Audible yawns in a non-human species may convey important social information

Most of us are familiar with yawn contagion, which is the act of spontaneously yawning when someone nearby yawns, often but not always audibly. For humans, yawning can come from fatigue or boredom, and seeing or hearing someone else yawn can start a chain reaction.

Many animal species also yawn when tired, and yawn contagion is known to occur among various social animals, including certain monkeys, monkeys, lemurs, pigs, wolves, domestic dogs, lions, spotted hyenas, etc. But aside from humans, only one species – the gelada (Theropithecus gelada), a species of Old World monkey – is known to yawn audibly.

Previous studies have explored yawn contagion within and between various species; for example, some work has shown that dogs yawn in response to the sound of humans yawning. But to date, there are no known studies of audibly triggered intraspecific yawning contagion in a non-human species.

Now a French and Italian research team has specifically studied how the sound of gelada yawns affects their peers. The team’s work is published in Scientific reports.

Geladas, also known as bleeding heart monkeys, are endemic to Ethiopia. They live in multi-level societies that include units, teams, groups, and communities. Primary units include either a single breeding adult male with several adult females, their young, and at least one follower male; or only young adults and adolescent males no longer living in their birth unit.

Similar to humans, geladas enjoy rich and complex vocal communication. According to the new study, “a similar evolving social landscape, with similar challenges (e.g., the need for group coordination with subjects not always in visual contact), has indeed likely led to the emergence of multimodal communication in both species.”

Existing findings show that yawning geladas, mostly males, emit a loud vocal sound that doesn’t just come from inhaling and exhaling. While the researchers behind this study knew that the visual contagion of yawning could affect geladas, they wanted to know if the sound of yawning without a visual cue would result in a similar effect.

Testing the Effects of Audible Yawning Sounds

The researchers hypothesized that, as in humans, as it evolved, yawning vocalization in geladas could have led to the possibility of yawning contagion in subjects who could not see ; and further, that such vocalization-based yawn contagion would adjust according to the perceived social value of the triggering sound.

To test these hypotheses, they conducted a study at NaturZoo Rheine in Germany, which hosts a colony of 106 geladas, the largest captive colony in the world. There, the animals live in five main units (four single-male units and one single-male unit) in two adjacent enclosures, divided by a pond, which include indoor and outdoor spaces in which the geladas can move freely. Animals in each enclosure can see and hear those in the adjacent enclosure, but cannot make physical contact with them.

Across all units, researchers randomly selected 33 test subjects (7 males, 26 females; 21 from one enclosure and 12 from the other) for reading experiments, conducted outdoors. Only adults were selected, because yawning contagion in geladas only appeared among adults.

Since audible yawns primarily come from males, the researchers first recorded male yawning sounds (which may help promote group coordination, according to a 2021 study) as test vocalizations and pairs of grunts from males. male affiliation as control vocalizations. Then, controlling for several confounding factors, they played the sounds back to test subjects under various conditions and observed their reactions. None of the males whose yawning noises were used as test vocalizations were tested with their own vocal recordings.

Yawning is a possible social signal

Notably, the results showed that an audibly triggered yawning contagion exists among geladas. This makes them the only species other than humans to experience this phenomenon. Writing that “(the) simple sound of a yawn can trigger contagious yawning in geladas,” the researchers note that compared to control grunts, audible yawns caused more yawning, a higher risk of yawning, and lower yawning levels. similar contagiousness in both males. and females.

The researchers also confirm that test subjects yawned more frequently in response to audible yawns emitted by individuals in their own group, indicating that group membership could be a contagion factor for audibly triggered yawning.

Although there is still much to explore in this area, the team believes that audibly triggered yawning contagion could serve a social purpose in both humans and geladas.

They write: “Given the importance of yawning in group synchronization, the adaptive value of auditory YC (yawn contagion) could be linked to the need for individuals to maintain acoustic contact when visual contact is not required. not possible, which makes this particularly relevant in societies characterized by modularity and flexibility, and in species with such rich communicative repertoires.

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