Human observers impact habituated samango monkeys’ perceived landscape of fear

Humans and human infrastructure are known to alter the relationship between predators and prey, typically by directly or indirectly shielding one of the species from the other. In addition to these overt changes to animals’ behavior, observers may have more subtle impacts on animals’ foraging decisions. However, the anthropogenic alteration of risk-taking behavior has rarely been acknowledged or quantified, particularly in behavioral ecological studies reliant on habituated animals. We tested the magnitude of the "human shield effect" experimentally on 2 groups of samango monkeys, Cercopithecus mitis erythrarcus, at a site with high natural predator density and no human hunting pressure. In general, giving-up densities—the density of food remaining in a patch when a forager leaves—were greatest at ground level (0.1 m) relative to 3 tree canopy levels (2.5, 5, and 7.5 m), highlighting a strong vertical axis of fear. When human followers were present, however, giving-up densities were reduced at all 4 heights; furthermore, for 1 group, the vertical axis disappeared in the presence of observers. Our results suggest that human observers lower monkeys’ perceived risk of terrestrial predators and, thereby, affect their foraging decisions at or near ground level. These results have significant implications for future studies of responses to predation risk based on habituation and observational methods.