ANTLER ABNORMALITIES: Understanding Non-Typcials

by John J. Ozoga

From a zoologist’s perspective, antlers are often viewed as “magnificent morphologies” and “complex processes.” Complex processes, indeed, and as a result, subject to frequent deviations from the norm.

Dr. Richard Goss, one of the world’s foremost authorities on antler evolution and regeneration, reminds us: “The more complicated a process is, the more chances there are for mistakes to occur.”

Therefore, it is little wonder that deer antlers vary so much in size and shape. No two antlers are ever exactly alike, although similarities from side to side, or even from one buck to the next, are sometimes striking. And, of the many questions asked of biologists, I suspect no subject draws more questioning than does the antler growth of whitetail bucks.

Unusual antlers take many forms. Sometimes they are stunted, or missing altogether, as in “hummels.” In other cases, they may be crooked, unbranched, or possess excessive branching and an unusually large number of points. Even the growth patterns, timing of maturation, and casting schedules are sometimes out of the ordinary. And, occasionally, even females grow them.

Reasons For Abnormal Antlers

The reasons for abnormal antler growth often are unknown, and highly speculative. But there are three basic probable causes:

(1)   The defects are coded in genes, and therefore hereditary.    

(2)   Defects are caused by physiological problems as the result of bodily injury, parasitism, disease, malnutrition, or other health-related factors, referred to as “systemic conditions,” that alter normal body functions.

   (3) Direct injury to the pedicle or growing antler causes deformed antlers.

The Role of Genetics

Many antler peculiarities such as rack shape, tine length and configuration, and other specific features are unquestionably hereditary – they’re strictly coded in genes.

Numerous examples exist of captive bucks reproducing successive antler sets very similar in appearance year after year. An individual’s antlers tend to get a little larger and may add extra points with advancing age, but otherwise closely resemble one another, from one year to the next – barring any injury or health-induced abnormalities, in the meantime.

Just as the normal size and shape of antlers are subject to inherited traits, so also are some irregularities. It is important to note, however, lack of antler symmetry normally increases in all species of deer with advancing age. The older the deer, the more likely it will grow antlers that are not evenly matched and of a form other than the typical configuration we are accustomed to seeing.

According to Goss, “Only the important antler traits are strictly coded in genes. Unimportant details can be left to chance.” Also, antler anomalies of genetic origin are expected to occur on both sides, whereas one-sided defects more likely result for other reasons.

Certainly, a buck possessing hereditary antler defects would tend to reproduce those same defects with each new set of antlers. It’s my observation, though, these abnormalities may not be evident until the buck grows his second or third set of antlers. Also, one would expect hereditary defects to be perpetuated and show up in future generations. The problem of determining which anomalies are genetic, however, and which are not, is not an easy task.

Remember, females may also carry genes for antler irregularities. Early European gamekeepers soon learned culling bucks with poor, mis-shapened antlers did not necessarily remove poor antler genes from the population. Ultimately, such culling accomplished little to improve trophy hunting.

Lack of brow times among mature bucks, for example, appear to be genetically controlled, at least in those individuals that repeat the condition year after year. Most I’ve seen were missing brow tines on both sides, which suggests the trait was genetic.

Third, antlers have been recorded sprouting from the dorsal surface of the snout and from the orbit of the eye, in mule deer and whitetails. And some deer have grown two nicely matched main beams on each side. But whether such deformities can be attributed to genetics is unknown.

Paired palmate antlers also seem to be hereditary. Several bucks I raised in the Cusino square mile enclosure grew palmate antlers when 2-1/2 years old, and the palmations became even more pronounced when the bucks reached maturity. Nonetheless, damage to the growing antler may also promote unusually heavy, wide antler beams, as we'll discuss later. The presence of drop times, coming off the main beam, might also be hereditary, or they may occur for other unknown reasons. Although scientists agree that drop tines occur most frequently among well-nourished, mature bucks, they seem to have no firm data regarding reasons for their occurrence in relatively few deer.

Of the several hundred bucks I raised, only one possessed what appeared to be genetically determined drop tines, first evident as small bumps when the buck was 2-1/2 years old. In subsequent years, although the buck had otherwise typical antler form, he grew identically positioned, paired, three to 4-inch long drop tines each year. Several other bucks I raised each grew a single drop tine, but each only during one year, suggesting that the growths occurred for other than genetic reasons.

Velvet Antlers and Cactus Bucks

Normal testes development and related seasonal rhythms in male hormone (testosterone) production are essential if a buck is to grow normal antlers and maintain a normal antler cycle. Just as in the case of castrates and so-called antlered does, insufficient threshold levels of testosterone commonly lead to prolonged or permanent velvet antlers in otherwise normal appearing bucks. Such hormone deficiencies may result from disease, ingestion of toxic substances, certain nutritional deficiencies, injury to the testes, or numerous other factors than might alter a males normal physiology.

Jack Ward Thomas and co-workers observed large numbers of whitetail bucks having abnormally small testes in the Central Mineral Region in Texas. During the late 1950s and early 1960s, three to nearly 10 percent of the bucks in that region suffered from some unknown malady that left them infertile and with permanent velvet antlers.

In northern regions, viable velvet antlers tend to freeze back during winter. In the South, however, continual growth of velvet antlers can lead to massive, tumor-like growths in some individuals. Such formations are known as a “peruke.” And individuals that grow them are commonly referred to as “cactus bucks.”

Certain diseases may also influence antler form. The lungworm (Dictyocauls sp.), for example, is known to cause spiral-shaped, or “corkscrew,” antlers in red deer. Other diseases may influence antler development in whitetails similarly.

In addition to causing abnormally small antlers, poor nutrition may also lead to poor antler form. In large-antlered, normally well-nourished mature bucks, a sudden shortage of dietary minerals can cause the tines, or even main beams, to collapse.

Contralateral Effects

Depending upon the severity and timing of the injury, bodily injury can have a profound influence upon antler size and form on whitetail bucks. Probably one of the most intriguing phenomenon is referred to as “contralateral effects,” where injury to one side of a buck’s body, usually a hind leg, results in antler deformity on the opposite side. The effects may appear for several years after the injury. When the injury contributes to a permanent disability, such as in the case of a leg amputation, the contralateral antler is generally shorter than normal through the remainder of the animal’s life.

Reasons for contralateral effects are unknown. In fact, some scientists refute such theory altogether. Many times uninjured deer, which appear normal and healthy, grow unequal antlers. Instances in which normal antlers are produced despite injuries, are equally common.

“If true,” says Goss, “it is difficult to explain...except perhaps in terms of a compensatory response to counteract the imbalance caused by the original injury. A crippled deer would be expected to have an altered gait, the effects of which might result in compensatory changes in the growing antler owing to alterations in the flow or pressure of the blood supply.”

Psychological Stress

Psychological stress can influence an animal’s physiology and, thereby, impact many basic body functions, including secondary sex characteristics and reproduction. For example, dominance-submissive relationships among bucks become important in the annual antler cycle. Dominant bucks that usually win contests with other bucks tend to become hormonally charged. They usually shed antler velvet first and become avid scent markers. Logically, habitual losers suffer the reverse depressing effects.

George Bubenik observed that severe psychological stress can contribute to unnatural antler casting schedules, a condition I also observed in the Cusino enclosure. Normally a whitetail buck will cast both antlers at about the same time, often only hours or even minutes apart. When severely stressed, however, one antler may be retained for several weeks, or even a month, longer than the other.

There seems little doubt that psychological factors can have an important influence on antler growth traits. Whether mental duress results in hormonal imbalances that contribute to abnormal antler form, to my knowledge, are unknown, but seem likely.

Pedicle Wounds

Before any deer can grow antlers it must first grow pedicles on which the antlers form; only the pedicle is capable of giving rise to a normal and complete antler. Typically, the larger the pedicle, the larger the antler outgrowth.

Damage to the forehead or pedicles of a deer is a powerful stimulus for antler growth. Accidental injury to the forehead bones can result in out-of-season antler growth among bucks, and even initiate antler growth among does.

Normally, injury to the pedicle will cause abnormal antler formation. Or, in severe cases, it may even lead to accessory antlers produced by the injured pedicle or nearby regions of the skull. Determining which antler abnormalities result from pedicle wounds, and which result from hereditary or systemic conditions, is often difficult.

Accessory antlers resulting from pedicle damage are usually malformed, being shorter than normal, and typically one-sided. In some cases splitting of the pedicle or antler bud can lead to the growth of a third antler beam. They may originate as a separate shaft from the antler base, or as short spikes from the forehead region or orbit of the eye. They tend to follow the usual antler replacement cycle, by shedding velvet in autumn, dropping off in winter, and re-growing in spring.

Velvet-Antler Injuries

The nervous system plays a key, but mysterious, role in antler development. The growing antler is well endowed with nerves, and the principle function must be to assist the deer in avoiding injury. Also, large-antlered bucks seem to possess a certain “kinesthetic sense,” which permits them to judge the position of their antlers, and avoid bumping them. Even the velvet hairs serve as sensitive feelers. Inevitably, however, growing antlers are sometimes damaged.

One of the most common injuries is a fracture of the antler. Sometimes the break is so severe that part of the antler dangles loosely, held only by the velvet, which eventually drops off. If the blood supply is not severed, such broken parts may remain alive, fuse, and continue to grow at a crooked angle.

If the antler is only cracked, it can repair itself. Such damage generally results in conspicuous swelling along the shaft where the fracture occurred.

Experimental de-nerving of the growing antler usually contributes to stunted antlers with abnormal form. This results because deer cannot avoid bumping into things and injuring the soft antler.

Although major damage, such as a fracture of the pedicle or budding antler, will usually heal fast, abnormal antler form generally results. Massive damage, such as splitting of the budding antler, may cause abnormally large, misshapen antlers with many extra points. In fact, early gamekeepers in Europe reportedly sometimes shot the velvet-covered antlers of deer with birdshot to cause extra antler points to grow. Even today, deer farmers in New Zealand sometimes cut off the first growth of red deer spikes to induce branching -- causing 8-point antlers to grow in just over one year.

The Trophic Memory Response

The late Anthony Bubenik observed maximum antler development arising on the damaged side the year after injury, with continued abnormal enlargement on both sides in subsequent years. Since the effects of trauma occur primarily on the injured side, this suggests that each antler has its individual antler growth center in the central nervous system. Based upon old evidence, and his experience, Bubenik proposed that such trauma was “remembered,” a condition he coined the “trophic memory response.”

George Bubenik emphasizes that the term “memory,” as used here, “is not ideal.” There is no mental effort to remember such injury, but somehow it is recorded in the brain and duplicated in subsequent years. He notes the following: (1) injuries to the pedicle and early stages of the young antler cause the most pronounced changes in antler shape, (2) the injury must involve nerves, (3) the response is slowly "forgotten," if not reinforced, and antler form will eventually return to normal, (4) the more severe the injury, the longer the memory lasts, and (5) re-injury, which reinforces the memory, causes massive antler growth of atypical shape. 

Needless to say, non-typical trophies are very rare in the wild. Certainly, that huge buck, sporting 20-point, non-typical antlers is an exceptional trophy, one of superior genetic ancestry  -- or is he?

Even if a particular buck has the genetic equipment to sprout massive, non-typical antlers, those traits will not be expressed unless he lives a healthy life, is exceptionally well nourished, and grows to maturity. On the other hand, many pampered, pen-raised bucks, more frequently achieve such stature. One must wonder, though, just how many of those non-typical trophies -- pen-raised or wild -- come to be “accidentally.”

Conclusions

It is often tempting to blame certain genetic quirks for unusual antlers -- some are. However, a review of the scientific literature reveals a multitude of possible reasons for abnormal antler size, shape, and growth rhythms. Unfortunately, few studies have been conducted specifically to unravel such mysteries. Instead, most records of antler abnormalities are based upon single observations, seldom duplicated experimentally. Even the scientists involved often disagree amongst themselves as to the reasons for growth of antlers that do not conform to normal expectations.