Rapid covering by shadow as a cue to predation risk in three lizard species

Because predators attacking from above may cast shadows on prey, diverse prey have evolved behavioural responses (fleeing, freezing, descending in water) to shadowing. I studied escape responses to shadow in three lizard species by passing my hand overhead, casting a shadow on them or not. Theory predicts immediate fleeing when a predator is detected closer than the optimal flight initiation distance (predator-prey distance when escape begins). Predictions that Sceloporus virgatus, S. jarrovii and Urosaurus ornatus are more likely to flee and to either flee or adopt alert postures when covered by shadow were verified. Probability of fleeing or alerting did not differ among age/sex groups in S. virgatus. For probability of fleeing, shadow and age/sex group effects interacted, juveniles seeming to be slightly less likely to flee from shadow. Sceloporus virgatus covered by shadow fled more frequently on horizontal than vertical surfaces, when fully exposed than beneath overhanging objects, and when shadow encroached rapidly than slowly. These findings are consistent with the hypothesis that probability of fleeing is greater when shadow is more likely to indicate attack. Responses to sudden covering by shadow conform to predictions based of escape theory based on predation risk.