 36 degrees latitude, for example, most whitetails breed between late September and late March. In some areas, peak breeding occurs in November, but in other areas not until December or January. Because the equator provides no seasonal photoperiod cues, deer in that region breed year-round.

Some late-breeding, late-birthing schedules common in Southern herds are caused by nutritional shortages and slow growth rates. In other cases, delayed breeding schedules apparently are caused by a shortage of mature bucks, which tends to be hunting-induced.

In milder Southern climates, an unbred adult doe might come into estrus as many as seven times in one season. This is one reason for the South’s potentially long whitetail breeding season. Therefore, in the South, late-breeding schedules might be advanced and better synchronized by changing deer harvest regulations. Those changes include carefully balancing deer numbers with available food and cover while allowing more males to reach maturity.

The Genetic Factor

There is some evidence that late breeding may be genetically linked and not easily altered. In Mississippi , for example, deer normally don't start breeding until November. Even when Michigan deer were transported to Mississippi , the Northern subspecies maintained its October and November breeding dates. Meanwhile, Mississippi pen-mates continued to breed in late December and January. Crossbred offspring of Michigan and Mississippi deer bred over the entire range of both parents.

Other researchers contend it isn’t known whether the breeding window is controlled genetically of it’s set after conception or birth. If photoperiod cues are set after conception, late breeding in some regions of the Southeast may be self-perpetuating. That is, a fawn’s timing would have been affected by when its mother’s cues were set, and her mother’s, and so on.

Nutrition

An individual doe’s breeding date might be affected by many factors. However, no single factor is likely more influential than nutrition. The quality of a doe’s diet will determine her growth rate, the timing of her sexual maturity and, therefore, her breeding date – no matter the region.

Doe fawns only breed under the most ideal conditions. Only those that are born early and grow big and fat by autumn achieve puberty and then breed. Even on Northern range, they generally don’t breed until mid-December, or even later in some cases. Therefore, this phenomenon can also cause a protracted rut.

Malnourished adult does, burdened with nursing fawns, and yearling does, commonly breed later than normal, especially on poor quality Southern range. However, they sometimes fail to breed on Northern range because of the narrow breeding window, especially when a harsh winter begins early.

Ironically, there is some experimental evidence that obesity might also delay a doe's estrus. In domestic livestock, for example, fat might infiltrate the ovaries so much that it hinders follicular development. The result is irregular or cessation of estrus, and a delay or failure to breed.

Although obesity might be a problem among penned-raised deer, especially 2 1/2 year olds that did not raise fawns, it probably rarely occurs among free-ranging deer.

Behavioral Stress

All deer compete aggressively for food, shelter and space. Losing is stressful and costly. Among does, one consequence of subordination, especially in high-density herds, is delayed breeding among low-ranking does.

A young doe that maintains close association with its mother will also play a subordinate, low-ranking family role. Social stress by maternal domination apparently causes a hormonal imbalance that interferes with a doe’s reproductive functions. Such stress tends to increase progesterone production from the adrenal glands, which can block the effects of estradiol-induced hormone surges during estrus. That interrupts ovulation and leads to late breeding.

Yearling does tend to remain subordinate to their mothers and, on average breed about one week later than mature does. However, even 2 1/2-year-old does that fail to raise fawns might revert to yearling behavior by seeking their mother’s leadership (and domination), and continue to breed late. Therefore, even when well-fed, social factors might delay breeding among young does when doe densities are high.

When poor nutrition is a factor, it's easy to see how the rut’s peak could be delayed by several weeks, especially in the South.

Rut Sign

Rubs, scrapes and other evidence of early buck rutting activity might indicate breeding among does. In the North, such activity usually starts in September and precedes active breeding by one or two months. In the South, bucks and does might be ready to reproduce about the same time, possibly as soon as early September. Therefore, a flurry of scraping activity in late September might indicate some Southern does are in breeding condition.

Most rubbing and scraping is done by bucks 2 1/2 years old and older. Therefore, delayed and less-intense buck rutting activity can be expected in socially unbalanced herds because few mature bucks are present. Such a tendency might have nothing to do with the timing of doe breeding.

In my northern Michigan studies, yearling bucks only made 15 percent as many scrapes and half as many rubs as mature bucks. Further, while mature bucks started scraping in September, yearlings made no scrapes until late October, about one week before the first does bred.

In northern Georgia , researchers led by Karl Miller recorded buck-rub densities ranging from 474 to 1,502 rubs per square mile. They also found rub density was closely related to the number of bucks older than 2 1/2 years in the herd.

Interestingly, in the latter study, rub density changed each year depending on acorn abundance, a critical autumn deer food in the study area. More rubs were recorded in years of good acorn production, when rubs were also more concentrated in oak habitats.

Priming Pheromones

Buck signposts, in the form of rubs and scrapes, are visual and olfactory signals. That is, they’re showy in nature and are scent-marked with various secretions. Unlike other communications, signposts are extensions of the animal, remaining functional for long periods, even in the maker’s absence. Thus, signposts convey long-lasting messages that likely have physiological and psychological effects on other deer.

Miller and fellow Georgia researcher Larry Marchinton propose that signposting by dominant bucks plays a vital role in maintaining a herd’s social harmony. These professors suggest that primer pheromones – which produce a physiological response – deposited by dominant bucks at rubs and scrapes help bring adult does into estrus early.

Even in northern Michigan , confining bucks and does together in autumn advanced the mean breeding dates by eight to nine days. The exact mechanisms involved were unknown, but observations suggested close, unnatural confinement of bucks with does had some type of biostimulating effect. It could have been caused by pheromones produced by males, which induced earlier ovulation.

Mount Holly – A Southern Study

Studies by Professor David Guynn at Clemson University found that delayed and protracted whitetail-breeding seasons can be advanced and shortened when nutritional and social factors are considered in herd management. They can, at least, in a Southern environment.

In Guynn’s Mount Holly study area in South Carolina , the herd was initially typical of many Southern herds; deer densities were high, but bucks had been heavily exploited. The unbalanced adult sex ratio heavily favored females, and the breeding season was late and long (96 days), with some fawns born in September.

The researchers came in and selectively shot deer. Their goals were to better balance deer numbers with food and cover resources, to decrease the proportions of does, and to increase the proportion of bucks. Within five years, the rut at Mount Holly became more intense and shortened to 43 days. Mean conception dates shifted from Nov. 11 in the first year of study to Oct. 15 in the final year.

Improved nutrition was one reason for the earlier and shorter breeding periods. A better balance in sex ratio also likely contributed to less estrus recycling. However, increased biostimulation of females by mature bucks was also considered important, because the availability of males to breed females could not by itself advance the breeding season.

Cusino Enclosure – A Northern Study

According to studies conducted by Miller and his co-workers, yearling bucks reach sex organ development and hormone production about one month later than prime-aged bucks. These findings closely mirror my studies that found delayed scent-marking behavior by yearling bucks.

The obvious question is this: Can inexperienced young bucks handle the job of sires in herds where intensive harvesting removes most older bucks?

We put yearling bucks to the test in the Cusino square-mile enclosure by removing all older bucks for three years. This study was possible because we could live-trap all deer from the area annually, release surplus animals outside, and shape reintroduced populations according to our study needs.

Compared to mature bucks, the yearling bucks fought more, even with does; failed to establish a strict dominance hierarchy before the rut; lacked a ritualistic courtship style by literally chasing every doe in sight during the rut; and exhibited poor scent-marking behavior.

Still, despite sexual inexperience and seemingly inferior behavior, yearling bucks serviced most does on schedule, and produced as many offspring as could have been produced by mature sires. Surprisingly, we saw a tendency by yearling does to breed earlier when serviced by yearlings instead of mature bucks, while some older does conceived unusually late when bred by yearlings.

These findings, of course, differed considerably from those at Mount Holly . In the North, it appears the rut’s timing is much more closely regulated by photoperiod. Scent-marking, priming pheromones and biostimulation than in the South, less influences it.

It is important to point out, however, that deer in our northern Michigan study were exceptionally well-fed. Had they been malnourished, the results could have been different.

Conclusion

The precise timing of the rut varies considerably north to south across the species’ range, depending on many poorly understood factors. Although the Northern rut is brief and rigidly controlled by photoperiod, poor nutrition and high herd densities can delay peak breeding by a week or two.

Southern herds seem more sensitive to social imbalances. When mature bucks are few, breeding in high-density, malnourished Southern herds becomes even more delayed and the rut more prolonged. Some believe this tendency for late-breeding and late-birthing then becomes self-perpetuating.

Whatever the reason for late breeding, many late-born Southern fawns become small, poor-quality adults. Such a scenario in  the North yields many small fawns ill-prepared for harsh winters, which lead to excessive winter-kill.