“Males basically have to attract and maintain the attention of females,” Dr. McGrew said. “One way to do this is leaf clipping. It makes a rasping sound. Imagine tearing a piece of paper that’s brittle or dry. The sound is nothing spectacular, but it’s distinctive.”

O.K., a distinctive sound. Where does the sex come in?

“The male will pluck a leaf, or a set of leaves, and sit so the female can see him. He spreads his legs so the female sees the erection, and he tears the leaf bit by bit down the midvein of the leaf, dropping the pieces as he detaches them. Sometimes he’ll do half a dozen leaves until she notices.”

And then?

“Presumably she sees the erection and puts two and two together, and if she’s interested, she’ll typically approach and present her back side, and then they’ll mate.”

My first reaction, as a chauvinistic human, was to dismiss the technology as laughably primitive — too crude to even qualify as a proper sex tool. But Dr. McGrew said it met anthropologists’ definition of a tool: “He’s using a portable object to obtain a goal. In this case, the goal is not food but mating.”

Put that way, you might see this chimp as the equivalent of a human (wearing pants, one hopes) trying to attract women by driving around with a car thumping out 120-decibel music. But until researchers are able to find a woman who admits to being anything other than annoyed by guys in boom cars, these human tools must be considered evolutionary dead ends.

By contrast, the leaf-clipping chimps seem more advanced, practically debonair. But it would be fairer to compare the clipped leaf with the most popular human sex tool, which we can now identify thanks to the academic research described last year by my colleague Michael Winerip. The researchers found that the vibrator, considered taboo a few decades ago, had become one of the most common household appliances in the United States. Slightly more than half of all women, and almost half of men, reported having used one, and they weren’t giving each other platonic massages.