In a new study, UK researchers established that "lip-smacking," a typical behavior seen in chimps, happens in the same timing range as human mouths when they speak.
It was previously observed in other ape species that their lip-smacking movements were measured at around five hertz, whereas spoken words from different human languages fall somewhere between two to seven hertz.
Although the mannerism in chimps has been observed in the past, the connection between the apes' lip-smacking and talking abilities in humans have not been previously made, until now.
According to Adriano Lameira, the lead author of the study from the University of Warwick, the rhythm of the practice in apes rooted deep from our primate ancestry. He adds that it is a keystone for speech evolution.
On the other hand, there were still significant gaps to fill as any sense of evolutionary continuity towards speech was not observed in African apes. Furthermore, the researchers say there was no indication for speech-like rhythm in gorillas, chimpanzees, or bonobos.
However, Lameira is confident that they are on the right path in discovering more about how these practices in animals shine light on human nature. The findings of the study were published in the journal Biology Letters on May 27, 2020.
Human Nature and Origins
The study traced two captive populations of chimpanzees, one in Germany and one in the UK. Additionally, the researchers also followed two wild populations in Uganda. In their findings, the authors observed lip-smacking at an average of 4.15 Hz.
They made all their observations whenever a chimpanzee was grooming another. They say that it was somewhat similar to a hairdresser interacting with its customer.
Lameria says that the validation of speech-like rhythm of the mouth in chimpanzees does not necessarily point out how language started in our own lineage. Instead, it gives assurance to other researchers that they are on the right track in revealing more secrets about human practices and their origins.
He added that the lip-smacking variability reflected how social translation, individual differences, and environmental factors affect how chimps communicate.
What Does Lip-Smacking in Apes Mean?
According to the Primate Rescue Center in Kentucky, lip-smacking is a social behavior that brings about friendly interactions between monkeys in a social group. Most of the time, a monkey will lip smack to a more superior monkey as a sign of submission.
It can also be a sign of affection or contentment. Some staff from the rescue center share that some of its macaques would lip-smack to the caregivers when passing out food.
Another study published in 2015 in the journal Scientific Reports explains that lip-smacks in apes function to coordinate and prolong social grooming. Researchers say the gesture is an indication of communicative behavior in chimpanzees.
Moreover, the researchers mentioned that lip-smacks were more likely to be done when the risk of discontinuation of the interaction by the recipient was high.
For example, chimps would lip-smack when grooming vulnerable body parts. The researchers said that groomers were also more likely to lip-smack during face-to-face grooming, where the visual facet of the gesture could be perceived.
