By Susan Milius, Science News
Don’t try this at home, but tickling a gorilla, orangutan, bonobo or chimp can inspire bursts of grunting sounds.
Yes, that’s laughter, says Marina Davila Ross of the University of Portsmouth in England. She and her colleagues analyzed sounds of ticklish great apes as well as human babies and traced a shared family tree of laugh sounds. Laughter’s roots go back at least 10 to 16 million years, Davila Ross and her colleagues suggest online June 4 in Current Biology.
Charles Darwin explored during the 1870s the possibility that human expressions of emotion, such as laughter, have evolutionary cousins in other species and reveal a deep, shared history. Davila Ross and her colleagues approached the old idea with a modern technique, creating a computer-generated family tree based on recordings of young animal tickle-sounds.
Romping great ape ancestors probably gave bursts of a few noisy, long, slow grunts, much as orangutans do today, the researchers say. As the great ape lineage continued to branch, tickle sounds grew shorter and faster. Today's chimps and bonobos, a relatively new lineage, give shorter calls with less time between them than do the modern representatives of the ancient orangutan lineage.
Airflow patterns also changed during laugh evolution, according to the reconstruction. Ancestral great apes probably gave grunting laughter with sounds on both the in and out breaths, much as orangutans do today. Humans tickle-laugh mostly on outgoing breaths, and researchers once speculated that apes just didn't have the nervous system to manage out-breath-only laughter. Davila Ross and her colleagues confirmed earlier hints that apes aren't so restricted. Recordings included examples of out-breath laughs from the apes.
And the team also contends that during laugh evolution, regular vocal cord vibrations became more important. The cords often vibrate regularly during human laughter, creating a melodic structure. Yet Davila Ross found a bonobo with regular cord-vibrating laughs.
“Fascinating stuff,” says Jaak Panksepp of Washington State University in Pullman. He does call the team’s results conservative. He argues laughter goes deeper into animal heritage than the great apes. Panksepp has studied high-pitched chirps coming from rats he tickles. “In my estimation, the empirical evidence already supports a much deeper evolutionary scenario for the emergence of a laughter-type response in brain-emotional circuit evolution,” he says.
Davila Ross recorded human laughter from babies of friends invited over for a tickle session. To acquire the ape sounds, she recorded tickle noises in European zoos and a Malaysian primate rehab center. Zoo apes had no history with her, so she left the tickling to their caretakers. Baby and juvenile apes were definitely tickled by the tickling in the course of this study, and they wanted more. “They would be still hanging around after we were finished,” she says. At the rehab center, she stayed long enough for animals to get to know her, and they willingly played and laughed with her.
Her observations about the positive responses of the ticklees mirror what Panksepp has observed in his rats. In demonstration videos, Panksepp wiggles his finger against the rats, flips them on their backs and ruffles their tummy fur. When he pulls away his hand, the rats chase after it as if eager for more.
He says the big test for looking at whether laughter emerged deep in the evolutionary history of life will be the identification of genes that contribute to brain circuits involved in the merriment. “This will provide the most rigorous evolutionary evidence, pro or con, for a deep continuity model.”
A baby orangutan grunts with apparent glee during a tickle session with a human friend. And a young, though substantial, gorilla plays tickle-my-foot.
Credit: University of Veterinary Medicine Hannover
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