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South Carolina’s low country—a 2,000-square-kilometer swath of coastal marsh and swampland—is gator country and has been for millions of years. That landscape has changed in recent decades with industrial development. Emissions from coal-burning power plants and other sources of pollution, like a large paper mill in the city of Georgetown, have contaminated the area with heavy metals.

“We’re lousy with mercury around here,” says Jay Butfiloski, a wildlife biologist with South Carolina’s Department of Natural Resources, referring to how widespread the pollutant is.

In the low country, as with other wetlands, bacteria living in the oxygen-starved soil convert the heavy metal deposited onto the landscape into methylmercury, a potent neurotoxin. Methylmercury accumulates up the food chain, concentrating in the bodies of plants and animals over time. That means American alligators—apex predators that can live up to 50 years in the wild—can serve as proverbial canaries in the coal mine. Though alligators seem unaffected by the pollutant, measuring the concentrations of mercury in their bodies can give scientists a rough assessment of the level of contamination in the environment.

There’s another good reason scientists monitor mercury concentrations in the reptile: fried, grilled, or boiled in a stew, wild alligator is on the menu for hunters brave enough to wrangle and cook one.

In South Carolina, eating wild alligator comes with a warning: healthy adults should consume it no more than once a week, and those who are particularly sensitive to mercury’s ill effects, such as pregnant women or young children, should eat it no more than once a month. (Those advisories don’t transfer to farmed alligator—the kind sold in restaurants and grocery stores.)

But because mercury accumulates over time, not all alligators pose the same risk when eaten—theoretically, older alligators contain the most mercury.

The trouble is, until recently, scientists had assumed alligators, like some reptiles, never stop growing, meaning the largest animals are also the oldest. In 2016, that perception was disproved by scientists who have been capturing and studying wild alligators in Georgetown’s Tom Yawkey Wildlife Center since 1979. By knowing each alligator’s approximate age, they found the reptiles stopped growing around middle age: 31 for females and 43 for males.

Once age and size were teased apart, Abby Lawson, a population ecologist at Alabama’s Auburn University, used the same decades-long data set to identify patterns between age and mercury concentration. The research led her to a puzzling finding: old alligators have less mercury in their bodies than middle-aged alligators. Somehow, the heavy metal stopped accumulating and then began to decline.

Lawson has a few hypotheses on why this is happening. She says the alligators could be offloading the contaminant through their skin or claws, which they lose throughout their lives. It’s also possible their diet changes as they age. Older alligators may consume less, or, as Butfiloski suggests, switch from fish and other aquatic prey to terrestrial mammals that harbor less mercury such as raccoons, deer, and wild hogs.

Though it remains unclear why mercury contamination in alligators peaks and then dips, Lawson says alligator meat, which has the texture of chicken and the flavor of fish, is enough of a novelty that hunters likely aren’t eating it often enough to be at risk of mercury poisoning.

Still, precaution should be taken since mercury levels vary geographically, Lawson says. For those willing to take a risk to indulge in some unconventional meat, stick to the oldest alligators. (Juveniles, the other age class that would be safer to consume, can’t legally be harvested in South Carolina.) But since elderly and middle-aged alligators can’t be distinguished by size, scientists will need to work out some other way of easily telling them apart—only then could South Carolina’s wild alligator consumption guidelines potentially be tweaked.