The Antler Cycle
Antlers - there is something magical and mystical about deer antlers. Part of their allure is that every antler is different and unique. However, our fascination with deer antlers has its roots deep in our psyche, from our ancestors, who have hunted antlered game since man’s existence. Our ancestors used antlers for tools and in religious ceremonies. Today, we are still captivated by antlers.
How antlers grow and why they are shed each year is a source of wonder and curiosity. Antlers are one of nature’s most remarkable accomplishments. Deer, which are capable of re-growing antlers every year, are the only mammals with this capacity to regenerate these complex appendages. The speed at which antlers grow, also makes them the fastest growing structures in the animal kingdom.
Antler growth in bucks begins when they are fawns. However, buck fawns never grow antlers larger than short “buttons,” or pedicles, which on rare occasions become hardened. These pedicles then develop into the buck’s first spike, or branched antlers, when he is a yearling (1.5 years old). Antler size then continues to increase each additional year until peaking at age 6.5 years old.
Deer farmers first discovered if buck fawns were castrated before pedicle formation, they would never grow antlers. If females are given testosterone, small pedicles develop on their forehead that occasionally further develop into hardened antlers. Scientists have even succeeded in growing antlers on the legs and hips of deer by grafting a portion of the skull bone, where the pedicle forms, to these areas!
Bucks begin growing their antlers in late-winter or early spring, within weeks of when the previous year’s antlers are shed. Antlers grow very slowly at first, but by late-May, antlers are rapidly growing. Antler growth is usually complete by the end of August. The velvet then hardens and falls off during September. The hardened, polished antlers remain until they are shed during January through March.
Why Are There Annual Cycles In Antler Growth?
Believe it or not, the 23 tilt of the earth’s axis is the ultimate cause for the annual cycles in deer antlers. If the earth were perpendicular in it’s orbit around the sun, there would be no changes in temperature and day length and no need for calendars. This tilt is what causes earth’s annually recurring seasons. Deer have adapted their physiology and behavior to these seasonal changes, including antler growth.
The environmental cue that regulates antler growth is the amount of day length, or photoperiod. The physiological cue is the male hormone testosterone. The way this works is complicated, but changing day lengths are sensed by the eyes, which send this message, via the optic nerve, to the pineal gland. The pineal gland, a pea-sized organ at the back of the brain, produces many different hormones. One hormone produced is the luteinizing hormone, which controls the amount of testosterone produced in the testes.
The pineal gland also produces a hormone called melatonin, which acts to suppress the production of luteinizing hormone, holding back production of testosterone. Melatonin is produced in higher and higher quantities as the amount of darkness during each day increases. During early summer, as the nights get shorter and day lengths increase, the production of melatonin decreases, allowing the pituitary to produce luteinizing hormone. Increasing amounts of this hormone then result in increased testosterone production, which triggers the velvet antlers to harden and the velvet to shed. Unlike antler shedding, velvet shedding usually occurs within a 24-hour period.
The antler cycle lags several months behind the changes in day length because the hormonal changes take time. During fall, decreasing day lengths cause melatonin production to increase, resulting in decreased production of both luteinizing hormone and testosterone. Decreasing testosterone levels then cause the antlers to shed.
Can The Cycles Be Altered?
When day length cycles are artificially changed, deer grow antlers according to the new cycle. Scientists have shortened day length cycles to six months instead of one year. This doubling of the annual cycle caused deer to grow two separate sets of antlers in the same year! When scientists tripled the annual cycle (one annual cycle completed every three months), bucks grew three sets of antlers in one year, but the antlers only grew to about half of normal size. Scientists have also lengthened day length cycles, causing deer to grow antlers only every other year. These antlers still only grow to normal size, but the length of time bucks are in velvet increases.
In the past, scientists have also relocated deer from one hemisphere to another, such as the transplants of different deer species from North America and Europe (northern hemisphere) to New Zealand (southern hemisphere). These transplanted deer adapt their antler cycles to the opposite day length cycles of the new hemisphere, even though temperatures at the new location decrease as day length increases. Deer that live on the equator, where there are no seasonal fluctuations in day length, are able to breed year-round. However, breeding and antler growth in individual animals still occur on 12-month cycles that depend on the animal’s birth date.
When Does Antler Size Peak?
We have very conclusive data when it comes to knowing when antler size peaks for bucks in south Texas. We have now randomly live-captured over 4,000 bucks from the wilds of south Texas. We have aged all of these bucks by tooth wear and measured all of the racks according to Boone and Crockett Club (BCC) guidelines. On average, antler size and gross BCC score does not reach its maximum until a buck is at least five-and-a-half years old. In fact, many bucks do not peak in antler size until they are six-and-a-half or seven-and-a-half years old. Many of the body characteristics we have measured during past buck captures have also indicated that several body measurements do not peak until these same ages.
Clearly then, if you wish to allow bucks the opportunity to reach their full antler growth potential, they need to be allowed to live until at least 5.5 years old. However, as noted above, the many bucks will die of natural causes before reaching full maturity.
Posted by Dr. Mickey W. Hellickson
Photo by Hardy Jackson