Debating reasons for variations in yearling buck antler size
June 01, 2009
Antlers grown by yearling (1.5 years old) bucks can vary in size from barely visible spikes to respectable multiple-tined racks scoring in excess of 100 points on the Boone and Crockett scale. Given this antler growth potential, wildlife managers and hunters become frustrated when a high percentage of yearling bucks carry only spike antlers.
Wildlife managers like to see fewer than 25 percent spike antlers among yearling bucks, but in some areas more than 70 percent of the yearlings are spiked.
According to James Kroll, director for the Texas-based Institute for White-tailed Deer Research, "While spikes are going to occur in every deer herd, anything more than 15% spike antlered yearlings is a symptom of serious problems."
Antler size among yearling bucks is generally related to the individual's body size. That is, the larger the buck in skeletal size and body weight, the larger the antlers and the longer the main beam. Although small-bodied individuals rarely grow huge antlers, large-bodied yearlings sometimes grow very small spike antlers.
Traditionally, investigators have agreed that nutrition, birth date, and genetics are the main factors that interact to influence young buck antler size. But, in special cases, density (social) stress, physical injury, and disease and parasites can hinder antler growth. Even the mother's nurturing ability (maternal environment) can be important.
Some investigators contend yearling buck antler size is governed chiefly by environmental factors, and is a good indicator of deer herd health and well-being; small antlers signal the need for changes in habitat and/or deer herd management strategies. Meanwhile, others insist genetics is the overriding factor influencing yearling buck antler size and that selective harvesting (culling) of inferior bucks is necessary to improve antler size.
Hence, there still is considerable debate among researchers as to why the antlers of yearling bucks tend to be so variable in size (and shape) and whether or not such variation influences subsequent antler development.
The deer antler actually consists of two parts, the pedicle and the antler. The pedicle, or stump, on which the antler develops, is a permanent structure. The antler is temporary, in that it falls off and is replaced annually.
Before a deer can grow antlers, it must first grow pedicles. In young deer, these are protruding, bony structures as much as an inch long. In adult bucks they become shortened and remodeled into the expanding skull.
The cellular make up of the pedicle differs from that of the surrounding bone in that the pedicle bone is spongy, instead of being composed of bony plates. It is the pedicle that gives rise to the antler. However, auxiliary antlers sometimes grow from surrounding parts of the forehead or upper orbit of a buck's eye when the pedicle is severely damaged.
Although the antler cycle is strictly controlled by seasonal changes in photoperiod (amount of daylight), growth of the pedicle is not. Pedicle development is linked more to the young buck's rate of sexual maturity. It is the production of the male hormone testosterone that stimulates additional bone deposition at the pedicle site.
Various circumstances might prevent a male fawn from attaining the critical body size necessary for normal sexual development and proper pedicle formation prior to winter. Primary factors affecting this rate of physical development include the fawn's birth date, level of nutrition, and social stress.
In white-tailed deer, pedicles usually don't become pronounced "bumps" until the fawn is about 4 or 5 months old. This is the age at which a healthy young male's testes develop and start producing enough testosterone to trigger pedicle development.
Initially, the pedicles are covered with skin and hair, similar to the rest of the deer's scalp. However, as the pedicles enlarge, the tips more closely resemble the growing antlers of adult bucks. The skin then becomes shiny and only sparsely covered with hairs.
According to noted scientist George Bubenik, "One of the unsolved questions in antler development is the fact that testosterone seems to promote growth of the pedicle but retards growth of antlers." That is, some threshold level of testosterone must be achieved before the young male can develop pedicles in autumn. However, in adult deer antlers start growing in spring when testosterone production is low, and high levels of testosterone during early autumn cause antlers to cease growing and mineralize.
Therefore, young male deer must be born on schedule and be properly nourished in order to achieve a certain threshold body weight and level of maturity before the testes can produce enough testosterone to initiate pedicle formation. Also, although poorly understood, even social (crowding) stress may lead to excessive production of cortisol or other hormones that block the effects of testosterone and impair pedicle development.
For example, we found that 5 percent of the buck fawns examined from a high density deer herd in Upper Michigan failed to develop pedicles by late November.
Buck fawns that fail to grow pedicles, or grow only small ones, then must make up additional pedicle growth the next spring before antler growth can commence. Since antler growth is controlled by daylight cycles, these retarded individuals simply do not have enough time to grow pedicles as well as sizable antlers, resulting in very small antlers at yearling age — regardless of their genetic potential.
While most bucks start growing their first antlers in spring, when about 11 months old, some male fawns may grow, polish, and cast small "button" antlers before 9 months of age. These so-called "infant antlers," which are generally less than one-half inch long, tend to be grown by early-born, well-nourished and healthy buck fawns — and are a sure sign of advanced maturity, indicative of puberty.
Because it is such a gradual process, it is impossible to determine when pedicle growth gives way to antler growth. Presumably, some upper level of testosterone is achieved to cause mineralization of the pedicle tip. At any rate, according to antler specialist Richard Goss, the infant antler is a "unique zoological structure," quite different from the adult antler.
Most biologists contend that fewer than 10 percent of whitetail buck fawns grow infant antlers. However, researcher Harry Jacobson reported that 20 percent of the buck fawns raised in research facilities at Mississippi State University grew infant antlers. He noted that none of the buck fawns "rubbed out" before the end of January, and some not until March.
In contrast, 84 percent of the supplementally-fed buck fawns in my northern Michigan enclosure studies grew infant antlers. But unlike in the South, the Michigan fawns polished their antlers from late November until early January, then cast them in February.
Early maturing fawns are undoubtedly physically superior in all respects as compared to fawns that fail to grow pedicles or do not grow infant antlers. In effect, young males that grow infant antlers then grow their second set of antlers at yearling age — and given favorable circumstances, likely produce superior antlers.
Clearly, infant antlers are the result of an early birth and favorable environmental conditions. While genetics may be involved, there is no evidence for an infant antler gene.
Unfortunately, the occurrence of short-spiked (less than 3 inches long) yearling bucks is widespread. Most are found in areas of high deer density, where deer experience nutritional shortage and where deer are socially stressed by high density, or where poor herd management contributes to late births.
In 1992, Michigan biologists examined more than 300 short-spike yearling bucks harvested by hunters throughout the state. Short-spiking rates were highest, ranging from 10 to 33 percent, in 7 counties where deer densities exceeded 50 deer per square mile. By comparison, short-spikes were rare in southern Michigan where deer had better nutrition (despite the occurrence of more late-birthing doe fawns).
In the central Adirondack region of New York, pioneer researcher William Severinghaus reported that more than 50 percent of the yearling bucks he examined in the late 1940s carried spike antlers less than 3 inches long. At the same time, he found fewer than 3 percent short-spikes among yearlings from the western Adirondacks where deer had better nutrition.
Likewise, in the flatwood country of northwestern Florida, Steve Shea and his coworkers found that 78 percent of the yearling bucks grew spike antlers less than 5 inches long. Despite a 75 percent decline in deer numbers, body and antler size among yearlings did not improve because of the low-quality forage growing on the infertile soil.
Even with unlimited, year-round supplemental feeding, once deer density surpassed 100 deer per square-mile in my northern Michigan enclosure studies, 23 percent of the yearling bucks grew short-spike antlers. Presumably, social stress prior to 9 months of age produced physiological consequences resulting in impaired pedicle development and led to unusually small antlers even among well-nourished yearling bucks.
There are two types of short-spike antlers, those with a flared antler base referred to as the burr (or cornet) and those without a burr. Antler cornets require an elevated production of testosterone, develop during late stages of antler growth, and presumably provide additional protection for blood vessels around the pedicle. The two types occur for different reasons.
Burrless short-spike antlers are small, pencil-like structures. They tend to have a uniform thickness, lack roughness (pearling) around the base, and have a blunt round tip. Since they are virtually elongated pedicles, similar to infant antlers, it's difficult to determine where the pedicle ends and the antler begins.
This condition arises most commonly among small-bodied yearling bucks that did not achieve proper pedicle development, or possibly had no pedicles at all, prior to 9 months of age. Late-born, physically stunted, and socially stressed fawns typically grow this type of short-spike. On northern range, few such animals likely survive their first winter, if it is a severe one.
Even some large-bodied yearlings on northern range may grow short-spike antlers with a totally different appearance. Typically, antlers carried by these yearlings are wide at the base, because of a prominent cornet, but come to an abrupt point.
Yearling bucks with this type of short-spike were probably born on schedule and experienced good nutrition during summer and autumn — they may even have grown infant antlers — but then endured extreme nutritional hardship during the winter and/or spring periods. Since most of their spring and summer nutrition had to be channeled into restoring energy reserves and body growth, few nutrients were left for antler growth.
Short-spike antlers do not appear to be hereditary. Instead, an abundance of short-spike yearling bucks is a sign of an unhealthy deer herd, one that is either nutritionally or socially out-of-balance.
The old adage, "The head grows according to the pasture, good or otherwise," holds true for whitetail bucks throughout their range. Also, when food resources are limited, body growth takes precedence over antler growth among young bucks — meaning malnourished young bucks will be retarded in sexual development and grow small antlers.
It should also be obvious that various factors, especially fawn birth dates, mother's age, autumn nutrition, and spring nutrition, interact to impact yearling buck body and antler size — and signal a deer herd's health status.
Biologists routinely record antler beam diameter, beam circumference, antler spread, and length of the main beam of yearling bucks harvested by hunters to determine the health status of deer herds. Regression equations using antler beam diameter measurements, in particular, have been used to predict yearling buck body weights and reproductive rates of females.
In New York, for example, when yearling buck antler beam diameters averaged only 12 millimeters, dressed weights of yearling bucks averaged 72.7 pounds and no doe fawns were expected to breed. At the same time, on average, yearling does
carried 0.47 fetuses per doe and older does carried 1.33 fetuses per doe. By comparison, when beam diameters averaged 22 millimeters, yearling bucks averaged 133.3 pounds dressed weight and doe fawns carried an average of 0.58 fetuses per doe; yearling does carried 1.98 fetuses per doe and older does 2.05 fetuses per doe.
Even when nutrition is unlimited, mature does tend to produce superior male fawns at weaning age. When deer are socially and nutritionally stressed at high population density, young does will breed and give birth several weeks later than normal, and raise smaller buck fawns, as compared to older does. As a result, even greater differences in yearling buck body weights and antler size can be expected, with more of them carrying small antlers, as deer become overly abundant.
Given a potentially wide breeding window, late birthing as the result of poor nutrition (or estrus recycling due to a shortage of bucks) can become a more serious and self-perpetuating problem in the South.
In the North, studies have shown that winter nutrition has minimal effect on antler size, whereas summer, autumn, and spring restrictions in diet will yield under sized yearling bucks with small antlers. Hence, improved yearling antler quality is predicted following abundant summer forage, heavy autumn mast crops, late onset of snowcover, and/or early snow melt and early green up.
In northern Michigan, buck fawns exhibit improved pedicle development during years of good acorn crops, and tend to grow superior antlers at yearling age.
Likewise, Michigan state University animal husbandry scientist Duane Ullrey found high protein and adequate energy supplies during March, just before the start of antler development, are especially important for antler growth in yearling bucks.
Ullrey also conducted a series of studies with captive fawns, simulating the effects of early or late green-up upon yearling buck antler development as related to their birth dates. As expected, he found that early (May) born fawns enjoying early (mid-March) green up grew the best antlers at yearling age. In contrast, late (June or July) born fawns subjected to late (mid-April) green up grew the poorest antlers at yearling age. Even so, birth date seemed to have more impact upon yearling antler size than did the timing of green up.
There is no denying that antler development is hereditary to some extent. However, the true genetic effects are often obscured by nutrition, maternal effects, and a host of other factors. And don't forget, the mother contributes half of the genes for antlers. As a result, the genetic basis of variation in antlers is complex and poorly understood.
Because antlers are expensive to grow, nutritionally speaking, Oklahoma State University researcher Stephen Ditchkoff believes antler size and configuration serve to signal a young buck's genetic quality — and permits a buck to advertise his "good genes."
Ditchkoff found that high quality, healthy young bucks (especially yearlings) usually grew well-balanced (symmetrical) antlers, with comparatively high Boone and Crockett scores. By comparison, genetically inferior individuals more frequently grew smaller, unbalanced (asymmetrical) antlers. Also, as relative antler asymmetry increased, yearling buck body weight, body length and skull length declined — suggesting a strong relationship between buck physical condition, antler traits and genetic quality.
Controlled breeding studies conducted by Donnie Harmel, in Texas, also revealed that a buck's body size and antler size can be influenced by genetics. Spiked yearling bucks he studied more often sired spiked sons than did fork antlered yearling sires. But many scientists question these finding because of small sample sizes and other experimental irregularities.
The Culling Debate
Selectively harvesting (culling) spike bucks, in hopes of removing inferior antler genes, has been widely practiced as a means of improving antler quality. But many biologists question the success of such practices, and some contend that selective harvesting can produce certain consequences.
The premise behind spike culling is that yearling bucks with spike antlers are genetically inferior and will never attain quality antlers typical of yearlings that grow forked antlers. Can selection against spike bucks be effective in changing the genetic character of the herd?
Not so says Harry Jacobson. To the contrary, he observed that spike antlered yearlings in his Mississippi research pens frequently grew exceptionally large antlers at maturity.
Jacobson and Texas A&M animal geneticist Steven Lukefahr teamed up, using a special computer program, to determine the genetic and environmental factors responsible for antler traits of 220 yearling bucks Jacobson had raised at his research facility.
In a nutshell, this research revealed that genetics accounted for only 5 percent of spike antler traits recorded among yearlings. Instead, the doe's nurturing ability and care of her offspring was more important, accounting for 29 percent to 34 percent of the variation in the yearling's antler points, spread, weight and beam length.
Jacobson and Lukefahr concluded the following: "Our results do not support the use of yearling antler records as criteria for selective breeding management or harvest schemes to alter the genetic quality of a white-tailed deer population."
More recently, Mitchell Lockwood and his cohorts from the Texas Parks and Wildlife Department completed an 8-year study to determine how genetic and environmental factors interact to influence yearling buck antler quality — and produced results different from those of Jacobson and Lukefahr.
In the Texas experiments, buck fawns were nutritionally stressed by being fed a restricted (half-ration) diet consisting of only 8 percent protein. Each year they then selected the best antlered yearling bucks as sires, to determine if such selective breeding would yield yearling bucks with superior antlers in subsequent years.
From 1993 to 1999, such selection produced improvements in all recorded antler measurements. For example, average Boone and Crockett yearling buck antler scores increased by 36.4 inches.
According to Lockwood and his group, "Our findings clearly indicate that under constant suboptimal environmental conditions, phenotypic change in antler quality can be realized with intensive selective harvest of yearling males."
These same researchers, and others, also contest the idea of protecting only small antlered deer from harvest, because it allows the harvest of yearlings with superior antlers but protects even older bucks with poor antlers. This (high-grading) tends to degrade antler quality in subsequent years.
Instead, they favor the so-called "slot-limit" approach, which protects medium-sized bucks. Such a harvest strategy allows for harvesting mature bucks with antlers larger than some pre-determined spread, and encourages the harvest of the poorest quality young bucks with unbranched antlers. At the same time this protects the best quality young bucks and minimizes the adverse effects of high-grading.
Clearly, many factors can interact to determine the quality of yearling buck antlers. Although inheritance of antler traits may be important, genetic effects are often masked by a multitude of environmental influences.
In my view, variations in climate, nutrition, social behavior, birth date and nurturing, as well as other factors that impact the young male's rate of sexual maturity, are more important than genetics in determining the quality of a buck's first set of antlers.
Ironically, after more than three decades of study, there still seems to be little agreement among researchers as to the relationship between the quality of a buck's antlers at yearling age versus those grown at maturity — probably because the animal's response varies regionally.