Antler Development: A New Look at an Old Obsession
By Bill Gray, Wildlife Biologist
Large antlers in male white-tailed deer are one of the natural world’s most revered structures. For the most part, large antlers are viewed within the context of human understanding. What then, is their significance when viewed within the context of their role in white-tailed deer ecology? One study may begin to shed some light on the significance of large antlers from a white-tail’s point of view.
The major histocompatability complex (MHC) is a grouping of genes closely related to the immune system in all vertebrate animals. Its relationship to the immune system is rooted in its important role in recognizing and responding to pathogens that enter the body. Consequently, the MHC is considered an indicator of genetic quality because of the severe threat posed by parasites and diseases. Researchers have long considered the possibility that secondary sexual characteristics such as antler development and body size may be an outward expression of genetic fitness. The relationship between certain physical characteristics and the MHC was the focus of a study completed with deer from the McAlester Army Ammunition Plant (McAAP) in southeastern, Oklahoma.
A wide variety of data was collected from 128 hunter harvested deer from October and November of 1995 and 1996 on the McAAP site. This data included chest girth, body weights, antler scores (using Boone and Crocket standards), skull length, overall body length, and both internal and external parasite loads. In addition, liver tissue was taken from each to provide material for genetic analysis. Upon genetic analysis, deer were placed in one of two distinct genetic lineages and were found to be assignable to three categories or “types” based on MHC specific genetic profiles, those being type 11, 12, and 22 deer respectively. Type 11 deer had MHC profiles from lineage 1; type 22 deer had MHC profiles from lineage 2; and type 12 deer had MHC specific genetic profiles from both lineages – thus indicating greater heterozygosity (genetic diversity) for MHC profile.
Interestingly, deer with both MHC profiles (type 12 deer) had 20% and 13% greater Boone and Crocket scores than type 11 or 22 deer respectively. Similarly, average number of points and average basal circumference for type 12 deer were 14% and 8% greater than for type 11 and 22 deer respectively. Type 12 deer also had a 13% greater average length of main beams than type 11 deer whereas this trait did not differ in comparison to type 22 deer. Other physical indices such as chest girth, body mass, and skull length did not differ significantly between the three types of deer with the exception of a 5% greater average body mass and skull length of type 12 deer when compared to type 11 deer.
The “good-genes” theory predicts that highly ornamented (i.e. large antlers) males should have lower internal parasite loads because they possess a superior immune system (MHC) better suited to cope with parasites. This may be further supported by the McAAP study in that the total number of stomach parasites present was found to be negatively associated with average antler score (more internal parasites/lower antler score). Additionally, Type 12 deer tended to have lower levels of both internal and external parasites than Type 11 or Type 22 deer. Conversely, the average number of external parasites showed a positive association with both antler size and body mass (more external parasites/higher average body mass and antler score). The latter finding may be supported by other studies indicating increased density of external parasites as host body size increases. Increased antler size relative to external parasite density is thought to be coincidental to a positive relationship between increased body size and increased antler development.
While there remains much research to be done, this study represents a crucial first step in documenting an association between antler development and the immune system (MHC). Because antler growth is such a biologically demanding process, this study suggests that antler development is an honest advertisement of an individual’s ability to cope with nutritional stress while simultaneously maintaining a properly functioning immune system. In short, individuals like those represented as type 12 deer, are able to advertise superior genetic “fitness” by maximizing antler development.
Antler quality in male deer has long been an area of fascination and prolific research. Most of the focus concerning antlers has been directed at the “how” of antler development in terms of techniques that may be used to enhance or otherwise produce large antlers. In this rush to produce a bottom line that can be measured in inches – deer managers have often overlooked equally important aspects that are only beginning to be addressed as we begin to consider the “why” of antler development.
Ditchkoff, S.S., Robert L. Lochmiller, Ronald E. Masters, Steven R. Hooper, and Ronald A. Van Den Bussche. 2001. Major-Histocompatibility-Complex Associated Variation in Secondary Sexual Traits of White-tailed Deer (Odocoileus Virginianus): Evidence for Good Gene Advertisement. Evolution 55 (3): 616-625
Ditchkoff, S.S., Steven R. Hooper, Robert Lochmiller, Ronald E. Masters, and Ronald A. Van Den Brussche. 2005. MHC-DRB Evolution Provides Insight Into Parasite Resistance in White-tailed Deer. The Southwestern Naturalist 50 (1): 57-64