ARE WE SHOOTING THE WRONG DEER?

Hunting May Have Genetic Consequences

by John J. Ozoga

         Whitetail deer are considered to be the most genetically diverse of the large grazing animals. That's extremely important to their overall health and fitness. Deer with high genetic diversity are typically more aggressive, achieve higher body weights and have greater reproductive success. Further, the bucks have larger antlers compared to others with less genetic diversity.

         This genetic diversity provides the whitetail deer its “plastic” behavior and highly adaptive nature, and likely accounts for the whitetail's spectacular success in North America.

         Because it can be highly selective, hunting mortality is potentially one of the most important factors that might disrupt whitetail genetics. Throughout the whitetails’ range, hunting mortality is one of the most important factors altering population density, sex ratio, age distribution and social structure. Hence, hunting could profoundly affect the whitetails’ maintenance of genetic variability — as various critics of sport deer hunting have charged.

The Debate

         We hunters emphasize the sporting value of deer hunting and fair chase, and we expound upon the vital role we play in controlling deer populations. Meanwhile, critics (anti-hunters) say we are mostly interested in producing a super abundance of living targets, that we farm deer for human benefits and that we kill the wrong deer — the genetically superior and healthiest — which will have long-term genetic consequences. Even hunting groups occasionally become involved in that debate.

         Those supporting traditional deer management, for example, criticize quality deer management advocates for employing antler restrictions. These deer managers say that letting genetically inferior bucks do the breeding results in high-grading.

         Meanwhile, proponents of QDM criticize TDM because it encourages nearly complete annual cropping of adult bucks. With such intensive buck harvesting, young bucks must prematurely assume the role of breeders. There is minimal contesting of dominance, and nearly all young males mate and sire offspring, with little or no mate selection for superior traits.

         Still other deer managers employ controversial culling practices, wherein young bucks with inferior antlers — and presumed inferior genetics — are removed in hopes of improving antler quality in the herd. In the process, however, the question is, do they also remove other desirable (adaptive) traits?

         Despite these debates, most wildlife managers have paid little attention to the long-term effects hunting might have on the genetic make-up of whitetails, as a review led by Richard Harris, a researcher from the University of Montana, revealed.

A Commentary

Harris and his associates reviewed literature to determine if there is sound scientific evidence that hunting has documented genetic consequences. Their findings were published in a 2002 issue of the Wildlife Society Bulletin. Although their literature search dealt with the effects of hunting in general, it included much of what we currently know — and don't know — about hunting effects on whitetail genetics.

         The investigators identified four potential effects of sport hunting:

 (1) It might alter the rate of gene flow among neighboring populations.

 (2) It might alter the rate of genetic drift through its effect on genetically effective population size.

 (3) It might decrease fitness by deliberately culling deer with traits considered undesirable by hunters or managers.

 (4) It might unintentionally decrease fitness by selectively removing deer with traits desired by hunters.

Hunting and Gene Flow

         Intensive buck harvesting can be detrimental to the genetic composition of a population. For example, as researcher Darrell Ellsworth and his co-workers said, “Bucks-only hunting ... may potentially alter dispersal patterns (and hence levels of gene flow) among populations if the harvest is severely biased toward males. Excessive removal of bucks may reduce the number of young males that would normally disperse and possibly reproduce in a newly established territory, and remove the dominant local breeders that usually displace subordinate individuals.”

         Among whitetails, males disperse more frequently than females and are responsible for most of gene flow from one subpopulation of whitetails to the next. That dispersal helps maintain an optimal balance between inbreeding and outbreeding. It tends to prevent the adverse effects of mother-son and brother-sister inbreeding, contributes to favorable genetic flow and promotes genetic diversity leading to greater population adaptability.

         “Limiting gene flow among populations may result in a reduction of the effective population size (number of breeding individuals) and the subsequent loss of genetic variation," Harris said. "Therefore, management policies should strive to preserve the pattern of sex-biased dispersal and maintain exchange of genetic information among populations by regulating the degree to which males are preferentially harvested.”

There is also concern that local genetic adaptations might be lost if the female social organization were disrupted through hunting. That is, female whitetails spend their entire lifetime on familiar ancestral range in close association with related females. They adapt to the local environment and exhibit orderly range-use patterns that enhance fawn survival. Excessive harvesting of female deer could disrupt the female social organization and remove important adaptive traits.

         Intense female harvesting has also been shown to alter male dispersal traits. In studies conducted by Georgia researchers Stefan Holzenbein and Larry Marchinton, fewer orphaned bucks dispersed from their natal range at yearling age compared to those raised with their mothers. Those findings suggest that brother-sister inbreeding — a deleterious effect — would be more common in herds in which adult does were heavily harvested.

         Other studies have produced contrasting results regarding the effects of orphaning. Some have shown higher dispersal rates for orphans, but others found no effects of orphaning on buck dispersal rate.

         All things considered, the Harris commentary concluded, “The available evidence suggested to us that alterations in naturally occurring patterns of gene flow would seem possible from any type of hunt; some level of social disruption must accompany any removal of individuals.”

Hunting and Genetic Drift

         Genetic drift is the random change in gene frequencies. It occurs in all populations, but its effects become pronounced only if genetically effective population is small. When the effective population size is small, genetic diversity is expected to decline, rare alleles (mutational forms of genes) are expected to be lost or adverse mutations might become fixed.

         We expect problems with genetic drift in small or declining populations rather than with large populations typically subjected to sport hunting. However, Harris and his group warn that highly skewed sex ratios in herds in which males are selectively hunted can lead to genetic consequences. For example, sex ratios of one adult buck to 10 adult does have been documented in elk and mule deer. As a result, small breeding groups of related individuals can occur within larger populations because related females show strong attachment to their established ranges.

         Therefore, highly skewed sex ratios may increase the frequency of inbreeding, even with little population-wide genetic drift. Most managers of ungulate populations attempt to prevent adult sex ratios from becoming too unbalanced to maintain normal breeding behavior.

Hunting and Deliberate Selection

         Culling yearling bucks with poor antler development is a recommended management practice in some areas, particularly the Southwest.

         “Although such management might be seen as a partial correction to practices where only the largest animals are taken, it is not without risks," Harris said. "By selecting for one particular trait of perceived value to humans, we believe it likely that management simultaneously (if inadvertently) selects against other traits potentially of adaptive significance for the species. In particular, relatively rare alleles that might be important in a long-term evolutionary perspective are vulnerable to loss when such selection for other traits take place.”

         That was proven with red deer in Europe. Culling yearling bulls with inferior antlers resulted in an increase in antler points but a sharp decrease in calf survival.

“In general,” the researchers concluded, “when one phenotypic trait is maximized, other traits are inevitably (and probably unknowingly) affected because life-history strategies inevitably involve trade-offs among various fitness components related to demographic equilibrium. Thus, human attempts to improve hunted species through selective culling seem certain to produce unforeseen consequences.”

         The authors voiced the same warning relative to the introduction of penned deer: “Releasing penned deer bred specifically for antler growth into the wild (to produce large ‘super bucks’) seems to us careless disregard for this fundamental concept.”

Hunting and Unintentional Selection

         Hunting regimes that concentrate on removing large bucks might inadvertently remove genetic traits hunters desire. Such selective harvest might adversely affect herd genetics in two ways: (1) by reducing genetic diversity or (2) by causing the loss of specific alleles.

         Although there is conflicting evidence, high genetic diversity in whitetails has been associated with larger antlers, higher conception rates, better fetal growth, and higher body weight and size. Despite the potential risks involved in selectively removing bucks with presumed greater genetic diversity, the authors found no evidence to support such concern.

         A slightly different problem might come into play with trophy hunting, in which hunters purposely remove bucks with the largest antlers.

“Such hunts may unintentionally select against those very traits by reducing the life span (and thus the reproductive contribution) of individuals carrying specific alleles,” Harris said.

         Although studies show contrasting results, Florida researchers Steve Shea and R.E. Vanderhoff observed a reduction in antler size among 2.5-year-old bucks five years after prohibiting the harvest of small-antlered deer. They attributed that change to high-grading of bucks born earlier during the year. That is, early birth was associated with larger antlers, leaving predominately late-born bucks to survive to 2.5 years.

         High-grading effects, for different reasons, were also reported in Missouri. In both cases, however, the researchers did not check to see if there was a genetic basis for such change.

         Hunters invariably prefer to kill large-antlered bucks. However, Harris and his associates point out that hunting does not always remove more fit deer because hunters are taking the oldest bucks, not necessarily those with superior genetics. Also, genetic changes caused by selectively taking large-antlered bucks might be buffered by the genetic contribution of females.

         If bucks with the largest antlers do most of the breeding but are selectively removed by hunters, the rate of loss of alleles affecting antler growth would be high. However, with whitetails, young and smaller bucks also do considerable breeding, as recent DNA studies show, meaning the rate of loss would be lower than expected.

When whitetails are selectively hunted, long-term changes in allele frequency can be expected.

         “It is difficult to see how it could be otherwise,” Harris said, "given that hunting often constitutes the largest source of mortality. However, because age and environment exert major influences on size, mating systems are often flexible, and gene flow among adjacent populations that vary in mortality patterns may replenish vulnerable alleles ... we expect such changes to occur gradually and to be undetectable for many generation lengths.”

Conclusions

         Clearly, hunting can result in certain genetic consequences. Unfortunately, the subject has not been intensively investigated, and the exact potential consequences are obscure. The effects tend to be subtle and difficult to detect without long-term monitoring.

         New genetic techniques that let researchers accurately identify the mothers and fathers of offspring will likely yield a much better understanding of the whitetails’ reproductive behavior — and a much better chance to determine how hunting affects the species' genetic well-being.

         Meanwhile, the Harris commentary urges managers “to manage hunting such that the age-specific survival pattern (and thus age-specific structure) emulates that occurring in the absence of hunting.”

         That is, attempt to inflict natural-type mortality — by killing young and old animals most intensively — to maintain populations that have natural sex and age structure.

“Such hunting regimes will generally produce little alteration in allele frequencies, have low chance of causing extinction of rare alleles, minimize extremely skewed sex ratios ... and still allow for the hunting opportunities we cherish,” Harris said.

         With that in mind, how best can we maintain natural deer populations? By using traditional (farming-type) management practices or by practicing “true” quality deer management? Or, should we adopt an entirely new hunting philosophy — by using harvest strategies that more closely mimic natural predation?