The DNR collects a tooth from all bear and the reproductive tracts from female bear. The tooth is used to determine the age of the bear and the reproductive tract is used to determine the reproductive condition of the bear. Gathered over a period of years, the information also can be used to estimate the number of bear in the state and whether the population is increasing, decreasing or stable. The tooth and reproductive tract should be collected at the time you dress-out your bear and turned in at the time the bear is sealed and registered. It is very important that we receive a tooth with each reproductive tract. Bear ages will be posted on the web by March. 

Tooth Collection

Bear can be aged using a microscope to count the number of cementum layers in cross sections of premolar teeth from either the upper or lower jaw. The cementum layers look much like rings in a cross-section of a tree; each layer represents one year. See details below.

The premolars are small, single rooted, peg-like teeth located between the canines and the molars. A premolar can be removed by cutting with a knife in front of and behind the tooth and pulling gently with pliers. It is okay if some of the gum adheres to the tooth. The aging technique actually works better if the tooth is not scraped clean. The age cannot be determined from the tooth crown; therefore, it is very important that the premolar be removed with the root as intact as possible.

The tooth can be wrapped in paper (not plastic or foil) or placed in an envelope and can be dried or frozen. Turn in the tooth when you have your bear sealed and registered.

Using Teeth To Age Black Bear

Figure 1. Bear jaw showing premolar used for aging bear.

The Wildlife Division has been collecting teeth from harvested bear since the early 1970's. In 1986 the Wildlife Division started using tetracycline injections in black bear research, and since 1989 has been putting out tetracycline baits as biomarkers for black bear. Many mammals in Michigan can be aged by microscopic examination of thin sections of their teeth. There are many different laboratory techniques for doing this, such as paraffin histology, cryostat, and Epoxy imbedding, but they usually involve decalcification of the tooth which destroys the tetracycline biomarker. The process of screening for the tetracycline has resulted in a lot of changes in our lab techniques. Many of the procedures we use were developed by Dave Johnston's group at Ontario's Ministry of Natural Resources so they could monitor the taking of experimental oral rabies vaccine by foxes in the wild. The basis for using tetracycline as a biomarker is its affinity for calcium. In the body it becomes bound to bone and teeth very rapidly, a matter of hours or days, depending on the type of tetracycline used. It remains in teeth permanently, and can be seen as a florescent line under ultraviolet light in thin sections years afterwards. Since the outer layer of a tooth root continues to grow, depositing yearly cementum layers throughout the lifetime of a bear, we can determine in which year tetracycline was given.

Figure 2. Extracted bear premolar prior to cleaning.

The teeth we prefer to use are premolars, which are small peg sized teeth located between the canine and the molars (Figure 1). The premolar is the easiest to extract, and can even be taken from anesthetized bears without harming their fitness. The root tip is where most of the cementum deposition occurs.

Figure 3. Premolar after cleaning and grinding.

The actual processing of the tooth begins with peeling off the excess dried gum tissue (Figure 2). Since the soft tissues interfere with the cutting of the tooth, we clean it further by soaking it in a warm enzyme and buffer solution for several hours, followed by a thorough wash in running tap water. This leaves a clean tooth with just the thin layer of the outer periodontal membrane still attached (Figure 3).

Figure 4. Cutting the tooth.

The last procedure before cutting is to grind the sides of the tooth crown so that it will fit into the holder securely for cutting. We use an ISOMET saw, made by the Buehler Company, which has been modified for our use. Two diamond edged wafering blades are used separated by a thin (50-micron) sheet of acetate. The tooth is oriented in the holder so that we obtain a lengthwise section. A shim is used to help control the width between the blades. Various weights on the arm also help control the cutting. The tooth is lowered onto the blades, which are rotating at about 100 revolutions per minute. It usually takes three to five minutes to cut a section from a premolar, depending on the size of the tooth and the pulp cavity inside (Figure 4).

Figure 5. Tooth section showing tetracycline.

After the tooth is cut, the thin section remains attached near the crown and must be broken out using a spatula. The raw section is what we need to screen for the presence of tetracycline, but first it has to be rinsed in water, followed by a 50:50 mix of glycerin and water, and then straight glycerin. The section is mounted on a slide in glycerin under a cover slip. Using reflected ultraviolet light on the microscope, the section is checked for tetracycline, which shows up as a yellow florescent line in the cementum. Figure 5is a 7 year old female, which received tetracycline injections three different times: At 3, 4, and 5 years of age.

Figure 6. A 5 year old which took tetracycline bait at 3 years old.

In order to figure out what years the bears were marked, the section is digitally photographed.  After the section is decalcified and stained for age determination, it is photographed again, and the two digital pictures are combined using a computer to find the year in which the tetracycline was given (Figure 6).

Most sections of course do not have tetracycline and are also processed for age determination. The glycerin is rinsed off with tap water.  The section is decalcified in 10% hydrochloric acid with chelators which help control the dissolving of the calcium.  After the calcium is removed, the section is rinsed in tap water, phosphate buffer, and tap water again and then placed on the slide and dried. The slide is then fixed with methanol and stained with Giemsa stain for four minutes each, followed by a rinse in tap water, and then dried on a warming plate.

Finally, back at the microscope, using transmitted white light, the cementum annuli are counted and the is age determined. Each dark stained line represents a cementum annulus. Females will have limited cementum growth during the times that they have cubs. This means the cementum lines during these years will be close together allowing us to get insight on black bear reproduction (Figure 7).

Figure 7. An 8 year old female that had cubs at 5 and 7 years old.