Male Guinea baboons have a curious habit. They will walk—or sometimes run—to another male baboon and say a quick hello in a very enthusiastic way: with a “mutual penis diddle”. Or sometimes it’s a quick mount from behind. Other times, they do a short dance-like “polonaise,” facing the same way, on their hind legs, hand on the other’s hip, and a few steps forward.

Clearly, this behavior needs an explanation. In some ways, it’s not all that much of a mystery: ritual greeting is actually fairly widespread among many primate species and takes many colorful forms. It's a behavior that's “common among males living in multi-male groups,” write the authors of a new paper exploring Guinea baboons’ greeting behavior.

So it’s no surprise that the Guinea baboons greet each other. But the intimacy of their behavior stands out. Unlike other species, where ritual greetings serve to cool down a tense or aggressive moment, for Guinea baboons, it seems to be more about keeping their social bonds strong.

Intense, reciprocated ritual greetings

To understand more about the strange behavior, primatologists Federica Dal Pesco and Julia Fischer gathered hundreds’ of hours of data in the Niokolo-Koba National Park in Senegal, where more than 400 Guinea baboons live in two “gangs:” the Mare and Simenti gangs. Within the gangs, the baboons live in smaller groups called “parties,” which contain around 45 individuals.

Dal Pesco and Fischer picked the two parties with the highest number of adolescent and adult males—24 individuals in all—and tracked those individuals closely for 894 hours. They recorded all their greeting actions, as well as data about the context in which they occurred. The researchers considered the greeting as “intense” if “an embrace, a penis diddling, a mount, or a polonaise was performed,” they write. Less intimate touches or gestures like a “head bob” counted as “not intense.”

That gave Dal Pesco and Fischer 1,981 greetings to analyze. They found that baboon bros don’t leave each other hanging: almost all the greetings were reciprocated. More importantly, the vast majority happened between party members; only 5.4 percent of all the greetings the researchers recorded happened between baboons in different parties. Within a party, almost all the male pairs exchanged at least one greeting with each other. This wasn’t to do with how closely the baboons were related, either.

In other species, ritual greetings seem to play a role in chilling-out tense moments. But in Guinea baboons, only 1.7 percent of greetings happened around an aggressive moment. Instead, a slight majority of them involved one baboon walking up to another very deliberately, just to say “hi.” This happened during all kinds of activities, including moving, feeding, and resting.

Social cohesion

The emerging picture was one of “greetings function[ing] to confirm and delineate group membership,” the authors write. That’s the kind of behavior that could be important in a species where males live closely together, dealing with the thorniness of competing for mates in order to reap the benefits that come from coalitions with other males. Of course, with this kind of animal behavior, there’s only so much insight humans are able to gain.

Although the sample of 24 baboons was small, the overall dataset was considerable, with hundreds of hours observed and greetings recorded. While decent sample sizes are essential in research that compares two groups on some measure (for instance, how well a drug works), the same problems don’t pop up in quite the same way in work like this. Still, the small population does limit generalization to the species as a whole, and getting data from other baboon parties will be necessary, as well as populations in other places, before drawing firm conclusions.

The question of baboon penis diddling isn’t just a curiosity but is also a way to understand how cooperative relationships can develop in a species. Ritualized greetings are “reported mostly for species characterized as rather tolerant,” the authors write. This evidence from Guinea baboons gives new insight into how those things are connected: the greetings seem to play a role in keeping social relationships running smoothly, which could improve group cohesion and enhance the benefits of living in a group. But working out the evolutionary story behind how this all emerges is still an open question.

Journal of Human Evolution, 2018. DOI: 10.1016/j.jhevol.2018.10.007 (About DOIs).