When it comes to finding a role model, vervet monkeys prefer the women in their lives. Researchers have found that the silver-furred primates imitate a useful new skill, such as opening a box with a treat inside, only when the monkey demonstrating it is female.
Vervet monkeys live in territorial groups of 10 to 50 individuals, which include adult males and females, juveniles, and infants. Females stay in their native group for life, whereas males join other groups when they become sexually mature, at about 4 years old. Typically, an alpha male dominates the group, although among females in the same group there is also a dominant individual. Males ascend the ranks by fighting, whereas females pass their status down to their oldest daughter.
Scientists have long wondered whether groups like these harbor some sort of culture. That is, can individuals pass down certain behaviors from generation to generation?
Erica van de Waal had a chance to probe this question at the Loskop Dam Nature Reserve in South Africa. A doctoral student at the University of Neuchâtel in Switzerland, she noticed that groups of vervet monkeys had strikingly different ways of resolving arguments. One group would use a scapegoat, whereas in another group the monkeys would console the losing party to keep the peace. She wondered whether ecological differences were responsible, or whether a particular strategy had spread among individuals of one group.
To find out, she needed to carry out learning experiments with wild monkeys, something that's been notoriously tough to do. It took her and her colleagues 4 months of following six groups of vervets to overcome the monkeys' shyness to humans. Once the monkeys were comfortable having scientists around, Van de Waal gave each group a wooden box containing a slice of apple. To get to the apple, the monkeys had to either pull open the door at one end or slide aside a door at the other. Half the box was painted black to differentiate the two ends.
As the researchers expected, one monkey consistently took charge when the box appeared. This alpha monkey was male in three of the groups and was female for the other three. With each group, the researchers sealed one end of the box and presented the task again and again over the course of weeks, thus teaching the monkey to go for only one of the doors. Meanwhile, the team took note of how many of the nearby group members paid attention to the alpha monkey.
Next, Van de Waal's team passed out boxes again, this time unsealed, to other members of the group. The mechanics of opening the door eluded the monkeys at first, and in groups that had watched an alpha male the monkeys seemed to have no preference for which side to try first, or even whether to try opening the box at all. But in groups in which a female had demonstrated the task, 80% of the monkeys followed the leader and went for the side they'd seen her open, the team reports online tomorrow in the Proceedings of the Royal Society B.
Females stay with their groups for life, giving them stronger bonds with the group and also a better knowledge of the resources in a particular territory. So it makes sense that other group members look to them for guidance, Van de Waal says. The results could also explain why different groups within a primate species have their own suite of behaviors, she says. If vervet monkeys learn from only the females in their midst, they'll ignore a foreign male who arrives with strange customs, thus preserving local traditions.
Although experiments on social learning in primates have been done in captivity, "it's a major achievement to have got the first one done" in the wild, says Andrew Whiten, a primatologist at the University of St. Andrews in the United Kingdom. "It suggests social learning is filtered by the social structure of a species."
He points out that if the researchers had focused only on alpha males, they would have seen no evidence of social learning. In future experiments, "it could be really important who you're looking at."