Human cultures have widely differing standards when it comes to kindness and politeness. Accidentally make eye contact with a Londoner on public transport and suffer mutual disgust, but go to South Africa and find yourself routinely embraced by complete strangers. For researchers who study human behavior, there is a strong urge to study a wider variety of different populations around the world in an effort to expand attention beyond wealthy Westerners. But when it comes to animal behavior, differences between populations have been viewed less critically.
A paper in PNAS this week they examined the differences in social behavior between four different populations of chimpanzees, finding that the groups had very different standards when it came to hanging out and grooming each other. They point out that this means that studying one population of chimpanzees isn’t always enough for accurate claims about the species as a whole.
Midwest vs. New Yorker chimpanzees
The Chimfunshi Wildlife Orphanage Trust in Zambia is an ideal place to research differences between chimpanzee groups. It is home to 120 chimpanzees, some of whom were born in the wild but needed to be rescued and some of whom were born in the orphanage. The sanctuary has several populations that are separated from each other but all live in the same ecological environment.
That’s important, because when it comes to studying population differences between chimpanzees, the environment can play an important role. If two groups of chimpanzees have different methods of cracking nuts, is that the result of passing on different traditions to their children or simply the result of the nuts and tools available in their kind of forest?
While some cultural traditions — such as nutcracking — have been established in chimpanzees, primatologist Edwin JC van Leeuwen and his colleagues wanted to see if there were wider cultural differences in social behavior. So they tracked 89 different chimpanzees in four populations and collected data over a two-year period. Since all chimpanzees lived on Chimfunshi, differences in habitat would not play a role in any cultural differences they found.
There could of course be other explanations as well. One of the biggest is demographics, and they found this explained some of the differences found: In the larger populations, individuals were less likely to get to know and care for each other — just like people know more about their small-town neighbors. than in a big city.
But they also found differences that couldn’t be explained by population size. “Some populations were found to be more sociable than others,” write van Leeuwen and his colleagues, “both in terms of the size of their typical congregations during the day and their tendency to socialize and interact positively.” In the friendlier chimpanzee cultures, social network analyzes showed a higher degree of social cohesion between individuals.
Not all chimpanzees
There may also be factors beyond group tendencies – individual chimpanzees have different personalities, and the group differences may break apart that caused chimpanzees to happen to be in each group. This can be a major problem, given the small number of chimpanzees involved. The real key is to see if the differences persist through generations: if the behavior changes when the members of the group turn around, then it’s probably a matter of personality. But if they stay the same over time, that means baby chimpanzees learn the social norms as they grow up and then perpetuate them. Long-term studies investigating this would be immensely valuable, the researchers suggest.
Another useful kind of experiment would be to look at what happens when unrelated groups of chimpanzees merge. This sometimes happens when a group is moved from one zoo to another, and it’s a useful way to see which bits of chimpanzee behavior are learned and which are instinctive.
But these results are enough to suggest that researchers studying one population of chimpanzees might have the same limitations as they would experience if they studied just one population of humans, the authors claim. Experimental studies of social behavior in primates, which often have very small sample sizes, “may be biased in their particular study population,” they write.
Researchers studying social behavior of our closest relatives often seek to understand the evolutionary roots of human behavior, so understanding the variability of the species we study is critical if we are to properly understand ourselves.
PNAS2018. DOI: 10.1073/pnas.1722614115 (About DOIs).