Alligators dine on many strange delicacies: sharks, kumquats, and stones. Scientists have long thought that, like birds, gators swallow stones to help them digest their tough-to-process meals, or accidentally ingest them in the chaos of consuming a live, thrashing dinner. But a new study supports another use for a belly full of rocks—as a way to boost bottom time on dives.

Crocodylians—which include alligators, crocodiles, and caimans—spend most of their time in the water, stalking prey and escaping from predators. Anything they can do to maximize their time below the surface is an advantage, and some experts wondered whether rocks might also serve this purpose.

To find out, researchers brought seven young American alligators (Alligator mississippiensis) into their lab and measured how long they stayed submerged before and after they voluntarily swallowed a set of small stones. Each alligator took 42 dives—21 before and 21 after their flinty meal. The stones—which were only about 2.5% of the alligators’ body weight—seemed to make a significant difference, increasing dive time by an average of 88% and up to 35 minutes, the team reported last month inIntegrative Organismal Biology.

The researchers think the stones work like a scuba diver’s weight belt, letting alligators and crocs stay deep under the surface even when their lungs are full of air. And the greater the weight, the more air they can take in before they dive deep. Because the current study used juvenile alligators, which have lighter, more cartilaginous tissues, the researchers want to run it again in adults. But for now, it appears that the mystery stones aid these powerful reptiles in doing what they do best: lying in wait, and out of sight.