Close encounters with vervet monkeys are a staple experience of tourists who stay at the grand lodges in African nature parks. These small, handsome green monkeys have a habit of darting up to poolside-dining tables and thieving food.
When I was monkey-watching in Kenya, my own experience in this regard was limited (after all, NSF tends not to fund leisure hours for dissertation work). While observing baboons in the bush, though, I would frequently see vervet groups passing by.
I was quite interested then to find a new study that asks whether contact with humans in the wild—not on the grounds of lodges—affects vervets’ acquisition of technical skills. The article became available online last week at the journal Folia primatologica. Written by Erica van de Waal and Redouan Bshary, it is called Contact with Human Facilities Appears to Enhance Technical Skills in Wild Vervet Monkeys.
Van de Waal and Bshary review studies that have used the now-popular “artificial fruit experiment.”Boxes containing food are given to monkeys or apes, with different ways of opening the box made available; the primates’ responses to the challenge are recorded. Rarely before this new study had the artificial-fruit approach been used on fully wild primates, and the hypothesis at issue here is new as well: Does exposure to human facilities affect the monkeys’ technical skill at the artificial-fruit-opening problem?
The research was undertaken at the Loskop Dam Nature Reserve in South Africa on six neighbouring groups of vervet monkeys, all habituated to humans. The groups, organized in matrilines (groups of related females) with multiple males, as is typical for the species, ranged in size from 13-23. Two of the 6 groups “were in regular contact with humans and their facilities.” That is, monkeys in both groups frequented areas with barbecue places, toilets, and dustbins; one of the groups also ranged through 3 fishermen’s huts. On weekends and holidays, these human areas were heavily used, but because they made up a small part of the monkeys’ human range, “contacts with humans and their structures were not constant and were spread in time.”
It’s regrettable that the authors “have neither quantified information about the time these monkeys spent around humans or their facilities nor of the type of contact”; however, they did observe humans feeding the vervets, vervets entering the dustbins and toilets, and in an echo of the game-lodge monkey thievery noted above, vervets stealing food from humans.
In their experiments, van de Waal and Bshary used “artificial fruits” devised of wooden boxes with Plexiglas doors on opposite ends; inside were eighth-pieces of apple. A monkey could open one door by pulling a knob, whereas for the other door, she or he had to slide the door to the left while holding a knob.
Demonstrators of the correct technique were vervets, not humans: alpha females in three of the groups, dominant males in the other three, all of whom had sole access to the box during a stage where only a single door was operative. The proper controls were employed, for instance, “the 2 groups that are in regular contact with humans differed with respect to the sex of the model and the type of door the model opened during demonstrations.”
For each group member, data were recorded as to whether she or he opened the box on the first trial and which door was first touched.
Neither sex of the demonstrator monkey nor door used to solve the task affected success on the box-opening, but group membership did: Vervets of the two groups with human contact fared better than vervets in the other four groups. Because length of the first manipulation at the box did not differ between successful and unsuccessful individuals, the authors reject an explanation of reduced neophobia (fear of new things or experiences) on the part of the human-contacting monkeys.
What is it, then, about proximity to our own activities that enhances the vervets’ capacities—or at least their responses? As the authors note, a next step would be to carry out this same test on captive vervet monkeys, to see if the captive animals’ were equally successful as the most successful monkeys in this experiment. Another idea, of course, would be to seek quantitative and qualitative information on what the wild monkeys actually do when in presence of humans.
Mainly, what grabbed me are implications of this research, not touched on in the article. Nonhuman primates are thrown, with every passing year, into greater proximity with humans. Sometimes the ensuing interactions are relatively neutral (think game lodges), sometimes they are uneasy (think crop-raiding monkeys). Is this landscape-sharing affecting the evolving behavior and cognition of other species in ways we don’t even realize?
It’s fascinating too to flip the question around: Could our own small-brained ancestors, living millions of years ago on the African savanna or in the rainforest, have learned certain skills by spending time near populations of non-human primates that were long-adapted to that local environment?