Horned lizards are difficult to detect because of their cryptic coloration and behavior, but often flee from approaching predators and use specialized behavioral, morphological, and physiological defenses at close quarters. Escape theory predicts that flight initiation distance (predator–prey distance when escape begins) increases as predation risk increases. We predicted that, despite relying on crypsis, Texas horned lizards (Phrynosoma cornutum) would have greater flight initiation distances when approached rapidly than slowly and when approached directly than indirectly. Flight initiation distance was greater for rapid than slow and direct than indirect approach, verifying the predictions for prey that do not rely exclusively on crypsis, forgoing escape. Effect size was larger for approach speed than for directness of approach, in part because the difference between minimum bypass distances was small (0.0 m for direct and 0.6 m for indirect approaches). We also investigated responses to a shadow passing over a lizard, which might be a cue to imminent risk. When a model accipiter passed overhead, lizards were much more likely to move and jump if and when the model cast a shadow directly on them than if the shadow passed nearby without falling on them. Some lizards fled when the shadow fell on them. We interpret these novel findings as indicating that P. cornutum assess themselves as being in immediate peril when suddenly covered by a shadow. They reacted primarily by immediate flight or jumping, possibly reflecting preparation to use alternative defensive strategies at close quarters or to delay escape while further assessing risk. Thus, although Texas horned lizards rely strongly on crypsis, they make escape decisions based on degree of predation risk.
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Vol. 66 • No. 1