Prey or predator? Body size of an approaching animal affects decisions to attack or escape

An animal must assess an approaching heterospecific to determine whether it is prey or predator. Because prey tend to be smaller than their predators, foraging responses decrease and escape responses are predicted to increase over a range of approacher size. Sufficiently, small approachers may be readily identified as prey and large ones as predators. However, more time and closer approach may be required to assess approachers of intermediate size, leading to predictions that predator/prey evaluations may differ among individuals, that attacks on the approacher begin from shorter distances than for smaller approachers and that escape begins at shorter distance than for larger approachers. In a field experiment, we dragged tethered model orthopterans of 3 sizes toward 2 lizard species that differ in body size. As model size increased, lizards were more likely to flee and had longer flight initiation distance. As model size decreased, lizards were more likely to advance toward and bite models and began advances from longer distances. At the intermediate model size, some individuals fled and others attacked intermediate models. Sceloporus jarrovii, the larger species, was more likely to attack and less likely to flee than S. virgatus at intermediate model size. The prediction of optimal escape theory that flight initiation distance increases with predator size due to greater risk was verified. Our evidence indicates that as approacher size increases, successive transitions occur between rapid assessment of the approacher as prey, uncertainty requiring closer inspection, and rapid identification of a potential predator.