New study of grooming behavior shows chimpanzees have ‘influencers’ too

The Beatles made half the world twist and shout and Elizabeth I popularized red lipstick. That’s how we humans are: a few individuals have the power to influence the habits of many. We forge our habits through social learning, but we do so selectively. It is not the same thing for Spanish superstar Rosalía to invent the term “motomami” as it is for a small-town priest to do so. In the same way, we are far more likely to acquire a custom if we see it adopted by the majority.

These biases in social learning are the engine of our cultural evolution and, to better understand their origins, science is studying them in other species as well: in particular, recent discoveries involving chimpanzees are very revealing, although they are the result of a story that began decades ago.

In the 1970s, most of human knowledge regarding chimpanzees came from two research sites. The first and best-known was in the Gombe National Park in Tanzania, set up in 1960 by the famous primatologist Jane Goodall, who was the first to observe chimpanzees using tools to hunt termites, eating meat and waging war.

The second site was founded five years later by Japanese primatologist Toshisada Nishida, in the Mahale Mountains, a little over 100 miles as the crow flies from Goodall’s location. There, Nishida’s team made important discoveries about the social life of chimpanzees, as well as detailing how they consumed medicinal plants when they were experiencing health problems.

One of the most striking findings about these primates occurred when scientists from both sites met. In 1975, two Gombe researchers named Bill McGrew and Caroline Tutin traveled to Mahale to learn about the work of the Japanese primatologists first-hand. What struck them most was that the chimpanzees there did not groom themselves in the same way as in Gombe, but displayed a very peculiar habit. Primates groom to their companions’ fur from parasites such as lice, but above all it is a way to strengthen social bonds.

In a paper published in 1978, McGrew and Tutin described the grooming of chimpanzees in the Mahale Mountains, which they termed the grooming handclasp (GHC). “Each participant simultaneously extends one arm above the head and both grasp the hand. Meanwhile, the other hand is engaged in grooming the other individual’s armpit area, revealed by the raised limb. To do this, the two chimpanzees sit facing each other on the floor in a symmetrical configuration,” the scientists recorded.

The Japanese primatologists had already observed this behavior but they did not pay it much heed as they assumed all chimpanzees groomed in the same way. However, the Gombe researchers had never seen their study subjects raise their arms and clasp their hands during grooming, as if they were performing a high-five.

The most interesting point about GHC from a scientific viewpoint is not the behavior itself, but the fact that some chimpanzee populations do it and others do not: this is a basic requirement for a given behavior to be considered culture, as is the behavior being passed down from generation to generation. Today, GHC is widely considered to be one of the most convincing examples of social culture in animals. Not only is it a behavior unique to some chimpanzee populations, but there are variations in how it is performed, with some individuals clasping hands and others placing their wrists or forearms together.

In each chimpanzee community where GHC takes place, there is a particular style that becomes more popular: in one group 90% of the individuals may prefer to clasp hands, while in another the majority will grip wrists. On the basis of this evidence, researchers Edwin van Leeuwen and William Hoppitt asked themselves: how are preferences for one style or another transmitted within groups? Do they have social learning biases like humans?

To try and find an answer, they spent several years studying the chimpanzees at the Chimfunshi Wildlife Orphanage Trust, a sanctuary in Zambia that is home to two distinct social groups. All of the animals come from wild populations where GHC is practiced and continue the custom at the sanctuary where they have a large forested enclosure at their disposal where they can freely develop social dynamics.

The first thing Leeuwen and Hoppitt looked for was the style preference that each individual displayed when performing GHC. The hand-holding variant proved to be the most popular overall, with one group preferring this method while the other also frequently held their wrists or elbows together. They then studied how these preferences developed and found that the older, more dominant chimpanzees exerted a strong influence on the others.

For example, two males named Peter and Rick would approach each other to initiate GHC. Peter is the more dominant male in the group and tends to hold hands while Rick, who is younger, does not yet have much social standing and tends to join wrists. In all likelihood, not only will they end up using Peter’s preferred GHC method, but Rick will also change his preferences and opt to join hands in their future encounters.

They also found that chimpanzees are conditioned by what the majority do. Among the younger chimps GHC styles are more varied but, as they grow older, the tendency is for them to gravitate toward the most popular method. This could explain why it is common for a single style to predominate in different groups. In addition, the researchers also revealed that social customs among chimpanzees are not only transmitted from mother to offspring, but also between unrelated individuals.

Leeuwen and Hoppitt have published their findings in Science Advances, providing significant evidence that chimpanzees have biases as humans do when it comes to acquiring a social habit. The researchers note that such biases can provide lead to evolutionary advantages, as copying the dominant individual in a group can bring greater social acceptance. This is a familiar concept for humans to understand. We have even invented a term for it: influencer.

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