Humans, along with most other vertebrates, yawn. It's likely an ancient behavior with some adaptive advantages. Although we most often associate yawning with boredom or sleepiness, it may serve more than one function. For instance, humans and other animals yawn both when relaxed and when aroused, such as during a tense moment of conflict. And there is some evidence that animals yawn in anticipation of important events and during behavioral transitions (such as between sleep and awake cycles).

With so many reasons to yawn, maybe it's not surprising that different kinds of yawns can convey different messages.

Gelada baboons are Old World monkeys native to Ethiopia. They have three easily identifiable yawn types: yawns with covered teeth, yawns with uncovered teeth, and yawns with uncovered gums and head movements.

Researchers from the University of Parma and the University of Pisa in Italy observed a colony of geladas at NaturZoo in Rheine, Germany. They collected over 5,900 bouts of yawning, noting the social context in which the yawns occurred and the behaviors associated with the yawns. Their results were published in the February 6 issue of Scientific Reports.

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The gum-bearing yawn was most common with males, especially high-ranking ones. This kind of yawn exposed the gelada's impressive canine teeth, which stood out against the reddish-pink color of their gums and the inside of their mouths. It was often accompanied by a loud call, and the researchers believe the yawn functions as a long-distance display. Males used this yawn during periods of tension, such as the time right before feeding, suggesting it may serve to intimidate other geladas.

The other two less intense types of yawns were seen most in females during friendly interactions. The researchers found these yawns to be more contagious, and observed females mirroring the intensity of other females' yawns. They believe these yawns are part of a complex communication system between geladas that often engage in friendly interactions. The contagiousness of the yawns in these contexts suggests the behavior might play a role in synchronizing the activity between two geladas, strengthening the emotional connection between them, or signaling the quality of their relationship.

Gelada males and females might use yawns differently, but all three types of yawn contribute to the smooth workings of gelada society; they function to let everyone know who's in charge and which geladas are friends.

Reference:

Leone, A., Ferrari, P. F., and Palagi, E. (2014). Different yawns, different functions? Testing social hypotheses on spontaneous yawning in Theropithecus gelada. Scientific Reports 4: 4010. doi: 10.1038/srep04010.