The Science Behind Sheds
(En anglais seulement)
By: Dr. Mickey Hellickson
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.
Antlers are one of nature's most remarkable accomplishments. 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 buck¹s first spike or branched antlers, when he is a yearling (1 1/2 years old). Antler size then continues to increase each additional year until peaking generally at age 6 1/2 or 7 1/2.
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 December through April, depending on location and managment practices.
Why Are There Annual Cycles In Antler Growth?
Believe it or not, the 23 degree tilt of the Earth¹s axis is the ultimate cause for the annual cycles in deer antlers. 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 base of the brain - produces many different hormones. One hormone produced is luteinizing hormone, which controls the amount of testoserone produced in the testes. The antler cycle lags 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.
In the past, it was believed that deer withdrew to secluded places to shed their antlers in order to avoid the loss of virility in 'public.' However, it is likely that deer are unaware of when they will lose their antlers. Antlers are shed when a thin layer of tissue destruction, called the abscission layer, forms between the antler and the pedicle. This layer forms as a result of the decrease in testosterone. As the connective tissue is dissolved, the antler loosens and is either broken free, or falls off on its own. This degeneration of the bone-to-bone bond between the antler and the pedicle is the fastest deterioration of living tissue known in the animal kingdom. In whitetails, a restricted diet has been found to cause bucks to shed their antlers early. It has been suspected that the lack of adequate nutrition somehow effects testosterone output. Nutritionally-stressed bucks may also grow their antlers and shed their velvet later. Older-aged bucks are thought to shed their antlers earlier than younger bucks. It has also been reported that higher-ranked (more dominant) bucks cast their antlers sooner than lower-ranked (subordinant) bucks. Older-aged, more dominant bucks probably shed their antlers sooner because of the high energy costs incurred in maintaining a higher dominance rank.
The farther deer are from the equator, the more defined their antler cycle. In other words, northern deer have a shorter "window" of when antler shedding can occur, compared to deer herds in southern states.
In addition, the specific date when a buck will shed his antlers may be determined more by his individual antler cycle than any other factor. This cycle is independent of other bucks and is believed to be centered on each animal's birth date. Penned deer studies have allowed scientists to measure the exact dates of antler shedding for individual deer year after year. One study in Mississippi found that individual bucks usually shed their antlers at the same time each year and almost always during the same week. Yearling bucks with only spike antlers shed sooner than yearling bucks with forked antlers, likely because they were more nutritionally stressed than fork-antlered bucks. This study also indicated there was no relationship between antler mass and date of antler shedding, although other studies have shown that bucks shed their antlers earlier as they grow older. Additional penned studies have also revealed that bucks usually shed both antlers within three days of each other.
Although there is no clear evidence that weather directly affects antler shedding, it is likely that severe winters may also cause bucks to shed their antlers earlier than normal because of the nutritional stress this causes.
Why Do Bucks Shed Antlers?
We have examined the environmental and physiological changes that occur to cause bucks to shed their antlers every year, but we still haven't addressed the question of why bucks shed antlers. Why do bucks spend so much energy in growing antlers, only to shed these antlers a few months later, forcing them to reinvest an enormous amount of energy to regrow the antlers again the following year? Why don¹t deer antlers stay attached and continue to grow throughout life like the horns of sheep, goats, and cattle?
Scientists have pondered these same questions for many years and they still do not know the answers. However, several theories have been developed to explain why antlers are shed every year. One of the most common theories is that bucks shed their antlers annually so that they have the potential to replace any damage to antlers that may have occurred in the form of broken tines, or a broken main beam. This theory seems valid because antlers are extremely important in display for acquiring females and because they are used during dominance fights with other bucks. If a buck breaks a main beam and is not able to replace that antler, it may not be able to acquire future breeding privileges.
A second related theory suggests that bucks shed antlers annually so that they can regrow larger antlers the following year, in order to keep pace with their increasing body size. This theory is based on the fact that antlers quickly mature into nongrowing structures before the buck is able to attain full body size. A third theory states that antlers are shed simply because of an accident of evolutionary chance. In other words, there is no real reason why antlers are shed. Antlers are different from horns, not because they need to be, but because of different evolutionary origin. A fourth theory suggests that antlers are shed in order to stop the die-back process, that occurs at the junction of the pedicle and antler, from traveling down into the skull.
Another theory suggests that antler shedding developed in the primitive antlers of ancestral deer from temperate zones. Antlers of deer in these colder climates would have been vulnerable to freezing in winter if they were not shed. The only way to prevent freezing would have been to stop the blood supply to antlers before the onset of winter. It is thought that ancestral males shed their antlers so that they were able to mimic the healthier, nonantlered females. In theory, this reduced their vulnerability to predation, because predators may have actively searched for antlered males due to their weakened condition.
One final theory simply suggests that antlers are shed each year as an energy-conserving measure, so that males don¹t have the added weight from the antlers to carry outside of the breeding season. Antlers are cumbersome and energy-expensive structures that are not needed after the breeding season. However, for this to be true, it must also be true that regrowing the antlers each year is less energy-expensive than maintaining the antlers through winter.
Which one of these theories is right? Who knows for sure? Maybe the true answer is a combination of these theories, or maybe none of these theories. Hopefully, scientists will be able to unravel this great mystery in the future. Until then, I¹m just glad that antlers are shed each year so that I can continue to enjoy my hobby of shed antler hunting.