May 01, 2013To put it mildly, the growth of antlers is a unique phenomenon. Only grown by members of the deer family, Cervidae, antlers are the only mammalian appendages that annually replace themselves and they're astounding growth rate even surpasses that of dreaded cancer cells.
As noted by the late antler expert, Dr. Richard Goss, "the most fundamental of living things is that they can repair themselves. However, true regeneration is the replacement of missing parts of the adult organism. The annual regrowth of antlers is a special case of this general phenomenon."
Antlers are actually two structures in one: the antler itself and the pedicle, or stump on the ends of which the antler develops. Antlers drop off and are replaced; pedicles are permanently attached to the buck's head.
Before any deer can grow antlers, it must first grow pedicles. These structures first show as a pair of cowlicks on the forehead of very young fawns, where they can be felt as small bony lumps beneath the skin. In the absence of the male hormone testosterone or the presence of the female hormone estrogen, no pedicles form and antlers fail to develop later. On northern range, buck fawns born late in the season or those poorly nourished during summer and autumn tend to grow small pedicles (or possibly none at all). Those subjected to severe social (psychological) stress, due to crowding at high herd density, might suffer the same consequences, due either to a deficiency in testosterone production or because of a hormonal imbalance that blocks its effect.
Pedicles usually don't become pronounced 'nubbins' until the fawn is about 4 or 5 months old. This is when the healthy fawn's testes produce sufficient quantities of the male hormone testosterone to stimulate the laying down of additional pedicle bone.
Interestingly, the timing of the young fawn's pedicle development is not affected by light cycles. Instead, it is more closely linked to his rate of maturity, testes development, and his ability to produce a substantial amount of testosterone. Subsequent adult antler cycles are more closely programmed by season and light cycles, which control the rhythm of hormone production.
Pedicles change with the buck's age. Each year more bone is laid down, as the pedicle becomes more concave in shape and increases in diameter to accommodate larger antler growth.
Given early birth and excellent nutrition, some buck fawns grow prominent pedicles topped with small button antlers less than inch ˝ long, during autumn when they're 6 or 7 months old. These so-called infant antlers, which are essentially mineralized pedicle tips, are cast during winter before new antler growth starts.
The process of growing infant antlers is different from antler regeneration. "Because it is such a gradual process,'' said Goss, "it is impossible to pinpoint the exact point when pedicle growth gives way to antler development. The (origin) of the first antlers is a phenomenon that is not conveniently classified. Although the histological events undoubtedly resemble those by which subsequent sets of antlers are regenerated each year, the process is not an example of regeneration because there has been nothing lost to be replaced…In this respect, the fawn's initial antler is a unique zoological structure."
Clearly, the presence of infant antlers is indicative of an advanced rate of maturity and achievement of puberty -- given the opportunity, these buck fawns are probably capable of breeding.
On the flip side, physically retarded buck fawns must first accomplish additional pedicle development the next spring and summer, before antler growth can start. In all likelihood, such an individual will carry very small diameter, pencil-shaped antlers lacking a flared basal burr (also called a cornet) at yearling age.
The Photoperiod Trigger
The annual cycle of antler growth, hardening, casting, and regrowth is controlled by the buck's endocrine system. In northern latitudes, white-tailed deer grow antlers in spring and summer, when testosterone production is at its lowest level. The cycle is dependent upon seasonal changes in the amount of daylight, or photoperiod, and follows the rhythmic rise and fall in circulating blood levels of testosterone.
The pineal gland, tucked deep within the mid-portion of the deer's brain, serves as the whitetail's nerve and hormone transducer. That is, in response to changes in day length, the tiny gland translates light signals into chemical signals that cause hormone changes responsible for setting the whitetail's seasonal rhythms. An increase in the amount of daylight during April stimulates greater production of a hormone known as prolactin that triggers new antler growth from the buck's pedicles.
Goss demonstrated the importance of changing light regimes in regulating antler growth in sika deer raised under controlled light. When light cycles were artificially shortened, the deer grew as many as three sets of antlers per year. When light cycles were lengthened, they only grew antlers every other year.
Because day length is nearly constant at equatorial latitudes, deer living in tropical and subtropical habitats experience only slight seasonal changes in the amount of daylight. In the absence of photoperiod cues, does may breed and give birth and bucks may carry hardened antlers, at any time of the year. But even in the tropics, bucks only grow one set of antlers each year, the timing of regrowth being largely determined by when the deer was born.
The swollen ring of skin around the pedicle grows over the stump of the antler -- a process referred to as “wound healing.’’ Kenny Darwin photo
Before antler regeneration can start, antlers must mature, die, and fall off. As soon as the old antlers drop, normally during winter, the swollen ring of skin around the pedicle grows over the stump of the antler -- a process referred to as "wound healing.'' This new skin is more like antler velvet, into which it will ultimately develop.
In the northern hemisphere, the exact timing of antler growth varies somewhat among individuals, depending upon their age and general health status. Healthy, mature bucks may show signs of bulging new antler growth in early April. By comparison, new antler growth among malnourished mature individuals and yearling bucks growing their first set of full-fledged antlers may be delayed from 2 to 4 weeks. (Coat molt generally follows the same pattern, with mature, healthy individuals being the first to don their red summer coat.)
Early in their development, antlers are quite soft and easily damaged. Growth occurs at the tips, whereas calcification (hardening) starts in the shaft of the antler. (The core remains moist and spongy until the antler is cast).
The growing antler contains an involved network of blood vessels and tissue which is covered by a hairy skin known as "velvet." Antler velvet is capable of enormous expansion, necessary to keep pace with the antler's rapid growth rate.
Although quite fragile and easily damaged, the live antler is richly supplied with sensory nerves. Even the delicate hairs serve as touch-sensitive feelers that warn of a pending collision.
As an added safeguard, bucks seem to possess a special (kinesthetic) sense that permits them to judge their own antler size and shape. So equipped, even large-antlered bucks can bound through dense forest cover without damaging their soft antlers.
Also, arteries supplying blood to growing antlers have unusually thick walls. When severed, the thick muscular walls quickly constrict, reducing blood loss.
Although the live antler may remain velvet covered for 4 to 5 months, most of the growth is accomplished during June and July. Regardless of deer body or antler size, complete elongation of the antler is normally completed within 100 days. This means large antlers grow at a much faster rate than small antlers. (For example, moose antlers may elongate at the rate of 3/4 inch per day).
The completion of antler growth, antler mineralization and velvet shedding are closely linked to autumn's shortening days. These changes are brought on by sharply rising testosterone production.
The antler's outer surface hardens quickly in a few weeks before velvet is shed. However, mineral deposition in the antler core is more prolonged. Bone formation starts internally in lower portions of the antler, and progresses from the base to the tips as antlers elongate.
By midsummer, the pedicle becomes semi-mineralized, restricting blood flow to the antler core. As a result, during late stages of growth, the antler is nourished primarily by arteries in the velvet.
As the blood flow shuts off in the velvet vessels, the bone gradually dies, dries and sheds the velvet, thus killing the antler. Velvet death normally takes several weeks, but can occur suddenly. Generally, death and stripping of the antler velvet signal death of the antler core itself.
The Role of Nutrition
Most whitetail bucks reach their maximum body size when 5 or 6 years old; maximum antler size is normally achieved when bucks are between 6and 8 years old. Antler size and body size go together -- large-bodied deer tend to grow large antlers.
Typically, (barring injury or ill-health for any reason) the mature buck tends to grow antlers that are remarkably similar each year. However, at old age, he may revert to growing only forked or spike antlers. Goss suggests that the degenerate antlers of old bucks are more like the stunted antlers produced under conditions of malnutrition -- which forecasts the old buck's pending demise.
Since body growth takes precedence over antler growth, any dietary deficiency can have strong negative effects on subsequent antler size. Also, nutrient requirements of mature deer are often less than those of younger deer due to the fact that younger deer have additional requirements for body growth. Regardless of their age, deer on deficient diets began antler growth later, grow smaller antlers, shed velvet later, and cast antlers earlier, as compared to well-fed bucks of comparable age.
The specific nutritional requirements for antler growth are only poorly understood. Logically, since antlers are regenerated bone, dietary factors that are important to bone growth must also be important in antler growth. Therefore, the amounts of digestible energy, protein, calcium, phosphorus, and vitamins A and D in the buck's diet are assumed vital for proper antler growth.
It would seem rather simple to determine the chemical composition of deer antlers, then estimate the nutrients required to build such structures. Unfortunately, antler mineral composition varies according to the stage of growth -- the composition of a mature antler is much different from that of an actively growing antler. Therefore, chemical analysis of antlers does not necessarily reveal the amount of dietary nutrients the animal must consume to build them.
Sharply contrasting seasonal patterns in food consumption by bucks also complicate matters when trying to determine nutrient requirements for antler growth. Bucks feed heavily during spring and summer, fast during the autumn breeding season, and sometimes endure harsh nutritional conditions during winter.
Interesting, deer do not store excess minerals in their skeleton in anticipation of antler growth. Instead, they accelerate turnover of important minerals -- such as calcium and phosphorus -- during the antler growth period. As a result, blood levels of most minerals change minimally on a seasonal basis. Even when fed rich supplemental diets, bucks still undergo mineral depletion in their ribs, skull and other bones that do not bear body weight, during antler growth.
Although the timing of antler growth is quite synchronized by photoperiod, generally starting during April and May in the northern hemisphere, the timing of antler casting tends to be quite variable. Some bucks might drop their antlers as early as late November, whereas others may carry them until late April depending upon their age and health status.
Antlers are cast when blood levels of testosterone decline. The maintenance of connection between the dead tissue of the antler and the living tissue of the pedicle is possible only during the period of high testosterone.
As soon as the testosterone concentration drops to a certain level the narrow bridge of bony tissue between the pedicle and antler erodes away and the antler falls off. This process by which the dead antler is separated from the living pedicle is unique resembling a form of self-amputation.
Any factor that helps maintain elevated levels of testosterone will prolong antler retention. A buck's nutrition and health status are factors of primary importance in determining the time of casting. If the buck does not remain in good physical condition his testosterone level will decline to the point of triggering antler casting.
According to Canadian researcher, George Bubenik, "There is some evidence that sex pheromones may activate and prolong the testosterone secretion in males…and delay the timing of antler cast."
Hence, excellent winter nutrition could account for mature bucks maintaining good physical, high testosterone levels, and prolonged antler retention as typically occurs in the Midwest farmland. But the presence of estrous doe fawns during winter might also be a factor contributing to prolonged antler retention via sex hormone stimulation.
Despite the frequent controversy surrounding aspects of antler growth, one truism concerning these so-called bones of contention prevails: "The head grows according to the pasture, good or otherwise."
To say that antlers are unique structures grown by remarkable creatures is indeed an understatement. In the words of Goss, "Antlers are an extravagance of nature rivaled only by such other biological luxuries as flowers, butterfly wings, and peacock tails. The antlers of deer are so improbable that if they had not evolved in the first place, they would never have been conceived in the wildest fantasies of the most imaginative biologists."