There have been other reports of self-medication in animals, including dogs and cats that eat grass or plants, probably to help them vomit, and bears and deer that consume medicinal plants, apparently to self-medicate. Orangutans have been seen applying plant material to soothe muscle injuries. But the researchers know of no previous report of nonhuman mammals using insects for a medicinal purpose.

In three instances, the researchers saw chimps using the technique on another chimp. In one case, they saw an adult female named Carol grooming around a flesh wound on the leg of an adult male, Littlegrey. She grabbed an insect, and gave it to Littlegrey, who put it between his lips, and transferred it to his wound. Later, Carol and another adult male were seen moving the insect around on Littlegrey’s wound. Another adult male approached, took the insect out of the wound, put it between his own lips, then reapplied it to Littlegrey’s leg.

One chimp, an adult male named Freddy, was a particularly enthusiastic user of insect medicine, treating himself numerous times for injuries of his head, both arms, his lower back, his left wrist and his penis. One day, the researchers watched him treat himself twice for the same arm wound. The researchers don’t know how Freddy got these injuries, but some of them probably involved fighting with other males.

There are some animals that cooperate with others in similar ways, said Simone Pika, who leads an animal cognition lab at the University of Osnabrück in Germany and is an author of the study. “But we don’t know of any other instances in mammals,” she said. “This may be a learned behavior that exists only in this group. We don’t know if our chimps are special in this regard.”

Aaron Sandel, an anthropologist at the University of Texas, Austin, found the work valuable, but at the same time expressed some doubts. “They don’t offer an alternative explanation for the behavior, and they make no connection to what insect it might be,” he said. “The jump to a potential medical function? That’s a stretch at this point.”