A trip to an unfamiliar part of the world is all you need in order to realize that humans have vastly different ways of eating, playing, talking, problem-solving, and so much more. Some of us use forks, while others prefer chopsticks, and still others simply eat with their hands. All three of these solutions emerged to the very same problem. But where do these cultural traditions come from? How do these differences persist over time? If we wish to understand the evolution of culture, then it is important to understand how how cultural traditions in other species emerge.
Chimpanzees appear to possess at least a basic form of culture: among six African chimp groups, for example, there are different traditions for throwing, inspecting wounds, and removing parasites. It is well known that chimpanzees use sticks to hunt for termites, but different groups eat the termites in different ways: some eat the insects directly from the stick itself, while others will run the termite-coated stick through their hands, and then bring their hands to their mouths. Different cultural traditions have also been observed in other primates, in dolphins, and across the animal kingdom.
For a long time, researchers thought that the main differences in traditions occur between groups: perhaps one group removes parasites with their teeth, while another prefers to snatch parasites with their fingernails. New research conducted with wild vervet monkeys (Chlorocebus pygerythrus), published today in the journal PLoS ONE, paints a more complicated picture of cultural evolution, and Mom is at the center.
Primatologist Erica van de Waal and her colleagues presented six different groups of wild vervet monkeys with a new problem: dirty grapes. They were interested in how how different monkeys went about cleaning the grapes prior to eating them, and how the cleaning methods might then spread to other monkeys.
One possibility is that monkeys learn new traditions by imitating dominant individuals; in female-dominated vervet monkeys, whatever cleaning method was used by the dominant females might then ultimately spread throughout the group. This would mean that the main cultural differences would occur between groups.
However, it is also possible that cultural transmission occurs at the family level, rather than at the group level. This would mean that cultural differences would emerge between matrilines - groups of monkeys related through females - rather than between groups. This would allow for multiple traditions to coexist within a larger group.
Finally, there could be a genetic mechanism driving cultural traditions rather than a social one. If this was the case, then adult sisters would be more likely to share a grape cleaning method than non-related females, despite the fact that each sister had her own matriline.
The researchers presented monkeys with clear plastic boxes full of grapes, covered with sand. Since dominant individuals always get to eat first, they made sure that there were always enough grapes in the boxes for even the most subordinate individuals to get some. They repeated this fifteen times over the course of two years. Throughout the experiment, they noted that four different cleaning techniques had emerged. Some rubbed the grapes in their hands to remove the sand, others rubbed the grapes against some other surface such as stones or branches, another set of monkeys peeled the grapes with their teeth but ate only the insides, and yet another group split the grapes open with their hands and ate the insides. Some monkeys didn't clean the grapes at all, and instead ate them as they were, sand and all.
Mothers and children were more likely to show the same cleaning technique, regardless of group membership or the status of the mother within her group's dominance hierarchy. As sessions progressed, each individual monkey became more likely to use the technique used within his or her own matriline, even if he or she began the experiment using a different method. And monkeys used their mothers' techniques even when other members of the matriline were absent, indicating that the effects were not the result of transient imitation.
Importantly, individual monkeys changed their strategies over time to match the techniques of their matriline. Since "convergence implies social learning," according to van de Waal, it is unlikely that genetic differences would account for grape cleaning techniques. And each adult female was more likely to share a technique with her sons and daughters than with her sisters, lending further support for the social origins for grape cleaning methods.
This raises the possibility that small differences among matrilines might eventually become stable cultural traditions. To see if these differences had the potential to become more persistent traditions, during each of the last five sessions, researchers left out a bucket with water next to the sandy grapes. None of the monkeys ever used the water to clean the grapes, suggesting that they were more likely to stick with their matrilineal techniques than to switch to a more efficient cleaning method. In fact, the researchers conducted a sixteenth testing session, one year after the fifteenth had taken place. Even one year later, monkeys were still using the cleaning methods that had initially spread through their families. Put another way, the cleaning methods had stabilized among matrilines. And stable differences in addressing identical problems is the very definition of culture. Just as some humans use forks while others use chopsticks to eat, some monkeys rubbed the sand off the grapes while others peeled the skin away.
Taken together, van de Waal's study shows that the mother may be a more important source of social learning for vervet monkeys than any other group member, regardless of the dominance status of the mother. However, they point out that this experiment can't discern whether children learned from their mothers, or if mom learned from her children. Perhaps van de Waal and her colleagues will return to these monkeys in several years to repeat the experiment and see if the next generation of monkeys still converge on matrilineal grape cleaning strategies. If so, that will provide convincing evidence that Mom does, indeed, know best.
van de Waal E, Krutzen M, Hula J, Goudet J, Bshary R (2012) Similarity in Food Cleaning Techniques within Matrilines in Wild Vervet Monkeys. PLoS ONE 7(4): e35694. doi:10.1371/journal.pone.0035694
For more on social learning: