Is Antler Expression Predictable - Effects of Environmental Conditions on Antler Development

Aaron Foley - Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville
Randy DeYoung - Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville

From a tree-stand, a hunter observes a 10-point, 130-inch Boone and Crockett score white-tailed deer walking through a draw. The hunter grabs his rifle but decides not to shoot because the buck appears to be 4-years old and may become a better trophy the following year. Elsewhere, a hunter observes a 7-point 130 inch 4-year old buck making a scrape. In order to improve "genetics" in the herd, the hunter decides to harvest the buck under the assumption that this male will continue to have below average antler size. The hunters in these 2 scenarios are making opposite assumptions -- antler size of one male will increase and another will remain identical. Which hunter made the correct judgment?

Images of 2 white-tailed deer bucks from a captive research herd photographed in consecutive years. The first male (top) has nearly identical antler characteristics indicative of high repeatability. Antler characteristics of the second male (bottom) varied, an example of lower repeatability

In areas of intensive deer harvest, deer managers often use antler criteria to guide harvest decisions. A manager can use antler criteria to protect young animals, or remove bucks with undesirable antler characteristics, either to provide more forage resources for the remaining deer, or in the hopes that selective harvest might improve the genetic potential for antler development. Conversely, several recent studies have raised concerns about the potential for "high-grading" a population by selective harvest of large-antlered bucks, in essence removal of the "best" genetic potential. These harvest management scenarios assume that antler size is predictable. One avenue to test the predictability of antler size is to use a quantitative genetics metric termed repeatability. Repeatability ranges from 0 to 1, with 1 indicating the exact same antler measurement is obtained every year an individual’s antlers are measured. A value near 0 indicates there is little similarity in a deer’s antlers when measured in subsequent years.

With the help of several colleagues, we used the helicopter net-gunning technique to capture 30-150 bucks annually on 7 south Texas study sites during 1985-2008. Using antler measurements collected from males aged 3-6 years old, we estimated repeatability of inside spread, main beam length, number of points, basal circumference, and total antler size. Total antler size is the gross Boone and Crockett score minus the inside spread. Inside spread was not included because it does not describe the size of an antler.

We amassed a large data set - more than 3000 bucks total! Overall, repeatability was moderate to high, ranging from 0.42-0.80 for all traits across all study sites. Main beam length (0.66) and inside spread (0.69) had the highest average repeatability, while number of typical antler points (0.53) had the lowest average repeatability.

Study sites were not equal with respect to nutritional resources for deer, and differed in spring rainfall variability (important for forage production) and presence of supplemental nutrition. We separated study sites based on rainfall variability (variable vs. consistent) and presence of supplemental nutrition. Average repeatability was lower in variable rainfall sites than consistent rainfall sites. The most pronounced difference was number of typical antler points; repeatability was 31% lower in variable rainfall sites. Presence of supplemental nutrition appeared to moderate some of the environmental effects on several antler traits, producing 18% higher repeatability compared to un-supplemented sites.

What do these findings mean for hunters and wildlife managers?

• Repeatability differed among antler traits, suggesting that bucks may invest resources disproportionately among traits during antler growth. Conversely, environmental variation may have a greater effect on some antler traits than others.
• Repeatability of antler points was low relative to other antler traits. Antlers are used in visual displays and fights with rival bucks. Within a range of antler points (e.g., 8 vs. 10 points) a buck’s antlers may appear visually similar to rival bucks. Therefore, in poor years, a buck may allocate resources to maintain the overall visual appearance and breaking strength of his antlers, but might invest less in points.
• The low repeatability of number of antler points suggests antler points alone might be a poor criteria in some harvest programs for mature bucks because the number of antler points may change from year to year.
• Because repeatability is lower in areas of variable rainfall, harvest in such areas is less likely to influence antler size in mature males, either positively or negatively. Efficiency of selection may be improved in areas with consistent environmental conditions and with the presence of supplemental nutrition.
• Because supplemental nutrition reduced environmental influences on antler development, habitat and forage management in variable environments may improve antler performance by reducing the effect of annual environmental variation.
• Decisions on whether to delay harvest of a mature buck until the following year will depend on variability of the environment. In areas with consistent environmental conditions, mature bucks are less likely to change antler traits from year to year. In contrast, when hunting in a variable environment during a poor year, not harvesting a mature buck may allow him to grow bigger antlers the following year if conditions improve.
• Antler repeatability differed for sites in close geographic proximity. Deer are continuously distributed in the region, so genetic differences seem an unlikely explanation. It appears likely that differences in habitat, nutrition, or other environmental factors are responsible.
• Finally, our estimates of antler trait repeatability were similar to previous studies on red deer and other species of deer. This observation suggests antler expression in other species of cervids may respond similarly to environmental conditions.

So, which hunter made the right decision - letting the 10-point buck walk or harvesting the 7-pointer? Who knows…it depends largely on the management goals, geographic location and what Mother Nature has planned for the following year!

For the published results, see:
Foley, A. M., R. W. DeYoung, S. D. Lukefahr, J. S. Lewis, D. G. Hewitt, M. W. Hellickson, D. A. Draeger, and C. A. DeYoung. 2012. Repeatability of antler characteristics in mature white-tailed deer in south Texas: consequences of environmental effects. Journal of Mammalogy 93:1149-1157.

About the author: Aaron Foley is a recent PhD graduate who worked under Drs. Randy DeYoung and David Hewitt. Collaborators on this research project: Steven Lukefahr assisted with the repeatability analysis. Mickey Hellickson, David Hewitt, John Lewis, and Fred Bryant contributed data from the South Texas Buck Capture Project and Stuart Stedman and Charles DeYoung contributed data from the Faith Ranch Buck Capture Project. Dan Friedkin and Donnie Draeger contributed data from the Comanche Ranch Buck Capture Project.

Trophy Points: Big Game Research On Line is complied and edited by David G. Hewitt, a Professional Member of the Boone and Crockett Club and the Stuart W. Stedman Chair for White-tailed Deer Research at the Caesar Kleberg Wildlife Research Institute.