Monitoring by prey that does not reveal awareness by turning toward approaching predators

Abstract

Prey monitor predator movements to assess risk, as required to make escape decisions and avoid being captured unaware. For prey that exhibit behavioral signs that they have detected predators, alert distance is the predator-prey distance when the prey performs the behavior and then continuously monitors the predator’s approach. Many other prey do not usually give any indication of having detected a predator prior to fleeing. This is especially likely in prey having laterally placed eyes that are approached from one side, as in typical studies of lizards. We conducted field trials to detect overt signs of monitoring by zebra-tailed lizards, Callisaurus draconoides, which usually exhibit no signs of monitoring. When a researcher walked in an arc starting at some distance from a lizard’s side and continuing until he was directly in front of or behind it, the lizard cocked its head and/or reoriented its body or fled and then reoriented. These behaviors allowed lizards to keep the researcher in view as he passed out of a monocular visual field. The findings demonstrate that monitoring occurs in these lizards, suggest that monitoring is so important that lizards risk being detected by moving, and suggest a possible method for studying effects of alert distance in prey that do not perform alerting behaviors when approached in full view. Alerting responses have been observed infrequently in lizards because researchers are in one of the wide lateral visual fields when they start to approach. Unless the predator moves out of view, prey with limited or no binocular vision have no need for postural adjustment to focus on the predator.