Any bear harvested is an achievement. Every hunt is a story, but two questions seem to invariably frame the telling of such stories: how old was it and how much did it weigh? There’s just something about harvesting a big, mature bear that seems to turn these achievements into triumphs and stories into legends. As a result, it’s tempting to speculate about a bear’s age based on its body size, weight, or visible tooth wear, and if you’ve hunted the same area for many years, your guess might come close. Still, nothing beats knowing for sure. Likewise, conservation agencies are eager for reliable bear age data. Understanding survival rates and age composition is necessary to model bear populations and inform sustainable management practices, but gathering age data for bears is a labor-intensive process. 

If you’ve had much success as a bear hunter, there’s a good chance you’ve been asked to provide a premolar from your harvested bear to a state or provincial conservation agency. Videos and instructions on how to remove the tooth are easily found online and many agencies will follow up with an estimated age for your bear, though it can take a year or more to receive results. What happens to the tooth in the interim and how the age is determined is a process known as cementum annuli aging.  

At its most basic, cementum annuli aging consists of examining thin cross-sections of tooth root and counting layers (annuli) reminiscent of the rings in a tree. But trees grow continuously, and a bear’s teeth do not. So how do the rings form? When we think about teeth, we typically picture the hard, light-colored enamel on the outer surface of our pearly whites. Under that layer is a softer, darker material known as dentin, which is visible in some species, such as deer, and may become more obvious with wear. Just like the toothpaste commercials have warned us, these two layers are not replenished or replaced. Below the gum line, however, there is a third layer we don’t typically see, known as cementum. While it’s plenty tough, cementum is not truly bone. It is produced throughout a bear’s life, coating the tooth root, becoming thicker with age and quite literally “cementing” teeth into their sockets. 

Perhaps its most important feature, cementum is not produced at the same rate throughout the year. During periods of stress or limited resources, cementum is laid down in thinner, denser bands. For bears in temperate climates, these periods of stress are believed to correspond with winter and hibernation. This creates a pattern of alternating light and dark bands in the cementum layer, one for each year of the bear’s life. To facilitate cementum annuli aging, ultrathin sections of tooth root are cut, stained with dye, and examined under a microscope. While it sounds simple enough, there’s as much art as there is science to interpreting these minute, and sometimes inconsistent, structures. 

To learn more about cementum annuli aging, I spoke with Arthur “A.J.” Stephens, Lab Manager for Matson’s Laboratory in Manhattan, Montana. Matson’s is the world’s largest wildlife aging laboratory. Since its founding in 1969, Stephens estimates that the lab has developed aging techniques for over 200 wildlife species. Annually, Matson’s receives more than 100,000 samples for aging, including teeth from 2,000-3,000 brown bears and 30,000-47,000 black bears. Less frequently, they are asked to age polar bears from subsistence harvests in Canada and to work with some of the more exotic bear species for research purposes. 

Stephens is quick to point out that Gary Matson, founder of Matson’s Laboratory, did not invent the cementum annuli method. It was first published in an article by fur seal researchers in 1950, but Matson came on the scene soon thereafter and became a pioneer in standardizing the now widely used method. 

“We’ve done our best to make a subjective process as objective as possible,” says Stephens, describing the way his team builds regional aging models and compares results. He notes that every species is different, but it gets even more complicated from there. Because the bands in the cementum come from seasonal stress, the method is notoriously less accurate for populations in warmer climates. As an example, cementum annuli aging for a northern population can be as accurate as 95% while the same species examined from Florida or South Texas may be closer to 60-70%. This doesn’t mean that the technique can’t be used in southerly populations. Stephens stresses that precision or repeatability of results can be just as important as accuracy in the long run. Instead of focusing on the exact age of each animal, being able to consistently group them into similar age classes still allows wildlife managers to monitor trends and changes over time. 

Since cementum is present on all teeth, I asked Stephens why it’s so important to use the first premolar for aging bears and other large carnivores. He acknowledged that any tooth can be used, but the first premolar offers several advantages. It’s a small tooth without an overly deep root. It can easily be identified and extracted, even from live bears. Its absence won’t negatively affect eating or self-defense and isn’t obvious on most taxidermy mounts either. Using the same tooth across the board also improves the overall consistency of the method for these species. 

Probably the most fascinating thing Stephens shared about cementum annuli aging is that the layers can reveal more than just an animal’s age. For female black bears specifically, the technique can also show their reproductive patterns. Again, because stress and resource availability aid the formation of the visible annuli, researchers have noted that female black bears tend to form narrower bands during the years that they raise cubs compared the years before they start reproduction and when they have yearlings at heel. By looking at her teeth, managers and researchers can get an idea of when a female black bear produced her first litter and how frequently she reproduced throughout her life because, just like her age, her body is keeping score. 

References: 

Schaffer VB. 1950. Growth Layers on the Teeth of Pinnipedia as an Indication of Age. Science 112(2907):309-311.