A team of researchers working in South Africa have a sneaking suspicion that they're being used as human shields. Monkeys who normally spend their time in trees avoiding predators like leopards and raptors seem to relax their vigilance a little around humans, venturing down to eat.
Humans, as well as human infrastructure, can alter the relationship between predators and prey by shielding one from the other. A stone wall filled with crevices could provide a refuge for a small critter, while a person’s presence might indirectly guard an animal against its would-be killer. Those who are used to us may actually begin to take more risks when we’re around.
To quantify this alteration in risk-taking behavior, Katarzyna Nowak of Durham University and colleagues tested the magnitude of the “human shield effect” on two groups of samango monkeys (Cercopithecus mitis erythrarcus) at a site with high natural predator density and no human hunting pressure.
Samango monkeys spend most of their time in trees avoiding predators on the ground, like large cats, and those in the canopies above, like birds of prey. They don’t stray very far, and they don’t climb too high. When observed by humans, however, the monkeys preferred to climb down to eat food from the forest floor. The work was published in Behavioral Ecology earlier this month.
The researchers set up feeding stations at various levels in the forest and looked at a fear measure known as “giving-up densities.” That’s the density of food remaining in a patch when a forager leaves. (It’s a little like, say, you were in a hurry and didn’t eat your whole burger.) With lots of predators at this site, the giving-up densities were greatest at ground level (0.1 meter) relative to the three tree canopy levels (2.5, 5, and 7.5 meters up). This highlights a strong vertical axis of fear, as they researchers say.
“The amount of food monkeys depleted from buckets over the course of the experiment varied with height, with the most food left uneaten at ground level, where there is a risk of predation by leopards and caracals,” Nowak tells The Independent.
When human followers were present, giving-up densities were reduced at all four heights. In one of the groups, the vertical axis totally disappeared in the presence of human observers. By passively keeping terrestrial predators away from the area, we seem to lower the monkeys’ perceived risks.
“When a human observer was following monkeys, they ate more food at every height, with the most notable differences at the bottom two levels,” Nowak explains. “Animals are expected to deplete more food where and when they feel safe [but] we had not expected human followers to have such strong effect!”