White-tailed deer are one of the most prolific deer species on Earth. Aside from being able to adapt to a wide variety of habitat conditions and environmental pressures, they can breed at a young age, produce more than one offspring per year and give birth at the most favorable time of the year. Given the proper circumstances, some whitetails can even breed when 6 to 8 months old. This special precocious trait allows deer populations to explode when conditions are right.

By John Ozoga

Whether a whitetail (male or female) reaches puberty and breeds in its first year will depend upon many poorly understood factors. Genetics, climate, nutrition, birth date, day length and social environment are some factors that can be involved. Although researchers still debate this phenomenon, they generally agree that late-born and malnourished individuals rarely achieve puberty and breed before 1 year of age. The mystical effects of deer social structure and density are not particularly clear. From a management perspective, it’s important to recognize that whitetails are capable of such an advanced rate of maturity, generally signaling favorable habitat and proper herd management. Most important, precocious fawns tend to become superior adults at a young age, making this phenomenon worthy of study and perpetuating when the management goal is to produce healthy and productive deer populations.

The Antlered Fawn
A buck fawn’s pedicles usually don’t become pronounced until the fawn is about 4 or 5 months old. That’s when the healthy fawn’s testes produce sufficient amounts of the male hormone testosterone to stimulate the laying down of additional pedicle bone. This process is closely linked to the fawn’s rate of maturity, testes development and ability to produce a certain amount of testosterone. It is not keyed to seasonal light cycles, as is future antler growth.

Some buck fawns might develop prominent pedicles topped with hardened button (that is, infant) antlers when 6 to 8 months old. Because such early antler development requires high levels of testosterone, buck fawns that develop infant antlers are presumably physiologically advanced and probably fertile. Conversely, late-born or severely stressed male fawns might not even grow pedicles during their first autumn.

The late antler researcher Richard Goss referred to infant antlers as a “unique zoological structure,” because it is impossible to pinpoint the exact time when pedicle growth gives way to antler development.

Infant Antler Research
Studies conducted at the Cusino Wildlife Research Station in Upper Peninsula Michigan found that well-nourished pen-raised buck fawns routinely grew infant antlers by mid-December. Likewise, 84 percent of the buck fawns raised in the Cusino square-mile enclosure grew button antlers when provided with unlimited supplemental feed. Most achieved antler calcification and polishing during early December, and all cast their small antlers before March.

By comparison, Harry Jacobson reported that only 20 percent of more than 200 buck fawns held in research facilities at Mississippi State University developed button antlers their first year.

Interestingly, Jacobson noted, “Not one of these was seen before the fourth week of January, and most developed these antlers in February and March. In other words, none could have been classified as legal antlered bucks during the deer hunting season.”

The frequency of infant antlers — hence achievement of puberty — among free-ranging buck fawns has not been intensively studied and is largely unknown. General observations suggest that only 5 to 10 percent of the buck fawns in most wild herds achieve polished infant antlers. But there are exceptions.

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About 37 percent of 130 buck fawns collected in January from the Crab Orchard National Wildlife Refuge in Illinois were presumed fertile based on the presence of polished button antlers. This high rate was obviously influenced by excellent nutrition. However, investigators concluded that late-born male fawns had minimal chance of achieving puberty.

Apparently, sexual maturity among buck fawns is a function of body weight and age. For male fawns to become fertile when 6 to 8 months old, they must be born early — during late May or early June — enjoy a high level of nutrition and weigh 90 pounds or more by December.

This phenomenon appears to be more prevalent in the Midwest and in Northern range, where it also tends to occur earlier, as compared to farther south. Although buck fawns have been known to impregnate doe fawns in captivity, their role as sires in the wild is unknown.

Researchers suggest that growth of infant antlers among buck fawns might be more prevalent than hunters realize, because most such development occurs after hunting seasons.

The Pregnant Fawn
Some female fawns achieve puberty and breed, a phenomenon that has been quite intensively studied with captive animals and documented in free-rang- ing deer populations. After intensive study of deer population dynamics in the 2-square-mile George Reserve deer enclosure in southern Michigan, Dale McCullough concluded that doe fawns breed only during superb conditions, when herds are well below carrying capacity and most newborn fawns survive.

To achieve puberty and breed at ages 6 to 8 months, female fawns must reach a certain critical body size. In the North, that size is about 80 to 90 pounds. For smaller Southern deer, it’s about 70 pounds. That doesn’t mean all animals reaching or exceeding these weights breed and reproduce, not even at low herd density.

Based upon studies at Cusino, it’s also important to recognize that a high incidence of precocious male fawns does not necessarily mean that a high proportion of female fawns in the same population breed. For example, in the Cusino enclosure, 84 percent of the male fawns grew infant antlers. At the same time, only 7 percent of the doe fawns conceived young.

Fawn Pregnancy Rates
The highest fawn pregnancy rate is in the Midwest. More than half of doe fawns typically breed in agricultural areas of Illinois, Iowa, Kansas, Missouri, Nebraska, Ohio, North Dakota and South Dakota, as well as in southern regions of Michigan, Minnesota and Wisconsin.

In Iowa, researchers found more than 70 percent of the doe fawns bred and that they carried an average of 1.25 fetuses per doe. Some even carried triplets.

Fewer doe fawns breed in Southern states. Only 10 to 40 percent reportedly breed in the Southeast, even in the best range. Likewise, less than 16 percent of doe fawns likely breed in Texas’ Llano Basin.

Doe fawns are also less likely to breed near the whitetail’s northern range limits. There, climate and range quality can vary greatly each year across relatively short distances — even in the same area.

In New York’s Adirondacks, only about 4 percent of doe fawns breed. But in southern New York, where forage quantity and quality is better, more than 36 percent of doe fawns become pregnant. Likewise, in southern Michigan, more than half of the doe fawns usually breed each year. In comparison, less than 5 percent become pregnant in Upper Michigan. Similar trends occur in Wisconsin and Minnesota.

Although some such differences are caused by nutrition, some researchers speculate that environmental factors — such as temperature and day length — play a role.

Genetics might also be a factor in some areas. However, genetics don’t explain the different breeding potential between fawns in northern and southern Michigan. The ancestral stock of south- ern Michigan’s George Reserve came from Grand Island, in Lake Superior.

Doe Fawn Research
Most of the whitetail’s seasonal physiological events — such as fattening, reproduction, antler growth, body growth and coat molt — are cued to seasonal changes in daylight, or photoperiod. This has led to speculation that regional differences in daylight amounts might affect puberty attainment in young deer.

Researchers in southern Michigan studied doe fawns raised in light-controlled chambers to evaluate photoperiod’s effects on puberty. Fawns raised on 16 hours of light and eight hours of darkness were placed in two test groups at age 4 months. One group was switched to eight hours of light in mid-October. The other group remained on 16 hours of light until December and then switched to eight hours of light.

The researchers found that fawns experiencing an earlier switch to short days were more advanced and more likely to breed. In fact, seven of eight early short-day fawns achieved sexual maturity in January or February, whereas none of those in the extended daylight group sexually matured.

Studies at Cusino, in Upper Michigan, produced contrasting results. My colleagues and I subjected one group of 4-month-old doe fawns to extended daylight by using overhead lights to illuminate outdoor pens to simulate longer days. Our control group was also confined outdoors but with natural daylight length. The study’s photoperiod phase lasted nine weeks. A mature buck was then placed in each pen until March.

During the study, 19 of 29 test fawns bred. Sixty-one percent of the long-day fawns bred compared to nearly 73 percent of the natural-day fawns. As result, the similar reproductive performance indicates factors other than photoperiod must cause poor breeding success in Northern doe fawns.

We found only one striking difference in reproductive performance between doe-fawn groups in our study: widely varying estrus dates according to experimental treatment. That is, long-day fawns had a prolonged breed- ing season, extending from Nov. 24 until Feb. 20. By comparison, all fawns exposed to natural daylight bred on or before Jan. 20.

Obviously, doe fawns must be fat and skeletally large to achieve sexual maturity. Under varying nutrition levels, it’s equally possible to find large, lean fawns and small, fat ones, none of which could be expected to breed. In our Cusino study, doe fawns that became pregnant were heavier and demonstrated greater skeletal growth, as compared to nonbreeders. We suspect, therefore, that doe fawns must achieve a certain critical fat to lean body composition to achieve puberty.

The timing and extent of fat deposits seem to play an important role in attaining puberty. Shortening days in autumn tend to trigger seasonal events such as fattening and pelage change. Therefore, long-day fawns studied in northern Michigan probably did not achieve the critical fat/lean body composition necessary to induce puberty until they were switched to natural daylight. This, in turn, likely accounted for their delayed puberty and breeding.

We concluded that body weights of wild fawns on Northern range become stable in mid-autumn because of marginal nutrition and the onset of harsh weather. For example, we found that as little as a 10 percent reduction in caloric intake for fawns in autumn seriously curbed their fattening and skeletal growth. Therefore, in Northern range, a natural decrease in high-energy foods, rather than decreasing daylight, probably halts the fawn’s sexual development.

Social Effects
In some cases, whitetail social structure and density stress can be just as important as nutrition in determining an individual deer’s well-being. In our enclosure studies, for example, as deer density increased to more than 100 deer per square-mile, antler development among yearling bucks declined, as did conception rates among yearling does and survival rates among fawns born to first-time mothers — even when the herd was supplementally fed. Hence, there is good reason to believe that social factors also play a role in puberty achievement among fawns.

As noted, female fawns seem to respond more sensitively to social effects than do male fawns. Even at high herd density and complex herd social structure, many male fawns grew infant antlers (that is, indicative of sexual maturity), but doe fawns rarely bred despite their favorable physical traits.

It’s important to note that Cusino’s enclosure deer lived more natural social lives than pen-raised deer. Having mature bucks in constant close contact with young females in small pens is unnatural. I suspect that differences in social behavior largely accounted for the gross difference in breeding performance of penned versus enclosure doe fawns recorded in our Northern studies.

Several possible social factors might explain why pen-raised fawns bred but enclosure fawns did not, despite compa- rable weather conditions and nutritious diets. The unnatural confinement of bucks with doe fawns in pens might have a biostimulating effect, which induced breeding.

In other studies, we observed that confining bucks and mature does together in autumn tends to advance mean breeding dates by eight to nine days. It’s possible the close (unnatural) contact with sexually active bucks also stimulated and advanced puberty onset in pen-raised doe fawns.

McCullough surmised that herd reduction in the George Reserve contributed to better physical condition and, hence, increased pregnancy rates among fawns. On the other hand, you could argue that fawns were freed from maternal suppression as the herd was purposely decimated by a mid- December hunt, and that social factors were just as important as nutrition in triggering puberty in doe fawns.

Also, McCullough observed that production of triplets by adult does and breeding by fawn does occurred at about the same herd densities. There- fore, he believed the occurrence of trip- lets among adult does might be a useful clue to breeding of doe fawns in wild herds.

However, in our Cusino enclosure studies, doe fawns seldom bred, despite the common occurrence of triplets raised by adult does.

A Texas study confirmed that orphaning plays a critical role in doe fawn breeding, even in a Southern environment. Two of four doe fawns deliberately orphaned in mid-autumn subsequently bred, but none of five control fawns achieved puberty.

Although southern Michigan deer numbers increased dramatically in the 1980s and 1990s, the incidence of doe-fawn breeding declined. Researcher Lou Verme argued that harvest rates did not keep pace with the expanding southern Michigan herd. As a result, he theorized that the decreased pregnancy rates among doe fawns stemmed from increased maternal domination, hence social stress not nutritional deficiency.

Conclusions
Achievement of puberty among whitetail fawns is a complex phenomenon. Clearly, fawns must be fat and skeletally large to achieve sexual maturity, whereas late-born or malnourished individuals seldom, if ever, achieve such an advanced physical state. Also, the sexes respond differently to stress factors.

Precocious fawns appear to be more prevalent in the Midwest, where deer herds are maintained below carrying capacity. However, researchers still debate environmental effects such as climate and photoperiod, as well as genetics and social factors.

Although birth date and nutrition are prime factors involved, biosocial factors coincident with herd eruption or decline can influence achievement of puberty among doe fawns. On the other hand, given good nutrition, male fawns seem less responsive to social stress.

There is little doubt that precocious fawns are superior in all respects, and more likely survive stressful Northern winters, compared to fawns that do not achieve puberty. Also, there is good reason to believe that they become physically superior and more productive yearlings. How this phenomenon influences whitetail physical development and productivity at maturity is unknown.

References
Miller, K.V. and R.L. Marchinton, (eds.). 1995. Quality Whitetails: The Why and How of Quality Deer Management. Stackpole, Mechanicsburg, Pa.
Durkin, P. (ed.). 2000. John Ozoga’s Whitetail Intrigue. Krause Publications Inc., Iola, Wis.