Frans Plooij was a graduate student at the University of Groningen in the Netherlands in 1971 when he went to the Gombe National Park in Tanzania to study chimpanzee migration with his new wife, Hetty van de Rijt-Plooij. She had hoped to teach in a local school. But upon realizing that there were no settlements nearby, she decided to keep busy by using recording equipment to analyze the vocalizations of chimpanzees.
With an outstretched hand holding a directional microphone, she diligently recorded the primates’ grunts, pant-hoots, and hoos, a sound like a whimper. The primates had gathered at a feeding site to eat bananas from a covered trench managed by scientists. Over 2 years, she recorded 28 tapes—more than 10 hours—of infant, juvenile, and adult chimpanzee calls. Now, for the first time, these calls are available to researchers in an article appearing in Scientific Data, a new open-access, online-only journal from Nature.
In 1973, Plooij joined ethologist Robert Hinde at the University of Cambridge in the United Kingdom, and the couple had neither the time nor the resources to capitalize on their unique collection. After earning his doctoral degree from Groningen in 1980, Plooij spent the next 3 decades working in child development, while van de Rijt-Plooij earned her doctoral degree from Cambridge in physical anthropology and co-wrote with her husband a parenting book called The Wonder Weeks. “Your life moves on,” Plooij says. “We collected far too much, and the sound recordings remained in the attic.”
Shortly before she died in 2003, van de Rijt-Plooij urged her husband to make the collection accessible to other researchers in hopes that the chimp calls might shed some light on the evolution of human language. And this week, Plooij was finally able to honor her wish, as information about the now-digitized recordings and her field notes is now freely available. “This is a unique collection that probably nobody will ever repeat,” he says. “So there was a great need, we felt, to make it available.”
In Gombe, female chimpanzees often carry their infants with them into trees, making it very difficult for researchers to get accurate recordings from the ground. Anne Pusey, a behavioral ecologist at Duke University in Durham, North Carolina, who also worked in Gombe, notes that the infants’ grunts are also “quite soft, so it’s difficult to get within range and record them.” However, at Gombe, mother chimpanzees would bring their infants as they checked the trench for bananas, giving van de Rijt-Plooij the chance to note their interactions and record their vocalizations.
Apart from the quantity of information, the new collection also includes two elements rarely captured: quiet vocalizations like a chimpanzee’s “hoo” of surprise, and young chimps’ soft grunts.
Lorraine McCune, a language researcher at Rutgers University in New Brunswick, New Jersey, analyzes human infants’ grunts and coos to understand the acquisition of language. From infancy to adulthood, chimpanzees come to use grunts for communication, similar to the way humans learn to communicate through grunts like “mm” or “mm-hmm” as they grow. Chimpanzees’ calls are more stereotyped and less complicated than human language, but McCune hopes that comparing chimpanzee infants’ sounds with those of human infants may help reveal what’s unique about human infants’ sounds.
“We don’t know how acoustically similar a chimpanzee infant’s grunt would be to a human infant’s grunt,” McCune says. “If [Plooij] has all the vocalizations and the field notes that say what was happening at the time, it would be much easier for me to see how those particular grunts related to what I find in human infants.”
The data now reside in Cornell University’s Macaulay Library and the Dryad Digital Repository, another online resource.