Moans, wobbles, yawns – the language of love for gelada monkeys in Ethiopia

A male gelada monkey making a wobble call. (Photo by Morgan Gustison)

ANN ARBOR—If a male gelada monkey wants to attract a female, grunting won’t work. The best pick-up lines involve moans, wobbles or yawns. Females who heard recorded playbacks of these calls lingered longer and spent more time near the speaker, a new study says.

Each one of the calls is acoustically interesting, the researchers say. Moans are long in duration. Wobbles have a high degree of frequency change.  And yawns use a large frequency bandwidth.

The findings, published in Scientific Reports, were based on observations of geladas in Simien Mountains National Park on the plateau of northern Ethiopia – a spectacular landscape with deep valleys and jagged mountain peaks. The research advances our understanding of the origins of highly complex forms of communication, such as human language.

“Females pay more attention to male vocal sequences that contain acoustically elaborate calls. Not only do the females look longer, but they also choose to hang around the area where they heard an elaborate sequence,” said Morgan Gustison,  a doctoral student in psychology at the University of Michigan and the study’s lead author.

Study subjects were 36 adult female geladas in the park in eastern Africa. Identified by their unique body markings, these animals—who lived outside the three groups studied by the U-M Gelada Research Project—were chosen so they would be unfamiliar with the 12 adult male geladas from which researchers recorded playback stimuli.

The playback consisted of 18 “grunt only” and 18 more complex “derived call” vocal sequences.

An adult female was chosen as a subject for the playback trial if she were stationary—either feeding or resting—and not engaged in social activity. The primate also had to be close to vegetation where the speaker could be hidden.

The females would hear the vocal sequences and look toward it, possibly trying to determine its origins. They spent more time in the general location when the sound involved a derived call, Gustison said.

“When the sound only involved the grunt, the females might be curious briefly, but not as interested as when they heard varied sounds,” she said.

The findings also indicate that male geladas use a more complex string of sounds to maintain social bonds with the females in their harem-like reproductive units. A unit usually has about a half-dozen females and one male, who does all of the mating until a younger male suitor is able to kick him out.

“The results of this study build support for the idea that sexual selection could play an important role in the emergence of complexity in primate vocal systems,” said Gustison, who worked on the study with Thore Bergman, an associate professor of psychology and ecology and evolutionary biology.

The work was funded by the National Geographic Society, the Leakey Foundation, National Science Foundation and the University of Michigan.