It’s a calm, cold winter afternoon just before dark and you’re in the stand, sitting exactly where you want to be during the peak of the whitetail rut. Just upwind of you is a lush food plot that is beginning to fill up with does, and you’re certain that the old bucks are going to be close behind on this perfect day. As the light begins to fade, you notice what you think is a nice buck stepping out on the far end of the plot. Through the scope you can see that it is in fact a mature buck; however, his rack does not quite meet your standards to be considered a trophy shooter. The management plan for this property requires that a buck must have at least 8 points to be a trophy; however, the rules also allow for certain mature bucks with smaller antlers to be removed, or culled, from the herd.
By Chad Newbolt &
Dr. Stephen Ditchkoff
This particular old buck has a wide spread but only 4 antler points, and he certainly has his mind on breed- ing as he chases does in all directions. The evidence is indisputable to you that this is a prime candidate of a cull buck, and you decide to take him before he has a chance to pass on those genes for bad antlers. You quickly place the crosshairs on the brute and pull the trigger, dropping the buck where he stands. Although this buck was not quite what you were after, you are congratulated by other hunters for doing your part in herd management by culling this inferior-antlered buck. As you reflect on the hunt, you can’t help but ask yourself, “What effect does removing these bucks really have on the antler growth of future generations of deer?”
TO SHOOT OR TO NOT SHOOT
Whitetail hunters across the country are frequently faced with scenarios similar to the one just described, where they must quickly use a set of visual clues about a particular buck to decide whether to shoot him or pass him up. The hunter’s ultimate decision is based upon a multitude of factors; however, individuals hunting on private land often selectively harvest male deer with the management of herd genetics in mind.
Most often, these selective harvest practices target “cull bucks” that are thought to have smaller than average or malformed antlers. The idea here is that removing these inferior bucks will prevent them from breeding and allow bucks with more desirable antler growth to make greater genetic contributions. Some intensively managed properties actually take selective harvest a step further by protecting a select group of “breeder” bucks with the most desirable antlers so that these prime bucks get plenty of chances to reproduce.
The use of selective harvest for the purpose of improving herd genetics requires an understanding of the factors that determine if and how much a buck will breed, and that a buck’s potential reproductive contributions to the herd can be accurately predicted. In other words, it’s predicated on the assumption that we can look at a buck and use a set of visual clues, such as age, antler size, body size and behavior to determine the likelihood and level that he will participate in breeding.
Whitetail researchers believed for a long time that breeding was limited to a small group of prime-aged (4 1⁄2 years old and older) bucks that had large antlers and big bodies. These assumptions were based upon a few observational studies of white- tail behavior and on studies of male reproductive patterns in somewhat similar species, such as red deer.
Technological advancements near the turn of the past century led to widespread use of genetic tools in wildlife studies, namely DNA micro-satellites, that could be used to more accurately determine the parents of offspring. Studies of white-tailed deer that utilized these advancements painted a much different picture of reproductive patterns than was previously thought. These genetic studies demonstrated that reproduction was, in fact, not heavily skewed toward a small group of “prime” males.
SEE ALSO: Where Do All the Bucks Go Each Year?
Studies from Michigan, Maryland and Mississippi all reported that yearling (1 1⁄2 years of age) bucks consistently produced fewer offspring than bucks in older age classes; however, they also demonstrated that lots of bucks of nearly all age classes were siring fawns!
Further, some of these studies also reported relatively frequent occurrences of multiple paternity, or cases where a female deer gave birth to a litter of fawns fathered by two different bucks. Of all the questions that were answered and raised by these seminal genetic studies, one thing was clear; we still had a lot to learn about what makes a white-tailed buck a breeder.
As you can imagine, studying male reproductive success in a smart, often elusive animal can be extremely difficult, and the few available studies of white-tailed deer that are related to this topic have been limited by these challenges. Studies that rely on DNA fingerprinting to determine parentage require multiple pieces of key information that are typically very difficult to collect in natural environments.
First, they require that researchers have reliable information regarding the demographics of the deer herd: specifically, abundances of both male and female deer, in order to estimate how much confidence can be placed on the parentage assignments. Despite all of the methods of counting deer that are currently available, it is still extremely difficult to determine herd demographics for free- ranging herds to the level required to be certain of parentage assignments.
Next, genetic samples must be collected from a significant proportion of the bucks on a piece of property. In order to determine why certain bucks are breeders, you must also understand why others are not, which requires that samples are collected for all types of bucks. If researchers are lucky enough to get their hands on sufficient numbers of bucks, the sampled bucks are typically classified into age groups, using the popular tooth wear and replacement method.
Unfortunately, this method has proven to be unreliable at estimating the ages of deer into anything other than four age classes: fawns, 1 1⁄2 years, 2 1⁄2 years and 3 1⁄2 years and older. I’m sure you can understand how grouping 3 1⁄2-year- olds and 6 1⁄2-year- olds in the same age class would be problematic when trying to figure out which group of bucks is breeding.
Finally, genetic samples must be collected for the offspring of the sampled group of bucks. Researchers must be nearly 100 percent certain of the ages of this group of deer or else all kinds of really bad errors can result. The bottom line here is that free-range environments pres- ent multiple challenges that make it almost impossible to determine the specifics of male reproductive success in white-tailed deer.
In order to overcome the difficulties facing studies of free-ranging deer, researchers needed a research environment where detailed knowledge of a captive herd was blended with the realistic deer behavior present in a natural setting. During October 2007, the Auburn University Deer Research Facility was constructed specifically to answer important questions about the biology and behavior of white-tailed deer that could not be addressed by conventional studies.
The facility is a 430-acre high-fenced property located about 20 minutes northwest of Auburn near the city of Camp Hill, Alabama. The deer herd is made up of wild animals captured when the fence was constructed and their descendants, which helped researchers maintain as natural a herd as possible. Since construction, we have used dart guns to capture and mark more than 300 unique deer, or about 90 percent of the herd, which allows us to be pretty certain of deer demographics.
The overall goal of this long-term project is to continue to get our hands on as many deer as possible each year so that we can collect DNA samples and physical measurements of antlers and body size for bucks. Currently, we have used the first six years of this data to take a look at how three important factors — age, antler size and body size — influence male reproductive patterns. The sex ratio and age structure inside the facility shifted progressively to include a greater ratio of bucks and older deer during the years following construction, which allowed us to also look at the role of herd demographics (i.e. sex ratio, male age structure, deer density) in patterns of male breeding success.
The first thing we discovered when looking at the collected data was that lots of bucks, of nearly all ages, regularly bred during each year of the project. On average, about 48 percent of bucks 2 1⁄2 years of age or older were producing fawns each year!
These findings were not entirely novel, because previous studies reported similar results. However, what we discovered about the role of demographic conditions was extremely interesting. We found that during years when the herd was young (comprised of males 3 1⁄2 years and younger) and female deer outnumbered males, yearlings and male fawns made significant reproductive contributions.
That’s right, we observed male fawns breeding during the same year they were born.
Although researchers were aware that it was possible for male fawns to breed, our study represents the first documented occurrence of this in a natural setting. Young males were still breeding during later years of the project when sex ratios were balanced and mature males were abundant, but the number of offspring attributed to young males was much lower under these conditions.
The next important finding of our study was that although lots of males participated in breeding, males with large body size consistently produced more fawns than their smaller coun- terparts. Large body size logically provides advantages during fights for mates, but may also allow bucks to carry more fat for energy reserve to be used when searching for females. Male white-tailed deer go through extended periods when they eat very little during the rut, reportedly losing about 18 percent of body weight on average. An abundance of stored fat might be just what a buck needs to make it through the breeding season in good condition so he can produce as many offspring as possible.
DOES ANTLER SIZE REALLY MATTER?
So what’s the answer to the question you’ve all been waiting for? What role did antler size play in male breeding success? Well, we did find that antler size was an important determinant in male reproduction. However, the role of antlers was heavily dependent on what was going on with herd demographics.
We found that when sex ratios were skewed toward females and the male age structure was young, antlers were really not all that important in determining if and how often a male reproduced. This is likely because young males typically don’t fight with other males to gain breeding opportunities, and these conditions don’t provide a lot of opportunities for bucks to encounter rivals to fight anyway.
Antlers became an increasingly important determinant of reproductive success as males became more abundant relative to females and more prime-aged bucks were present. Basically, lots of old bucks and fewer females to go around resulted in a very competitive environment where big antlers provided an advantage.
Does it make sense to selectively harvest males for the purpose of improving herd genetics? Our results suggest that managers who attempt to alter herd genetics through selective harvest are, at the very least, fighting a Mt. Everest scale uphill battle. Although antlers do appear to be an important factor influencing male reproduction under certain conditions, lots of bucks, of all ages and antler sizes, are likely breeding on most properties.
Lots of breeding males means that protection or removal of any male likely has a small net effect on herd genetics. The reason why so many white-tailed bucks are breeding likely lies in the fact that the mating system of white-tailed deer makes it very difficult for a select group of males to monopolize females. Deer have a highly synchronous estrous period, which simply means that lots of does are receptive to breeding at the same time.
The fact that most deer are bred during brief periods means that their fawns will also be born during short periods, and these short fawning periods are thought to help aid fawn survival. The idea here is to swamp predators with more fawns than they can consume so that more fawns make it through these critical early days. Although short breeding periods are great for predator swamping, they are a real hindrance to managers looking to alter herd genetics through harvest. Individual males can tend only one receptive female at a time, which limits the number of females they can breed during the brief rutting period. No matter how big and bad a buck is, it’s physically impossible for him to breed all of the receptive does at any one time.
Our results also suggest that culling deer for genetic gains makes little sense, particularly on intensively managed properties, due to the fact that smaller-antlered males may already be in a disadvantaged position. What good does it do to remove small-antlered deer that were breeding less frequently anyhow?
In theory, our findings more strongly support passing up exceptional trophy bucks that are likely making a better than average number of reproductive contributions as a means for managing herd genetics for antler growth. But how realistic is it to expect most hunters to pass up the buck of a lifetime so that he can do some more breeding?
Managing herd demographics for a balanced sex ratio and older male age structures might create a competitive environment where large-antlered males gain a greater share of breed- ing opportunities, which could possibly influence herd genetics. Once again, the fact that so many bucks would likely still be breeding under these conditions makes it difficult to foresee these management actions having any measurable effect.
— 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 25 years.
— Chad Newbolt began working with the School of Forestry and Wildlife Sciences, Auburn University, as a research associate in 2007, and has since assisted with various wild- life research projects throughout the United States and abroad. His main research interests include white- tailed deer reproductive ecology and improvement of deer census methods.