From a microscope on the third floor of the University of Wyoming Biological Sciences Building, Miranda Strand peeks into wild lives of Wyoming’s big game animals.
Strand, a tooth-aging coordinator for the Wyoming Game and Fish Department, works in the department’s Wildlife Forensics and Fish Health Laboratory, which is located on the UW campus.
By the thousands, she prepares and examines teeth collected by hunters and biologists around the state, counting their years like tree rings. The information she gathers is a clue for measuring herd-wide demographics, but it also offers a glimpse of the narrative of each individual animal’s life.
The tooth-aging process begins in the field, when a hunter harvests a mule deer, bison, elk, moose, pronghorn, black bear or mountain lion. If the hunter stops at a check station, the tooth might be pulled by a Game and Fish employee. Biologists will also mail boxes to hunters with certain tags, or collect teeth themselves.
They’re usually interested in the first incisor, one of the two front bottom teeth, which comes in first as an animal grows its adult teeth.
“Normally, hunters don’t really mind giving that tooth up,” Strand said.
As the teeth come into the laboratory, they’re cut so that only the root remains. This portion of the tooth is soaked in hydrochloric acid to soften it. Then it’s cut in half.
“The solution removes the calcium and makes it flexible, like an eraser, so we can cut into it,” Strand said.
The softened tooth is then sliced into sections eight micrometers thick. That’s eight one-millionths of a meter, or about one-tenth the thickness of a human hair.
These thin sections, cut lengthwise to expose the root from top to bottom, are placed on a slide. Then they’re stained purple to reveal the layers of growth Strand will later count to determine the age.
“It’s quite a lengthy process,” Strand said of the preparation.
As an animal ages, a substance called cementum is deposited on the root of the tooth, thickening the walls from the outside in.
“When the tooth is younger, it’s hollow, and as they get older it fills in the space,” Strand said.
During the winter, when food supplies are scarce and the animal is under stress, the layers of cementum are dark. In the summer, when food is plentiful and stress is minimal, growth happens more quickly and a lighter layer of cementum forms.
The contrast between light and dark shows up under a microscope as rings called annuli, allowing a scientist to count their years. Accounting for the age when an animal grows its adult teeth then leads to the age of the animal when it died.
Some of the 3,000 teeth a year that come through the lab are sent from hunters and biologists in the Laramie region.
Biologist Lee Knox said knowing the ages of animals provides a variety of insights into the condition of a herd.
Moose, for example, are notoriously hard for biologists to keep tabs on. They don’t bunch up in the winter to be counted from a helicopter, so teeth provided by hunters are valuable.
“If you’re lucky enough to be a moose hunter, you’re required to hand in teeth,” Knox said.
Wyoming hunters can’t harvest a cow who’s with a calf, so most cows from successful hunts should be either too young or too old to have young.
“If we start seeing a lot of prime age cows being harvested, then that’s cause for concern,” Knox said.
Game and Fish manages moose to provide hunters with opportunities for older bulls of better trophy quality. Biologists can look at the ages of harvested bulls to learn if their management goals are on track.
“We’re looking for a median age over five years old,” Knox said.
When the department can collect a lot of teeth from a herd, such as the elk herd near Laramie Peak, the information augments other measurements about herd condition. Ultimately, the information affects seasons and licenses.
“Because there are a lot of cow licenses in that area, it can tell you the age structure of your herd,” Knox said.
Ungulates such as mule deer or elk usually have a fawn or calf every year. Black bears, however, usually have cubs every other year.
At the lab, Strand pulled up a slide of a female black bear and pointed out where the annuli grew close and spread out. In years when a bear births young, her annuli are closer together, suggesting nutritional resources diverted away from growing teeth and into nurturing young.
“Black bears are really fun to age,” Strand said. “We can tell how old a sow is when she has her cubs.”
The bear whose tooth was on the slide had had cubs when she was 5, 7, 9 and 11 years old. Strand said she’s seen teeth from black bears in their 20s, but they’re rare.
“It’s cool when they get into their 20s and they’re still having cubs,” she said.
The story of a mother and her young is also the story of herds growing and shrinking, and it’s counted on their teeth.