Chimps and bonobos signal "hello" and "goodbye" to one another when entering and exiting social encounters, a new study finds.
In other words, these apes, which share about 99% of humans' DNA, politely greet and bid adieu to each other, just like humans do. Until now, this behavior hasn't been documented outside of the human species, the researchers said.
"Our findings show that two species of great apes habitually go through the same process and stages as humans when establishing, executing and terminating joint actions" of hi and bye, the researchers wrote in the study, published online Aug. 11 in the journal iScience.
Related: 8 human-like behaviors of primates
Granted, the apes didn't just give their equivalent of a vocal "What's up?" during social visits. Rather, they had a slew of nonverbal cues. This happens with humans, too. For instance, when people approach to interact, they often orient their bodies toward each other, look at each other and display the intention to touch, hug or kiss before they start talking, the researchers wrote in the study. When leaving an interaction, people often turn their bodies away from each other.
These behaviors amount to a "joint commitment," which is partly a feeling of obligation that we feel toward one another, but also a process of setting up a mutual interaction and agreeing when to end it, the researchers said.
To determine whether chimpanzees and bonobos practice these behaviors, the researchers analyzed 1,242 interactions of apes at zoos, and they discovered that these primates often communicate with one another — often with gestures that include gazing at and touching each other, holding hands or butting heads — before and after encounters such as grooming or play. Of the two species, however, the bonobos were definitely the more polite ones, greeting each other more often than the chimps did, the researchers found.
When beginning a joint interaction, bonobos exchanged entry signals and mutual gazes in 90% of cases, whereas chimps did so 69% of the time, the researchers found. During departures, bonobos also outshined chimps, displaying exit behaviors 92% of the time, whereas chimps showed it in 86% of interactions.
The research team also investigated whether these behaviors changed when the apes interacted with close confidants. They found that the closer bonobos were with one another, the shorter the length of their entry and exit behaviors. This isn't so different from human behavior, said study lead researcher Raphaela Heesen, a postdoctoral researcher in the department of psychology at Durham University in the United Kingdom.
"When you're interacting with a good friend, you're less likely to put in a lot of effort in communicating politely," Heesen said in a statement.
In contrast, the length of the chimps' entry and exit behaviors was "unaffected by social bond strength," the researchers wrote in the study. This might be because in comparison with the hierarchical chimp society, bonobos are largely egalitarian, socially tolerant and emphasize friendships and alliances between females and mother-son relationships, the researchers said. As such, it makes sense that the bonobos' social relationships would have strong effects on their "hellos" and "goodbyes," the researchers wrote in the study.
Meanwhile, there was no significant effect of rank difference on the presence of entry or exit phases in either ape species, they noted.
The findings suggest that perhaps a common ancestor of apes and humans practiced similar behaviors, the researchers said.
"Behavior doesn't fossilize. You can't dig up bones to look at how behavior has evolved. But you can study our closest living relatives: great apes like chimpanzees and bonobos," Heesen said. "Whether this type of communication is present in other species will also be interesting to study in the future."
Originally published on Live Science.