One morning in South Africa's mountainous Lajuma Research Center, an adult female samango monkey came down from the trees to search for peanuts in an experimental food dispenser. Every once in a while she scanned her surroundings for predators, but she never bothered to look behind her once she realized that Katarzyna Nowak was there.

Animals that are not at the top of their food chains are adept at avoiding their predators. Samango monkeys, for example, stay up in trees. But to retrieve peanuts from the center's dispensers, they have to be on the ground—and that makes them vulnerable. Only when it is certain that no predators are around will a monkey spend time looking for food. So why did this one stop checking for danger behind her? Nowak, a biological anthropologist at Durham University in England and at South Africa's University of the Free State, suspects that the monkey figured that if a human was around, then a leopard was probably not. “[It was] as if she was thinking that I had that area covered,” Nowak says.

Nowak put her suspicion to the test. She and her colleagues watched 100 individuals in all and found that they ate more food available on the ground when humans were present than when humans were absent (and observing them via camera). “Researchers were perceived as shields against terrestrial predators,” the team writes in the journal Behavioral Ecology.

Although researchers have purported to see many animals change their behavior while being watched by humans—from zebras on the African savanna to moose in North American forests—Nowak's study is one of the first to subject the “observer effect” to scientific scrutiny.