The researchers studied the way wild samango monkeys studied them — specifically, they compared the monkeys' behavior when humans were and weren't hanging around. Not only did the monkeys behave differently in the researchers' presence, but they capitalized on people's tendency to spook terrestrial predators like leopards. These monkeys have realized human observers "create a temporarily safe, predator-free environment," lead researcher Katarzyna Nowak tells MNN.
"This means that these arboreal monkeys can then exploit the forest's understory and ground level for forage, and may, for instance, obtain a more varied diet by consuming fungi or insects in the leaf litter when human observers are around," says Nowak, who studies zoology and anthropology at South Africa's University of the Free State and at Durham University in the U.K.
To shed light on this, Nowak and her colleagues examined two groups of samango monkeys at a site with high natural predator density and no human hunting pressure. These monkeys normally spend lots of time in trees, where they display a "vertical axis of fear": Climbing too high makes them vulnerable to eagles, but dawdling near the ground exposes them to leopards and caracals.
A Sykes' monkey ventures onto the ground near Ngorogoro Crater, Tanzania. (Photo: Shutterstock)
Nowak first demonstrated this elevation anxiety by setting up buckets of food at various heights in the two habitats. After vacating the area to let the monkeys feed, she found they had left significantly more food in buckets near the forest floor — a sign they were less comfortable letting their guard down to feed there. When the researchers stuck around, however, the monkeys who were already "habituated" to humans became bolder about eating from ground-level buckets.
That shows how observant and resourceful these monkeys are, but it also shows why habituating wildlife to humans may not always offer a window to their natural behavior. We tend to assume wild animals will go about their business once they've grown accustomed to human observers, but some just adapt their normal activity to capitalize on human company. And while that's impressive, it could also modify ecosystems by favoring animals that aren't wary of people.
"Human observers don't only displace monkeys' natural predators while they follow the monkeys," Nowak points out. "Observers may also displace unhabituated monkey groups, making habituated groups dominant and facilitating these groups' access to resources outside their core range."
On top of that, she adds, a healthy fear of humans is in many species' best interest. "Habituating wild animals to human presence must be decided upon with great caution. If these same animals are threatened by human activity in the form of poaching or poisoning, then through habituation for research, we may make them more vulnerable to such harmful activities."
Some primates, elephants and other animals can distinguish between groups of people or even individuals, so it's plausible they could tell hunters and scientists apart. Many others can't, however, and "we shouldn't be banking on this," Nowak says. "Habituation remains an ethical issue."
Nowak and her colleagues have also begun branching out their research, re-running the experiment in an area with few natural predators but lots of human-monkey conflicts. By comparing those monkeys' foraging rates in native forests versus people's gardens, they hope to test the "risk-disturbance hypothesis," which suggests risk from humans can be similar to natural risk from predators.
And among the samango monkeys who are more at ease with people following them, the researchers are trying to better understand that trust by (harmlessly) violating it. They needed to do that anyway, Nowak explains, by briefly trapping habituated monkeys for tagging.
"Following our initial study, there was a short period of live-trapping of the samango monkeys at our field site," she says. "This live-trapping was aimed at ear-tagging monkeys to aid in individual identification. We decided to re-run our experiment following this live-trapping period to see if trapping monkeys changed their perception of researchers as 'shields.' Joel Berger, who has conducted a lot of valuable field research on animal fear, would call trapping of habituated animals a 'violation of their de-facto trust' which they have developed for us over time, so our next analysis will examine this."
That may sound harsh, but in addition to providing insight on animal behavior, it's a relatively benign way these monkeys can learn a vital lesson for wildlife worldwide: Trust humans at your own risk.
Related wildlife stories on MNN:
- Elephants may have a specific alarm call for 'human!'
- 5 reasons why you shouldn't raise wild animals as pets
- Monkeys' lip smacks are eerily similar to human speech