The discrimination ability of Texas Horned Lizards (Phrynosoma cornutum) during antipredator responses was tested with snakes of two genera having distinctly different prey foraging and subjugation strategies; Western Diamondback Rattlesnakes (Crotalus atrox) are “sit-and-wait” predators with a venomous strike from ambush, whereas whipsnakes (Masticophis spp.) are nonvenomous, rapid pursuit-and-grasp predators. Neither snake constricts prey; both ingest prey whole. Lizards were watched for reactions during close approaches by moving snakes. All unapproached and some approached lizards remained alert and motionless. Approached lizards that reacted either (1) ran rapidly to a distant point in the large enclosure, or (2) maintained their position but dorsoventrally flattened their body and tilted their stance, orienting a “dorsal shield” posture toward the snake. The distinctly contrasting responses of the lizards to the two snakes were significantly different, relocation running from rattlesnakes and stationary-body reorientation toward whipsnakes. For slow-running, broad-bodied Texas Horned Lizards, running is an appropriate escape response to a nonpursuing venomous predator, whereas the nonrunning body-conformation/orientation change is an appropriate defensive response, advertising size and spiny defenses, to a rapid-pursuit snake that must grasp prey with its jaws to effect capture and subjugation. Apparently horned lizards visually recognized, probably innately, the two snake taxa as different categories of predator threat.
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