By Jeff Rohlman
-- How old is my mule deer? That's one of the more common questions asked by the public at a Fish and Game check station this time of year.
The answer is not only of interest to the hunter, but essential to the biologist for determining the recent history of the mule deer population as well as its current status and possible future fate. Rough estimates of deer age, such as whether the animal is mature or juvenile, can be obtained from overall body development and antler growth.
Two other proven techniques—examining tooth replacement and wear on deer jaws and annual growth ring aging—provide more accurate estimates of an animal's age.
Because deer start out with a set of "milk" teeth that are gradually replaced by permanent teeth, biologists can use tooth replacement in the lower jaw to age deer into one of four categories: fawn, yearling (1 1/2 years old), two-year-old, and three years or older. A combination of incisor replacement (eight lower front teeth, four on each side of the jaw), the premolars (three larger, mid-jaw teeth on each side of jaw), and molars (three rearmost grinding teeth on each side of the jaw) is used to more accurately assess a deer's age.
Deer fawns will have their permanent middle incisors by October of their first year. Tooth replacement is not as critical to aging fawns. Their body size and shorter head are fairly reliable indicators of their age.
Yearlings, deer 15 to 17 months old in October, will have most, if not all, of their permanent incisors. But they will still retain the three-cusped third premolar, which will be well worn.
Two-year-old deer, 27 to 29 months, have all of their adult teeth, but the permanent, third premolar back will have only two cusps and may be partially or fully erupted with little or no staining or wear.
Three-year-old deer have all their permanent teeth with dark staining and some wear visible. It is difficult to accurately age a deer older than three years using the tooth replacement and wear method.
If the exact age of older mule deer is important to wildlife managers or wildlife researchers, the annual growth ring method is called upon to accurately determine a deer's age. Like many other wildlife species, deer add a layer to the root of each tooth each year of their lives. By examining a cross-section of an extracted tooth under a microscope, these growth rings can be counted, providing an accurate age for the deer in question.
--Jeff Rohlman / Regional Wildlife Manager, Idaho Southwest Region