Aging White-tailed Deer by Tooth Replacement and Wear: Accurate or Unrealiable
(En anglais seulement)
By: Dr. Larry Marchinton, Kent Kammermeyer, Brian Murphy
As a QDMA member, you understand the importance of collecting a lower jawbone from every deer harvested. By comparing each jawbone collected to established tooth eruption and wear criteria, an age can be assigned. The ages assigned allow important comparisons both within and between age classes and provide valuable insight regarding management success. Additionally, the population models used by many state wildlife agencies are based on the estimated ages of deer in the harvest. It is obvious that if the technique used to assign ages was flawed, it would have significant implications for many aspects of whitetail management.
There is a growing perception among wildlife biologists that aging deer by the tooth eruption and wear technique — the most widely used aging method — is unreliable. This follows several studies that have shown that the ability of wildlife biologists to correctly determine the age of deer based on this technique varies greatly.
For example, a 1989 study by Jacobson and Reiner found that the average estimates by 55 southeastern wildlife biologists were correct for 71.9 percent of a sample of 98 known-age whitetail jawbones. The range of scores was large (i.e., many did poorly) and they tended to underage deer over 3 1/2 years old. In 2000, Hamlin and others found that four Montana and two Washington biologists correctly aged from 23.8 percent to 66.7 percent of 21 known-age whitetail jaws.
A Colossal Aging Error Discovered
In attempting to understand the variation in how biologists age deer, we discovered an apparent error in the wear description for 3 1/2-year-old white-tailed deer in each of the last four editions of the Wildlife Techniques Manual. Nearly all colleges, universities, and state wildlife agencies use this manual to teach the aging technique to wildlife biologists and students. In all editions, the implication is that the white-tailed deer tooth wear drawings are based on the 1949 Severinghaus article, which originally outlined this technique. The Severinghaus article is highly respected and one of the most cited research papers in all of wildlife management.
A deviation from the Severinghaus wear description first appeared in the second edition published in 1963. It was apparently a simple labeling error in the drawing. Regardless, the error has been repeated in all subsequent editions. As you can see from the diagram in the fifth edition published in 1994 (below, right), the labeling states that the dentine line in crests of the first and second molar is wider than the enamel. And, the arrows on the drawing point to the second molar’s lingual crests not its buccal crests. However, Severinghaus stated something quite different in his description of the 3 1/2-year-old age class. "The lingual crests of the first molar were blunt and the dentine of these crests was wider than the enamel." "…the lingual crests of the second and third molars formed a narrow line about the width of the enamel." He also states that the buccal crests of the molars sloped laterally downward and the dentine in the crests of the first and second molars was wider than the enamel." Failure to note that he was referring only to the buccal crests here is probably how the error was made by whoever prepared the first illustration published in 1963.
Severinghaus and many others have clearly stated that the dentine line of the second molar’s lingual crests does not become wider than the enamel until 4 1/2 years old. We agree. However, use of the aging technique as described in the Wildlife Techniques Manual would result in aging most 4 1/2-year-olds as 3 1/2, and many 3 1/2-year-olds as 2 1/2. This could help explain the variation in the ability of biologists to correctly assign ages to known-age jaws. This would be especially true for young biologists who did not receive intensive "hands-on" training in their university coursework and have relied solely on the Wildlife Techniques Manual for their training.
Has the Error Affected Management Decisions?
As a result of the growing interest in Quality Deer Management (QDM), many deer managers desire to move more bucks into the 3 1/2 and older age classes. Many have reported difficulty obtaining this goal and several reasons have been proposed including heavy legal and illegal harvest, high dispersal rates, and mortality due to disease, fighting, and accidents. Are underaging errors another factor? We believe so. As such, it is likely that more bucks are reaching the older age classes than previously believed.
Also, significant aging errors likely affect population models such as those used to track deer populations in many states. A relatively small error in age estimation can produce significant changes in population estimates, potentially resulting in inappropriate season lengths and bag limits.
The good news is that the error was not repeated in QDMA’s 2001 jawbone removal and aging poster. In addition, this poster provides more detailed information on this aging technique than is provided in the Wildlife Techniques Manual. It is widely regarded as the most up-to-date, educational resource on the subject.
Has Tooth Wear Gotten a Bad Rap?
Some studies have indicated that the cementum annuli technique is more accurate than the tooth eruption and wear technique, at least for deer in the older age classes. The cementum annuli technique involves counting the growth rings in a cross section of the roots of a deer’s incisor tooth. This technique obviously involves specialized equipment and training as well as being more time consuming and costly than the tooth wear approach. Moreover, the best aging results with the cementum annuli technique have been achieved by Dr. Gary M. Matson of Matson’s Laboratory in Montana — the nation’s only commercial aging laboratory (www.matsonslab.com). Matson’s unique ability with the annuli technique can be attributed, in part, to his extensive experience. He has had access to more known-age tooth samples than anyone in the country. It is doubtful that even those on Matson’s own staff can duplicate his level of accuracy.
Maybe the lack of practice and experience with the eruption and wear technique could help explain the variability in age estimations among wildlife biologists.
Precision, Accuracy and Bias
For most management and research purposes, aging precision is less important than aging accuracy. In this example, precision relates to how consistent the person is when assigning ages versus how accurate those assigned ages actually are. For example, if we concluded that a group of deer harvested from a particular population had a certain age structure, it would not be very important if we over-aged 20 percent and underaged 20 percent. On the other hand, it would be a problem if we over-aged 30 percent and only underaged 10 percent. In both cases we incorrectly aged 40 percent, but in the second scenario our aging technique was biased — consistently under- or overaging — creating the wrong conclusion. In the first case, the resulting population age structure was correct even though we incorrectly aged 40 percent of the jaws. The point being that the elimination of bias could be more easily achieved and more important for obtaining accurate conclusions than improving precision.
Where Do We Go From Here?
We believe much can be done to improve the accuracy of the tooth eruption and wear technique and improve its value for deer management purposes. In fact, with so many aspects of deer management being dependent on reliable age estimations, the QDMA has agreed to fund a graduate study on this very subject beginning this August. The 2-year study will be conducted through the Warnell School of Forest Resources at The University of Georgia and be directed by Drs. Karl Miller, Bob Warren, and Larry Marchinton. Long-time QDMA research intern, Jeremy Meares, will be the lead student on the project.
The goals of this study are to pull together as many known-age jaws as possible and examine them for previously undiscovered aging clues. The study also will test a large number of wildlife biologists to determine the top few percent. If even a small number of biologists can make the technique work with an acceptable level of accuracy — then it proves the technique does work.
The top-scoring biologists will then be interviewed to determine the characteristics they use to assign ages. Ultimately, the information gathered from this study will be used to develop a training module, likely on CD-ROM, that can be used by universities, state wildlife agencies, and hunters to learn the "improved" tooth eruption and wear technique. Progress reports on this landmark study will be featured in future issues of Quality Whitetails.