Vervet monkeys follow different social “norms” and respond to “peer pressure,” new long-term study shows
IMAGE:
THREE VERVET MONKEYS WITH AN ADULT GROOMING A JUVENILE
view moreCREDIT: CHARLOTTE CANTELOUP
People living in different communities follow different social customs or norms. In some places, for instance, it might be standard practice to greet each person you see on the street, while in others that simply isn’t done. In some cases, such differences may even vary from one neighborhood to the next. Now researchers reporting in the journal iScience on December 19 have found similarly varied social traditions and styles among neighboring groups of vervet monkeys.
“We report the existence of behavioral traditions of social customs in vervet monkeys that are stable across 9 years,” says Elena Kerjean (@KerjeanElena) of the University of Lausanne in Switzerland and Paul Sabatier University in Toulouse, France.
The researchers found that such differences in social traditions set up differences in “social atmosphere” that could be passed on from one generation to the next through social learning. Intriguingly, they also found that dispersing males quickly adapted their social style to that of their new group, suggesting that they may experience a phenomenon akin to social conformity driven by peer pressure.
Kerjean and colleagues including Erica van de Waal and Charlotte Canteloup knew that other animals, like people, follow traditions. But most studies had focused primarily on the presence or absence of traditions such as tool use. They hadn’t explored more subtle, quantitative differences in social behavior that can lead to important differences in a group’s social atmosphere.
To explore further in the new study, the researchers analyzed more than 84,000 social interactions between almost 250 vervet monkey individuals collected over nine years in three neighboring groups. Their analyses revealed an unexpected difference: in one of the groups, dubbed Ankhase (AK), the monkeys were more likely than in the other two groups to trade off when grooming each other.
“We found that individuals in one group—AK—display significantly more affiliative behaviors than in the two other groups, and this difference was stable over 9 years of study,” Kerjean says.
The AK group was not only more social than the other two, but they also exchanged grooming more reciprocally. When a monkey groomed another monkey, that monkey would usually repay the favor. As a result, grooming was exchanged more fairly in AK compared to the other two groups.
“You can think about it like massage exchanges between individuals,” Kerjean explained. “If I give you a massage 100 times a year but you only did it two times, I may feel that our relation is quite unfair. That’s the kind of differences we observed between our groups.”
The researchers also found that six males who moved from one group to another adapted their social interactions to better match their new groups. Those changes were also seen in both directions. Upon moving out of the AK group, males became less social and less likely to groom a partner fairly. Males leaving one of the less social groups for AK showed exactly the opposite trend.
“Males adapted their sociality to the group they integrated with, which we believe is a good example of social conformity,” Kerjean says. “This normative rule—act like others—probably helps them to get better integrated in a new group. This conformity effect had been previously shown through a novel food experiment, but this is the first time that we observed that with social behavior.”
The findings show that groups not only have different social traditions but that those traditions also can be stable over time in ways that are likely mediated socially. The researchers suggest that these differences are passed on through social learning, although they can’t rule out that there may be other differences in the environment at work, too.
Now that they know about the existence of these social traditions, they say they’d like to learn more about how they are introduced and passed on. They note that they’re especially curious to explore the role of key individuals or leaders in pressuring newcomers to follow along.
Three vervet monkeys with an adult grooming a juvenile
CREDIT
Charlotte Cantelou
iScience, Kerjean et al., “Social dynamics of vervet monkeys are dependent upon group identity” https://cell.com/iscience/fulltext/S2589-0042(23)02668-8
iScience (@iScience_CP) is an open access journal from Cell Press that provides a platform for original research and interdisciplinary thinking in the life, physical, and earth sciences. The primary criterion for publication in iScience is a significant contribution to a relevant field combined with robust results and underlying methodology. Visit http://www.cell.com/iscience. To receive Cell Press media alerts, contact press@cell.com.
JOURNAL
iScience
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Social dynamics of vervet monkeys are dependent upon group identity
ARTICLE PUBLICATION DATE
19-Dec-2023
Would you like to groom me?
Researchers at UNIL’s Department of Ecology and Evolution reveal the existence of social traditions in vervet monkeys. This work was published in iScience on December 19, 2023.
British tits have learned from each other how to pierce the lids of milk bottles left on doorsteps. On the island of Koshima, Japanese macaques started washing sweet potatoes to rid them of sand. The animal realm is full of examples of traditions linked to food, tool use or hunting techniques that have spread within specific communities. However, few traditions of social nature, i.e. how individuals interact with each other, have been described.
A study led by Erica van de Waal, Associate Professor in the Department of Ecology and Evolution (DEE) at UNIL's Faculty of Biology and Medicine, and Charlotte Canteloup, a former DEE post-doctoral fellow, documents the existence of socially transmitted customs in three groups of vervet monkeys from the same population.
Erica van de Waal's team has been studying the cognitive and social abilities of these primates for almost fifteen years, as part of the "iNkawu Vervet Project" initiated by the professor. In the field, in South Africa, biologists follow these animals daily, identifying each one, observing them and recording their behaviours. These can be affiliative (so-called "positive" social attitudes), such as time spent sitting side by side or playing together, body contact and grooming. Or agonistic, such as conflicts, threats or aggression.
Genetics and environment out of the equation
First author of the study, Elena Kerjean a former master's student under the supervision of Erica van de Waal and Charlotte Canteloup, analysed 84,702 social interactions involving 247 monkeys between 2012 and 2020. She then calculated a sociality index estimating the propensity of individuals to be rather affiliative or agonistic, an index measuring the reciprocity of grooming and an index quantifying social behaviours between members of the same family.
"The results show that one of the three groups, called AK, was generally more affiliative than the other two, and that grooming was more reciprocal. This was true throughout the nine years of the study", says Charlotte Canteloup, now a permanent researcher at the CNRS and the University of Strasbourg. This observation was independent of socio-demographic variations between the three groups, including sex ratio, number and age of individuals. "Furthermore, as the three groups share a very similar habitat with overlapping territories, and gene flow is ensured by the movement of males between groups, it is highly improbable that these variations in sociality can be explained by ecological and genetic differences only. All this suggests a social origin for these differences", adds Elena Kerjean.
According to the researchers, AK members may have developed a more affiliative "social tradition", under the social influence of the community or specific key individuals. "The monkeys could thus behave like others through a process of behavioural mimicry acting like a social glue", suggests Charlotte Canteloup.
Males embrace their new home’s habits
Interestingly and surprisingly, according to the authors, some animals - in this case six adult males who dispersed to their neighbour groups - adapted their behaviour to match that of their new home. Thus, those who left the most social group to join one of the other two became less social. And vice versa.
In a previous study, the team looked at how a new eating habit spreads and is maintained in vervet monkeys. "We had found that males abandoned their initial preference for one food and conformed to the local dietary norm of their new community. This time, we show that this conformity may extend to social behaviour, which has never been reported before", comments Erica van de Waal.
More generally, the research published in iScience contributes to a better understanding of the traditions of vervet monkeys in their natural habitat, which are far less studied and documented than those of great apes such as chimpanzees. "Primate cultures are made up of a greater variety of customs than what was believed even a few years ago. The more research progresses, the more we discover that 'basic' and universal social behaviours in apes, such as play, body contact and grooming, form the basis of cultures", concludes Charlotte Canteloup.
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JOURNAL
iScience
SUBJECT OF RESEARCH
Animals
ARTICLE TITLE
Social dynamics of vervet monkeys are dependent upon group identity
ARTICLE PUBLICATION DATE
19-Dec-2023
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