The Causes of Antler Asymmetry | Deer & Deer Hunting

You have hunted that special spot for years, maybe even decades, and killed some nice bucks, but have yet to see that true once-in-a-lifetime giant. But early this fall you captured a trail camera image of a legitimate record book class deer — a “Booner!” — and you’re beyond fired up!

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As hunting season approached, you put in countless hours of patterning that buck — you developed a plan, constructed the perfect treestand setup, and then waited for conditions to be just right. The moment finally arrives and you make the perfect shot on that once-in-a-lifetime buck! Once the excitement wears down, you measure the antlers and sure enough, the gross score is well above the minimum required for entry into the record books. Later, the rack is officially scored, and you find out that deductions dropped the final net score several inches below the minimum, leaving you frustrated and disappointed.

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Most of us have spent time looking at, even obsessing over, sets of antlers. We’ve all heard those stories about that monster buck that somehow didn’t make the record books because the net score wasn’t quite good enough, or we’ve seen that odd-looking buck with massive main beams and long tines on one side and a spike on the other.

Whether they were from that last buck you shot, the pictures of giants you see every month in Deer & Deer Hunting magazine, or that matching pair of sheds from the buck that got away, you might have wondered why there is variation between the right and left antler and what causes it. In many of these antler sets the variation is only a few inches, while in other extreme cases, it might be 100 inches or more.

These differences between the right and left antlers are commonly seen as deductions from the gross score, ultimately creating the reduced final or net Boone and Crockett Club score. This measurable variation is known as asymmetry in scientific circles.

What is Asymmetry?

Differences in size or orientation in physical traits between a left and right side is referred to as asymmetry. Genetically, we would expect both sides to be exactly the same, or symmetrical, but consider any set of antlers. There is always some difference (it might be great or small) between the left and right side. The number of points might be different. The brow tines might be different lengths. The main beams might curve at different angles. Or maybe all of the above is true. These are examples of asymmetry.

One type of asymmetry is fluctuating asymmetry (FA), which refers to small random differences between the right and left sides of bilateral traits. Bilateral traits simply refer to physical traits that can be divided into symmetrical (right and left) halves. Some examples in humans are your ears, hands, arms, feet, etc.

FA will be present in all bilateral traits regardless of species or the traits being measured. This includes humans as well. Think of the slight differences between the fingers on your right hand and those on your left. We usually imagine our right and left hands and fingers as mirroring one another (the same with our feet). However, most of us are aware of slight differences between the left and right. These are examples of FA.

As previously mentioned, the degree of asymmetry between animals can vary greatly, but with FA the differences are minor and are to be expected. In most cases with deer antlers, FA will be minimal, with each antler having maybe a couple inches of variation in each antler characteristic. For example, if the right G1 (brow tine) is 6 1/8 inches long and the left G1 is 6 inches long, there would be 1/8-inch of FA in that buck’s brow tines. This sort of variation or asymmetry is normal, and you can expect to find these minor differences between the right and left antlers for all antler measurements. In other words, good luck finding that perfectly symmetrical deer, the one where there will be no deductions when calculating its net B&C score.

All deer species have some degree of asymmetry present in their antlers. Generally, deer species that use their antlers primarily as weapons for dominance and are more solitary, such as the pudu, have less complex antlers and those antlers will have less FA. However, social deer species that utilize their antlers more for displaying to potential mates, such as caribou, have more complex antlers and thus, greater levels of FA. White-tailed deer fall in the middle of this spectrum, with strong antlers to aid in fighting for dominance, yet are complex enough to serve as ornamentation for displaying quality to potential mates.

Injuries Can Cause Asymmetry

The most obvious and striking forms of asymmetry in antlers are caused by injury. The three most common types are damage to the pedicle, damage to the antler while it is still in velvet or after the antler has mineralized, and leg injuries. The antler pedicle is essentially a permanent extension of the skull frontal bone from which the antlers are regenerated during the spring and cast during mid-late winter.

If you look at a buck’s skull, such as those European mounts we all have on display, the pedicle is the small extension of the skull that is at the base of the antlers. Physical trauma to the pedicle and/or skull is primarily caused by impact trauma or an antler that has shed irregularly. Impact trauma can be caused by normal activities (e.g., sparring/fighting and rubbing) or from external sources (e.g., collisions with vehicles).

Antlers that cast irregularly, often occurring earlier in the season than normal, can detach with a portion of the pedicle attached to the shed antler. If you have ever found a shed that does not have a flat, uniform base, but rather a jagged edge, chances are good that the shed antler contains a portion of that buck’s pedicle.

Damage to the pedicle or the frontal bone can lead to gross variation in antler development and substantial asymmetry when compared to the other antler. Damage to the pedicle and/or skull often leads to malformed antlers throughout a deer’s life, such as bucks with “spike on one side” characteristics, and in some cases can eventually lead to death.

In a study conducted in our lab at Auburn University, pedicle damage was the leading cause of the infamous “spike on one side” anomaly. While many hunters interpret this antler malformation as a sign of poor genetics, our data demonstrated that in some age classes up to 75% of antler abnormalities may be caused by pedicle damage. In more extreme instances, damage to the pedicle or frontal bone might cause intracranial abscesses (a bacterial infection of the brain) resulting in illness or death.

In a study conducted at the University of Georgia, researchers found that intracranial abscesses caused illness or death in 2.2% (87% of which were males) of examined white-tailed deer skulls in 12 states and four Canadian provinces. Pedicle damage can become increasingly prevalent in populations that are managed for older age classes of males. Older males fight for dominance more aggressively, and therefore have a greater chance of damaging their pedicles or skull through combat. Damage to the pedicle often results in permanent damage to that antler side, and generally will result in asymmetry for the remainder of that deer’s life.

In contrast, damage to antlers in velvet or the breaking of tines after the antler has hardened often results in a temporary malformation and asymmetry (i.e., that annual antler set). Antlers that are still in velvet are especially subject to breakage and compaction damage. Unlike the hardened antlers on the bucks we hunt during the fall, growing, velvet antlers are soft, full of nerves, and highly vascular and cartilaginous, especially in the growing antler tips (similar to your ears).

A minor impact to an antler in velvet is much more likely to lead to breakage than hardened antlers, yet the antler might heal and continue to grow. A healed break might appear as a tine or main beam that suddenly makes a sharp turn after it is mineralized. Contrastingly, hardened antlers are mineralized and lack the nerve endings and blood vessels present in growing antlers. Hardened antlers are utilized for sparring and fighting for dominance and are subject to breakage through combat.

Our research at Auburn University has shown that hardened antlers in one population had an estimated breakage rate (at least one broken tine or main beam) of around 50%. However, these data were collected from a population that had a male-skewed sex ratio and a high density of mature males. Deer populations with more typical population characteristics might not have such high antler breakage rates.

Contralateral Antler Malformation

Contralateral antler malformation refers to the neurological condition where a leg injury causes abnormalities in antler development. Contralateral refers to the injury and the malformed antler being on opposite sides of the body. When a hind leg sustains the injury, development of the opposite antler might be negatively affected (i.e., an injury to the left hind leg can cause antler malformation on the right antler). However, when the front leg sustains the injury, the antler on the same side might be negatively impacted. Some causes of leg injuries that might lead to contralateral antler malformation are vehicle collisions, old gun shot/arrow wounds, or injuries sustained while fighting other bucks.

In a study conducted in Texas examining 32 hunter-harvested bucks with antler malformations, 22 (69%) of the malformations were attributed to either old gun shot wounds or healed leg fractures. Scientists believe that contralateral asymmetry is neurologically caused, however, and the exact mechanisms are currently unknown. Depending on the severity of the injury and the animal’s ability to heal that injury, contralateral asymmetry may or may not result in permanent asymmetry to the antler.

Environmental Factors

In addition to the factors listed earlier that lead to gross antler malformations, there are other less obvious influences that can cause asymmetry in developing antlers. Antler size is controlled by three main factors: age, nutrition and genetics. Age is something that deer managers have considerable control over (just don’t shoot him and he’ll get older), and genetics is essentially out of their control. However, nutrition is something that we are able to influence, but there are also a variety of factors out of our control that affect nutritional availability. Because nutritional availability is greatly influenced by environmental conditions, therefore, so is antler development.

Antlers are quite possibly the fastest growing tissue in the animal kingdom, and thus require a substantial quantity of nutrients during a short time period to sustain their production. Research has shown that the highest-quality males in the best condition (e.g., those that have the greatest immune health, those that can secure the most nutrients, etc.) will grow the largest antlers. Additionally, research has identified a negative relationship between the degree of FA and the size of antlers, suggesting that bucks with more symmetrical antlers are of greater quality. Higher quality males (i.e., those with better genetics, stronger immune systems, etc.) are better able to cope with the nutritional stress imposed by growing large antlers, and thus have antlers with less fluctuating asymmetry.

Diseases and parasites are another major source of environmental stress for wild animals. They influence antler asymmetry by forcing a buck to utilize energy and nutrients to cope with pathogenic challenge, thus leaving less energy and nutrients to devote to antler growth and development.

The degree to which a buck can handle the stress of a disease is indicative of his immune health. Any particular disease (e.g., epizootic hemorrhagic disease or EHD) or parasite (e.g., ticks) has the potential to negatively impact antler development, as recovery will influence the availability of resources that can be devoted to developing antlers. Those that can handle and recover quickly from a disease (or are immune entirely) will be able to devote those valuable resources to antler growth and should have greater symmetry in their antlers.

Age Influence

As we know, annual antler size in deer increases rapidly until they reach their maximum size, when a buck is around 6 1/2 years old. Research in our lab suggests that white-tailed deer have less antler asymmetry with increasing age. The most likely explanation for the negative relationship between age and FA is that low-quality males (those that would likely have high FA) are more likely to have already experienced mortality before reaching prime age. As we discussed, low-quality males produce antlers with less symmetry, due to a lack of being able to cope with environmental stresses. These lower-quality males are more susceptible to diseases, parasites, predation and malnutrition. Thus, we would expect the proportion of low-quality males (those with more antler asymmetry) to experience greater levels of mortality and decrease in a cohort as it gets older.

Genetic Potential

No antler discussion would be complete without a look at genetics. Genetics determines the potential size and configuration of a set of antlers. The true antler potential for an individual can be expressed only if that individual has reached an old enough age and has had adequate nutrition available to grow large antlers.

The interesting aspect about antler asymmetry and antler genetics is that researchers believe there is no separate left and right antler gene. In other words, genetically, the left and right side of an antler set should be perfectly symmetrical. External stresses (the environment) and damage to pedicles, velvet and hardened antlers, and injuries to the limbs are responsible for the deviations from perfect symmetry.

However, genetics do play an indirect role in causing increased asymmetry. The susceptibility of an individual to those environmental stresses we discussed earlier is greatly influenced by an individual’s genetics. An individual’s ability to combat diseases, parasites and periods of nutritional stress is often genetically controlled.

Two bucks experiencing identical environmental conditions are likely not to respond to a given environmental stress in the same way because of variation in their genome. For example, one of those males might have the genetic foundation needed to better cope with the stresses caused by an outbreak of EHD, and therefore will have more resources to devote to antler growth and development, and should produce antlers with less asymmetry.

While our understanding of the influence of genetics on asymmetry is in its infancy, we do know that regardless of an individual’s genetic quality, the environmental conditions they face, or the injuries they might sustain, one thing is certain — no deer will ever have a perfectly symmetrical set of antlers.

— Dr. Steve Ditchkoff is a professor in the School of Forestry and Wildlife Sciences at Auburn University. He manages the deer research program at Auburn and has been conducting research on white-tailed deer for more than 25 years.

— Nicholas Deig is nearing completion of his M.S. degree at Auburn University under the supervision of Dr. Steve Ditchkoff. His thesis research is focused on antler characteristics of white-tailed deer.

Literature Cited

Baumann, C.D., W.R. Davidson, D.E. Roscoe and K. Beheler-Amass. 2001. “Intracranial Abscessation in White-tailed deer of North America.” Journal of Wildlife Diseases 37(4):661-670.

Ditchkoff, S.S., R.L. Lochmiller, R.E. Masters, W.R. Starry and D.M. Leslie. 2001. “Does Fluctuating Asymmetry of Antlers in White-tailed deer (Odocoileus virginianus) Follow Patterns Predicted for Sexually Selected Traits?” Proceedings of the Royal Society of London B: Biological Sciences 268:891-898.

Karns, G.R. and S.S. Ditchkoff. 2012. “Antler Breakage Patterns in White-tailed Deer.” The Proceedings of the Southeastern Association of Fish and Wildlife Agencies 66:114-119.

Karns G.R. and S.S. Ditchkoff. 2013. “Trauma-induced Malformed Antler Development in Male White-tailed Deer.” 37(4):832-837.

Marburger, R.G., R.M. Robinson, J.W. Thomas, M.J. Andregg and K.A. Clark. 1972. “Antler Malformation Produced by Leg Injury in White-tailed Deer.” Journal of Wildlife Diseases 8:331-314.

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