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Herpetological Review, 2008, 39(2), 156–162.© 2008 by Society for the Study of Amphibians and ReptilesBody-flip and Immobility Behavior in RegalHorned Lizards: A Gape-limiting DefenseSelectively Displayed Toward One of Two SnakePredatorsWADE C. SHERBROOKESouthwestern Research Station, American Museum of Natural HistoryPortal, Arizona 85632, USAe-mail: wcs@amnh.organdCLAYTON J. MAY5814 South Hopdown Lane, Tucson, Arizona 85746, USAIn 1937 Howard K. Gloyd led a herpetological expedition tosouthern Arizona (Gloyd 1937). Film records show a body-flippingreaction, termed “plays dead,” of a Regal Horned Lizard(Phrynosoma solare) to prodding with a thin stick and subsequenthuman handling. An adult lizard repeatedly flipped itself onto itsback, nine times within 25 s (immediately righted by Gloyd’s handafter each flip). We found only one subsequent mention of thebehavior in P. solare, without functional explanation (Parker 1971).Death feigning, letisimulation, or tonic immobility, has beenconsidered an antipredator response across invertebrate and vertebratetaxa without clear identification of its adaptive significance(Carpenter and Ferguson 1977; Greene 1994; Honma et al. 2006;Ruxton 2006; Ruxton et al. 2004), although in some predatoryfish the use of death feigning appears to be clearly adaptive duringaggressive mimicry (Tobler 2005). Many hypotheses offeredto explain immobility responses of prey tacitly assume that preymanipulate predators by sending false information that they aredead and that this information interrupts prey-subjugation behaviors,thus providing opportunities for prey escape (Honma et al.2006; Ruxton 2006). Honma et al. (2006) proposed that an inducibledeath-feigning response of a pygmy grasshopper (Criotettixjaponicus) is a specific antipredator response against a gape-limitedanuran predator to avoid being swallowed. The grasshopper’scharacteristic rigid posture, with body parts physically extended,interferes with prey manipulation and does not mimic death, butdirectly enhances prey survival.In the case of P. solare, it is difficult to identify an evolutionarilyadaptive advantage to body-flipping and re-flipping, to upside-down,by an immobile, death-feigning, animal. This leavesthe behavior lacking a clear biological explanation. Our study attemptsto place body-flipping and immobility behavior by P. solarein the context of adaptive antipredator behaviors that are effectiveresistance against specific predators that rely on jaw capture ofprey which they ingest whole, such as a non-venomous snake.We report several additional encounters (rare) of body-flippingbehavior in P. solare in response to human handling. We then describefield trials aimed at elicitation of the body-flipping andimmobility response or alternative responses (such as runningflight) in specific predator-context encounters involving twosnakes, one non-venomous (Masticophis flagellum) and one venomous(Crotalus atrox). The two snakes present the lizards withtwo different threats based on their prey-subjugation strategies(Endler 1991; Sherbrooke 2008), 1) M. flagellum: search/wait,identify, rapidly pursue, physically jaw-capture, subjugate, andingest, and 2) C. atrox: wait, identify, envenomate (strike), track,and ingest carcass. We use our observations to propose that thelizards distinguish between two categories of predator threat, thetwo snakes, and respond to each with distinctive antipredator behaviors(flipping or running) that appear appropriate for selectivelyenhancing survival in response to each predator’s subjugationskills. We also use the differences in responses of P. solare tothe two snakes to propose a hypothesis for the previously unexplainedbody-flipping and immobility behavior, noting the predatorcontexts in which it is employed and not employed, and wediscuss aspects of body-flipping and immobility that may functionas antipredation defenses with M. flagellum.METHODSCarpenter and Ferguson (1977) reviewed literature reports andnumerically catalogued lizard behaviors (termed “act systems”)involving body inversion (act system #26, turn over) andletisimulation (act system #150) in various lineages oflepidosaurians. Similarly, Greene (1994) enumerated several categoriesof antipredator behaviors (#3, catalepsy, letisimulation,death feigning, tonic immobility; #22, invert body). It is difficultto unequivocally assign our observations to a particular categorydue to the paucity of examples, diversity of descriptions, and frequentlack of meaningful context for the reported behaviors. Wesimply use descriptive terms, body-flip and immobility behavior.In a body-flip followed by immobility a lizard rapidly raises oneside (by extending its legs on that side) to effect a role over alongits nose-to-vent axis, landing upside down where it remains motionless(see Figs. 1 and 2).Following an observation of repeated body-flipping and immobilityof a captive P. solare in response to human handling (Fig. 1,A–C; 30 June 2006), we reviewed our field notes and summarizedadditional records of this behavior.We then studied the behavioral responses of adult P. solare duringfield trials utilizing a known ophidian predator of P. solare,the Coachwhip (M. flagellum) (Kauffeld 1957), that also preys onother similarly-armored horned lizards (Sherbrooke 1981). Theindividual M. flagellum (SVL 128 cm, tail length [TL] 47 cm;mass 787 g) utilized had previously been observed to capture andeat a P. solare (SVL 88 mm, TL 48 mm; mass 36.4 g) on the studyarea (May, unpubl. data). Our trials involved four P. solare fittedwith radio-transmitters (Holohil PD-2; approximately 3 g), whichwere relocated in the field, and six lizards encountered in situ whiletraversing the study area. Fourteen trials, involving 52 encounters(presentations), occurred between 26 August and 2 September 2006(Table 1): 0930–1200 h MST (12), and 1730–1900 h MST (2).The study area is immediately adjacent, on the west and northwestsides, to a small volcanic hill in the Altar Valley, Pima Co.,Arizona (32°02'11.5"N, 111°23'46.6"W, datum WGS 384).During trial encounters, the M. flagellum was restrained in glovedhands at mid-body, allowing the anterior third or more to movefreely as it was held to the ground approximately 1 m from thelizards. It was then allowed/encouraged to approach and contacteach lizard (Fig. 2, A). During each trial, an attempt was made toexpose the lizard four times to the snake. Contact by the snake156 Herpetological Review 39(2), 2008