https://phys.org/news/2018-07-monkeys-benefit-nut-cracking-abilities-chimpanzees.html

"After studying both species for many months, I remain intrigued by the puzzling relationship between mangabey monkeys and chimpanzees. At some moments the monkeys seem to be scared of these potential though infrequent predators, while at other times they approach chimpanzees within meters to profit from their tool-using skills, as if they had learned to anticipate the other species 'mood'," says Karline Janmaat of the University of Amsterdam and the Max Planck Institute of Evolutionary Anthropology.

Mangabeys showed elevated levels of vigilance, measured by noting down how often the monkeys watched their distant surroundings at the chimpanzee nut-cracking sites compared with other foraging sites, suggesting that they perceived elevated danger at these sites. This could be explained by the tendency of chimpanzees to hunt monkeys. The elevated level of vigilance during scavenging is therefore consistent with the possibility that the mangabeys encounter a dilemma between an opportunity to gain a high amount of "free" energy and increased predation risk. "The decisions of mangabeys to approach food left-overs from potential predators, may imply contextual knowledge that helps the monkeys to judge when they are safe and when they need to be vigilant or move into a safe position", adds Janmaat.