Don't look now, but is that dog laughing?

Amid all the panting, a dog at play makes a distinctive, breathy exhalation that can trigger playfulness in other dogs, says a Nevada researcher. Yes, it might be the dog version of a laugh.

"To an untrained human ear, it sounds much like a pant, `hhuh, hhuh,'" says Patricia Simonet of Sierra Nevada College in Lake Tahoe. However, this exhalation bursts into a broader range of frequencies than does regular dog panting, Simonet discovered when she and her students analyzed recordings.

They observed the bursts during play but not in aggressive clashes, Simonet reported in Corvallis, Ore., last week at a meeting of the Animal Behavior Society.

Gordon Burghardt of the University of Tennessee in Knoxville, who theorizes about the evolution of play, says Simonet's presentation caught his interest. Her dog-laughing proposal needs more testing, he cautions. But he notes that other scientists have proposed that nonhuman primates and even rodents laugh.

Simonet's team investigated the question by standing in parks with a parabolic microphone that enables them to record dog hubbub from a distance. "People kept coming up to talk to us, so we finally had to wear signs explaining that we were trying to record," she says.

Simonet differentiates a broader-frequency exhalation from pants by calling it a laugh. With recordings of such laughs and growls, the researchers tested 15 mostly young dogs in an observation room. When the researchers broadcast the laugh, a puppy often picked up a toy or trotted toward a presumed playmate, if a person or another dog was in the room. Simonet's own best attempt at the laugh likewise prompted dogs to look for a romp. Broadcasting growls elicited no such effects.

This dog-exhalation study reopens many questions about whether animals laugh, comments Brian Knutson of the National Institutes of Health in Bethesda, Md. He has recorded chirps that laboratory rats give as they wrestle with each other. Rats also chirp before receiving morphine or having sex. He interprets the sound as indicating "the rat expects something rewarding."

Such phenomena help neuroscientists trace the brain's reward circuitry, Knutson explains. He says he's unsure about how to compare the chirp of a romping rat to the guffaw of a person. "I think we've done a decent job of figuring out what it means in the rat," he says. "Now the onus is on the human researchers."

Another analyst of rat chirps, Jaak Panksepp of Bowling Green (Ohio) University, has recorded the animals' ultrasonic squeaks while he tickled them. "Of course, you have to know the rat," he cautions. He says he is open to the possibility that the rat chirps amount to laughter in the animal world. Also, he suggests that Simonet's team could search for animal laughter by recording the sound dogs make when they are tickled.

Yet another student of play, Marc Bekoff of the University of Colorado in Boulder, says he thinks he knows the panting sound Simonet describes. "When I get down on all fours and go up to dogs and go `hhuhahhuhahhuh,' they get very solicitous," he says. "Whether it turns out to be like a laugh or not doesn't matter in the end, because what's important are all the questions it opens up about how communications work."