Factors increasing snake detection and perceived threat in captive rhesus macaques (Macaca mulatta).

The primary predators of primates are all ambush hunters, and yet felids, raptors, and snakes differ in aspects of their ecology that affect the evasive strategies of their primate prey. Felids and raptors can traverse long distances quickly, thus the urgency of threat they present increases as they come closer in proximity to primates. In contrast, snakes do not move rapidly over long distances, and so primates may be reasonably safe even at close distances provided snakes can be detected and monitored. We investigated the ability of captive rhesus macaques (Macaca mulatta) to detect snakes at distances ranging from 15 to 1.5 m. We also examined variation in intensity of perceived threat by applying a Hidden Markov Model to infer changes in underlying state from observable behaviors, that is, increased attention and mobbing. We found that the macaques often failed to detect snake models but that closer proximity improved snake detection, which is necessary before threat can be perceived. We also found that having only one individual in fairly close proximity (≤ 7.5 m) was sufficient to alert the rest of the group and so the chances of detection did not increase with increasing group size. Finally, we found that when the snakes were perceived, they did not elicit greater intensity of response with closer proximity. These results provide evidence that the threat from snakes is greatest when they are in proximity to primates but are unseen. When snakes are seen, however, distance appears not to affect primates' perceived risk, in contrast to their perceived risk from raptors and felids.

Scales drive detection, attention, and memory of snakes in wild vervet monkeys (Chlorocebus pygerythrus).

Predatory snakes are argued to have been largely responsible for the origin of primates via selection favoring expansion of the primate visual system, and even today snakes can be deadly to primates. Neurobiological research is now beginning to reveal the mechanisms underlying the ability of primates (including humans) to detect snakes more rapidly than other stimuli. However, the visual cues allowing rapid detection of snakes, and the cognitive and ecological conditions contributing to faster detection, are unclear. Since snakes are often partially obscured by vegetation, the more salient cues are predicted to occur in small units. Here we tested for the salience of snake scales as the smallest of potential visual cues by presenting four groups of wild vervet monkeys (Chlorocebus pytherythrus) with a gopher snake (Pituophis catenifer) skin occluded except for no more than 2.7 cm, in natural form and flat, the latter to control for even small curvilinear cues from their unusual body shape. Each of these treatments was preceded by a treatment without the snakeskin, the first to provide a baseline, and the second, to test for vigilance and memory recall after exposure to the snakeskin. We found that (1) vervets needed only a small portion of snakeskin for detection, (2) snake scales alone were sufficient for detection, (3) latency to detect the snakeskin was longer with more extensive and complex ground cover, and (4) vervets that were exposed to the snakeskin remembered where they last saw "snakes", as indicated by increased wariness near the occluding landmarks in the absence of the snakeskin and more rapid detection of the next presented snakeskin. Unexpectedly, adult males did not detect the snakeskin as well as adult females and juveniles. These findings extend our knowledge of the complex ecological and evolutionary relationships between snakes and primates.