Human observers impact habituated samango monkeys’ perceived landscape of fear

Abstract

Humans and human infrastructure are known to alter the relationship between predators and prey, typically by directly or indirectlyshielding one of the species from the other. In addition to these overt changes to animals behavior, observers may have more subtleimpacts on animals foraging decisions. However, the anthropogenic alteration of risk-taking behavior has rarely been acknowledgedor quantified, particularly in behavioral ecological studies reliant on habituated animals. We tested the magnitude of the human shieldeffect experimentally on 2 groups of samango monkeys, <i>Cercopithecus mitis erythrarcus</i>, at a site with high natural predator densityand no human hunting pressure. In general, giving-up densitiesthe density of food remaining in a patch when a forager leaveswere 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. Whenhuman followers were present, however, giving-up densities were reduced at all 4 heights; furthermore, for 1 group, the vertical axisdisappeared in the presence of observers. Our results suggest that human observers lower monkeys perceived risk of terrestrialpredators and, thereby, affect their foraging decisions at or near ground level. These results have significant implications for futurestudies of responses to predation risk based on habituation and observational methods.