Are unbalanced antlers indicative of poor genetics?
Asymmetry Index To Buck Quality...
November 01, 2007
Deer hunters generally regard large nontypical antlers as outstanding trophies and the mature bucks that carry them as examples of genetic superiority. And they may well be. But what about young bucks with uneven (asymmetrical) antlers? Surprisingly, research conducted by Stephen Ditchkoff and his colleagues at Oklahoma State University suggests that unbalanced antlers grown by young bucks are indicative of poor quality.
Although no two antlers are ever exactly alike, even on the same animal, their similarities from side to side are often striking. When a buck's antlers are well-balanced, we refer to them as having "bilateral symmetry" or being of "typical" form when measured for trophy status.
When a buck has one antler that does not closely mirror the other, we say they exhibit "asymmetry". In such cases, one antler may possess more tines than the other, its branches may be of different lengths, or they may sometimes grow out at variable angles. When measured for trophy status, they more likely fall into the "nontypical" category.
Given what is known about antler evolution and morphology,
well-balanced antlers — those having bilateral symmetry — are considered normal.
According to the late Richard Goss, one of the world's antler experts, "...minor variations from one side to the other represent the results of unknown, but nongenetic, factors that influence the normal developmental processes by which antler growth is achieved."
The Oklahoma investigators contend that exaggerated deviations from antler bilateral symmetry more likely occur among bucks that can not cope with stress. Environmental stresses most often include such things as parasites, disease, poor nutrition, harsh climate, pollution, and so forth. Or they may be the result of physical damage.
Antlers are secondary sexual traits that serve two primary functions. For one thing, they are ornamental, and allow the carrier to advertise himself to prospective mates. Antlers also serve as weapons and as visual threats to potential combatants during the breeding season.
When food resources are limited, body growth takes precedence over the growth of such secondary sexual characteristics as antlers. Since antlers are expensive to grow, from a nutritional standpoint, the healthiest and physically superior bucks tend to produce the best antlers — those that are large and well-balanced.
There is good reason to believe that antler asymmetry may serve as an indicator of the level of stress experienced by a buck and the genetic quality of that individual in terms of his ability to eliminate or cope with stress. High-quality individuals should be those best able to handle stress and grow antlers that are large and well-balanced.
Assuming inferior bucks most likely carry asymmetrical antlers, the degree of antler imbalance should be most prevalent among young bucks. That is, as a buck age class progresses through time, selective pressures and natural mortality should serve to eliminate inferior animals — those with asymmetrical antlers. As a result, the proportion of bucks with uneven antlers should decrease and the average quality should increase among older bucks.
|When bucks have one antler that does not closely mirror the other, it's called "asymmetry". In this case, one antler possesses a drop tine. Kenny Darwin photos|
Also, within a buck age class, the frequency of antler asymmetry should be related to antler size and body size, since antler size and body size normally go together. On average, young bucks with asymmetrical antlers — being inferior — should have a smaller body stature and carry smaller antlers, as compared to those with well-balanced antlers.
In recent years, Ditchkoff and his associates have conducted extensive deer research at the 70 square-mile McAlester Army Ammunition Plant in southeastern Oklahoma. In this particular study, carried out in the late 1990s, they gathered antler and physical data from bucks in order to determine whether antler asymmetry may be an index to buck quality.
They collected data from 359 hunter-harvested bucks and from 80 bucks that were captured, measured and released for further study. Antlers were measured according to the Boone and Crockett scoring system. This system measures the length of each tine, length of the main beam and circumferences around the main beam at the base of the antler. It provides separate scores for right and left antlers, as well as a total score indicative of overall antler size. Chest girth, skull length, hind food length, body length and tail length were also measured on each animal. Dressed weights were recorded for all hunter-harvested animals. Because of difficulty in aging older animals, bucks 6-1/2 years of age and older were grouped for data analysis.
Measurements of asymmetry were determined for antler score, number of antler points, main beam length and basal circumference.
The absolute amount of asymmetry was calculated as the difference between measurements of the right and left antlers. The relative amount of asymmetry was calculated by dividing the absolute measurement by the score of the larger antler. This was done to adjust for the size of the trait and to express asymmetry as a percentage of trait size. That is, a 10 inch difference between the large antlers of a mature buck would be proportionately much less compared to a 10 inch difference in the spikes of a young buck.
It's important to recognize that the total score for one antler was compared to the total score of the other as an index to asymmetry. This means, for example, that an 8-point rack could show considerable asymmetry if tine lengths varied a great deal from side to side. At the same time, a 9-point rack might yield similar total scores from side to side and show minimal asymmetry.
So, Ditchkoff's methods did not necessarily identify nontypical antlers. Nonetheless, one would expect most nontypical racks to show relatively high asymmetry.
Antler Asymmetry and Antler Size
As hypothesized, Ditchkoff and his associates found a negative relationship between antler size and degree of asymmetry — as antler size increased within an age class, relative antler asymmetry decreased. Interestingly, the relative asymmetry of antler score, main beam length and basal circumference was greater among small-antlered deer than medium- or large-antlered deer, but the relative asymmetry of the number of antler points did not differ among groups.
This pattern suggests that large-antlered bucks are able to cope with stress better than their counterparts with smaller antlers. It also suggests that the frequency of antler asymmetry may serve as an index of buck quality. Since the nutritional requirements to produce large antlers are great, only high-quality bucks should be able to produce large, well-balanced antlers.
It is reasonable then to assume that individuals with large antlers show the least amount of stress-induced antler asymmetry. This would allow the healthiest, genetically strongest bucks to advertise their superior status to competitors and prospective mates. If so, superior bucks would likely attract more mates, do most of the breeding, and enhance genetic fitness within the population.
If antlers are a true signal of quality, and strength, then superior bucks should also be able to intimidate inferior bucks, thereby achieving dominance with minimal energy wasted in fighting.
Antler Asymmetry and Buck Physical Condition
If antler asymmetry is a measure of quality in whitetails, there should be a relationship between antler asymmetry and certain physical measurements. In this study, relative antler asymmetry was negatively associated with buck body weight, body length, and skull length.
According to Ditchkoff, "Body mass is a good measure of condition because it ultimately represents how effective an animal has been at meeting its nutritional requirements, not only for growth, but for competing life-history demands. A strong negative relationship between body mass and relative asymmetry supports the hypothesis that asymmetry in antler morphology signals the quality of an individual."
The Age Factor
Relative antler asymmetry was negatively related to antler size for all deer and within age groups up to 5-1/2 years of age. It was greatest in 1-1/2 year old bucks. However, bucks 6-1/2 years of age and older had no significant relationship between relative asymmetry and antler size. In fact, while all other bucks showed decreasing antler asymmetry with increasing antler size, antler asymmetry among bucks 6-1/2 years of age and older increased slightly as antler size increased.
Obviously, natural selection would account for a decrease in relative antler asymmetry with advancing age. Since bucks with asymmetrical antlers are of lower quality, they would more likely succumb to such stress factors as disease, parasitism, malnutrition or predation. As a result, the proportion of poor quality bucks — those with asymmetrical antlers — would naturally decrease among older buck age classes.
Ditchkoff speculates that, "If antlers are to be an honest signal [of quality], then there must be a cost associated with their production." Therefore, "...secondary sexual characters [such as antlers] should increase in size to some threshold where the benefits of the character are balanced by the costs of production."
Keep in mind, although a buck may achieve his maximum body size by 5-1/2 years of age, he probably will not grow his largest antlers until he is 6-1/2 years of age or older.
Hence, it may be that antlers of these older bucks illustrate this upper threshold phenomenon. Because of their advanced age, some of these older bucks may demonstrate increased antler development, in order to "look their best," attract more mates, and achieve final high reproductive success. In doing so, however, they may experience considerable nutritional stress. This added stress could therefore account for an increase in antler asymmetry among large-antlered, aged bucks.
This means that different rules may apply to the mature buck with uneven or nontypical antlers. That is, while the young buck sporting unbalanced antlers is probably one of inferior genetic quality, the mature buck with large nontypical antlers is more likely a testosterone-charged buck possessing superior
An individual buck's status, relative to physical quality and antler configuration, may change with advancing age. If a young buck survives, he may very well overcome any deficiencies he exhibited, or hardship he experienced, during early life. His antler configuration may change accordingly. Likewise, a high-quality buck could decline in physical condition as he aged due to an injury or some other malady. For example, aged, senile bucks commonly grow spindley uneven antlers.
Since bucks raised in captivity seem to have a higher incidence of antler asymmetry, psychological stress due to crowding and strife may also play a role in this interesting phenomenon. This is a question Ditchkoff hopes to explore in the future. His goal is to vary deer herd density in a large enclosure, while measuring antler size and asymmetry as related to buck physical and physiological condition.
Theoretically, even with unlimited nutrition, bucks unable to cope with crowding and social conflict (psychological stress) should suffer certain physiological imbalances and be hindered in their physical development, including the production of smaller than normal antlers with greater asymmetry.
Bodily injury can also lead to antler asymmetry, and sometimes results in a phenomenon referred to as "contralateral effects." In such instances, an injury to one side of the buck's body — usually a leg — causes antler deformity on the opposite side.
My wife and I once raised a captive fawn that broke its left hind leg when only a few weeks old. Although the leg healed well, and he lived to be 7-1/2 years old, he remained on the runty side and always grew a deformed right antler. At least in this one instance, the early injury seemed to have a permanent effect on antler configuration.
The late Anthony Bubenik also suggested that a sharp decline in diet quality can be sufficiently stressful to cause antler abnormalities. For example, he sites the case of a buck producing excellent antlers when optimally nourished. But when switched to a poor diet, the buck grew "voluminous antlers" on which the tines and beams collapsed due to inadequate consumption of calcium and phosphorus.
It appears that bucks experience a variety of stressful circumstances that may or may not cause the growth of unbalanced antlers. Apparently high-quality deer can cope with stress more effectively than poor-quality individuals.
Within an age class, up to 5-1/2 years, uneven antlers will more likely be carried by the inferior bucks — those with small bodies and small antlers. On the other hand, among bucks 6-1/2 years of age and older, those with the largest antlers are sometimes inclined to grow nontypical racks that are not well-balanced.
Therefore, asymmetry in the antlers of white-tailed deer serves as an effective signal of genetic quality and capacity of a young buck to cope with stress. These signals undoubtedly play an important role in establishing dominance during the breeding season and may be important in mate selection in order to maintain genetic fitness within the population.
It's my guess, a more precise measure of antler asymmetry than that employed in this study will reveal even more intricate relationships between a buck's antler condition and his breeding performance.