Anti-Snake Behavior and Snake Discrimination in Vervet Monkeys

Anti-predator behavior can inform how animals classify potential threats and which cognitive mechanisms might be involved in risk assessment. Snakes are common predators for many primate species, yet most of our knowledge on primate anti-snake behavior stems from predator model experiments. Only some studies could investigate natural predator–prey interactions. Here, we combine an observational study on anti-snake behavior in free-ranging vervet monkeys, Chlorocebus pygerythrus, with an experimental test of snake classification to achieve an integrated understanding of the monkeys' responses to and classification of snakes. Over 25 months, we gathered data from over 200 individuals in five groups and recorded natural encounters between vervet monkeys and five different species of snakes. We assessed whether the time that monkeys spent in close proximity (≤ 10 m) to snakes varied with the encountered species and whether age and sex classes differed in their propensity to approach snakes (≤ 10 m) or produce alarm calls. Encounters with pythons lasted longer, and monkeys were likelier to approach them than other species. While adult males were less likely to approach snakes, we found no effect of age or sex class on alarm-calling probability. Alarm calling was restricted to individuals close to snakes, apparently facilitating the recruitment of nearby group members to a snake's position. In experiments, we tested whether vervet monkeys discriminated snake models by length or diameter, but found no effect of model size. While the experimental data showed the same trends as data from natural encounters, the model pythons were approached by a much larger proportion of subjects than real pythons. This contrast between responses in natural predator encounters and simulated events stresses how assessing the variation in natural encounters can provide valuable context when evaluating experimental data.