The Myth of Managing Genetics in Free-ranging White-tails
Captive buck in velvet
Courtesy of Texas Wildlife, Magazine of the Texas Wildlife Association
Article by Randy DeYoung, Caesar Kleberg Wildlife Research Institute
Trends in deer management place increasing emphasis on genetics. One oft repeated phrase is that large-antlered bucks are the product of “age, nutrition, and genetics.” While all three factors are important, the exact role of genetics remains murky. From a scientific perspective, the importance of age and nutrition are well established, but some management strategies aimed at affecting genetics are questionable for free-ranging deer.
What do we know about deer genetics, and can we manage genetics in free-ranging deer? The answers to these questions involve an understanding of nutrition and habitat quality, buck breeding success, and deer behavior.
White-tailed deer range from Canada to South America and vary extensively in body size and other traits over this vast range. Some of these differences are due to genetic factors, but many environmental factors can influence body size, growth rates, and physical characters. In fact, within regions, deer populations can vary extensively in body and antler size.
A recent study in Mississippi revealed that average body and antler size of both bucks and does was associated with soil quality. Mature bucks in the best soil regions averaged 25 pounds heavier and 20 B&C inches larger than bucks in poor soil regions. Furthermore, deer in the best soil region approached their maximum body and antler size at a younger age than deer in the poorest soil region. Molecular data indicate that there is little genetic difference between deer in these soil regions. Thus, soil and habitat quality play a large role in physical differences among populations of deer within and among regions.
There has always been a mystique about the ability of south Texas to produce big deer. Really, the main factor has been the ability to control the age of bucks over large areas by controlling harvest. Many areas of the Southeast, Midwest and elsewhere in the U.S. could consistently produce big bucks if they would allow young bucks to mature, instead of harvesting them at 1.5 to 2.5 years old. This simple fact underscores the importance of age and nutrition!
Many management strategies aimed at improving population antler size through manipulation of genetics involve selective harvest or introduction of deer with desirable antler characteristics. Several recent studies question the basic assumptions of many selective harvest or introduction programs.
One such assumption is that bucks with desirable antler characteristics will breed and sire most or all of fawns produced. However, studies of genetic parentage in deer indicate that breeding is spread among many different bucks. Although mature bucks will sire about 70 percent of the offspring in balanced populations— those with reasonable sex ratio and age structure—young bucks (1.5 and 2.5 years old) may collectively sire about 30 percent of fawns.
There appears to be no clear relationship between antler size and breeding success; some large-antlered bucks sire fawns, some apparently do not. Thus, it is nearly impossible to predict which wild bucks are breeding or how many offspring they will sire. It is clear that managers must exercise a great deal of control over a population before they can be assured
that only desirable bucks will breed, which means that every undesirable buck must be removed before the rut each year, a difficult prospect for large properties.
Another factor severely limits the potential for managing genetics in free-ranging populations: bucks do not stay where they are born. Most bucks permanently disperse at 1.5 to 2.5 years of age, traveling from two to 25 miles, or more. Thus, it does little good for landowners with low-fenced land to attempt to affect genetics of a deer population. Most buck fawns born on a property will disperse, while older bucks were probably born miles away. Large property size has less of an effect than you might think. On a low-fenced ranch of 25,000 acres, only 70 to 80 percent of bucks born on that property will remain there.
A long-term study in Scotland of red deer breeding success and heritability of antler traits has raised some serious questions about whether one can achieve dramatic success in free-ranging deer via selective harvest. The study concluded that age, year of growth and permanent environmental effects had a large influence on antler size. A red deer stag had to be mature and
in good condition to grow big antlers; it was this combination of large, healthy body and antler size, plus intangibles such as aggressiveness, that allowed these stags to compete for and fight to win and keep harems and breeding rights.
Any factor that affected a stag’s health or body condition affected antler size. Stags born in a bad year could be stunted permanently and would never grow large antlers at maturity regardless of their genetic potential. The environment had such a large effect that antler size alone was not a reliable predictor of genetic quality for antler development.
Are these results relevant to white-tailed deer? Probably yes, in most cases, although there is not enough evidence to indicate whether these permanent effects caused by early life experiences are as dramatic for white-tailed deer as they are for red deer. Studies of wild deer have indicated that yearling deer with poor antlers— spikes—tend to have smaller antlers at maturity than yearlings with forked antlers. On average, forked-antlered deer turn out to be 10 to 20 B&C inches larger than spikes in a sample of south Texas bucks.
Are these differences due to genetics, or do they simply indicate that these particular yearlings had a poor start and could not overcome early life challenges? At this time, no one can say for sure, but the red deer study suggests that genetic potential of wild deer is not always easy to judge.
What about captive deer? One common response when the feasibility of genetic management techniques is questioned is that selective breeding works in captive deer, so it should work in wild deer, as well. Deer breeders are clearly successful in producing impressive bucks. However, the critical question is whether the results achieved in a pen are easily transferable to free-ranging populations.
Why does genetic manipulation work in pens? In a pen you can: 1) control exactly who breeds; 2) select both the bucks and does as parents; and 3) perform controlled inbreeding (or line-breeding) to concentrate good traits in a lineage. While breeders have produced some outstanding successes, the process is inexact, and not all offspring develop large antlers. Thus, a process some have termed the “corral continuum” must be considered: the ability to achieve any genetic manipulation will increase as the situation approaches the level of control by captive breeders and decreases as the property approaches free range.
If a manager cannot easily affect genetics of free-ranging deer, is there still a role for selective harvest or culling programs? It depends. Removing undesirable bucks can make food and other resources more available for remaining deer. However, recruitment and fawn survival must be high to sustain the harvest. If fawn survival is variable among years, such as due to frequent droughts in arid regions, removing a large number of bucks may only result in fewer deer. One additional caveat: although spike-antlered yearlings tend to be smaller than fork-antlered bucks at maturity, many spikes will gain respectable antler size at 5.5 years (120 to 140 B&C inches), and a few will be larger. Mature bucks of this size have harvest value.
As scientists, our role in management is not to develop new management techniques but to develop knowledge to ensure that the principles of management rest upon a solid foundation. It appears that some of the foundations of genetic manipulation are not supported. Why, then is genetic management such a compelling topic? Many have claimed success when large-antlered bucks are produced within a few years of implementing genetic management in free-ranging deer via selective harvest or introduction plans. However, the same managers typically use many other management practices simultaneously— feeding, habitat improvements, balancing sex ratio and age structure.
One can measure pounds of feed consumed, acres in food plots or brush control, or sex ratio and age structure. Yet, the one factor that cannot be measured — genetics — tends to receive most of the credit. Perhaps there is an unconscious desire for a quick fix to a complex problem. Perhaps active management always feels better than sitting back and watching the deer grow. Perhaps any management strategy that involves shooting large numbers of bucks is just too compelling! Managers should critically examine the assumptions behind each management strategy and ascertain the potential benefits and liabilities of that approach in relation to their specific situation and management goals.
