Responses to Chemical Cues from Plant and Animal Food by an Omnivorous Lizard, Gerrhosaurus validus

Abstract

Many lizards use lingually sampled chemical cues to locate, identify, and perhaps assess the quality of foods (Cooper, 1995, 1997). Responses to prey chemicals have been studied in species representing all of the major lizard taxa. Primarily because most lizards are insectivorous/ carnivorous, far less is known about response to chemical cues from plant foods by omnivorous or herbivorous lizards. It seems probable that in species derived from insectivores that now exhibit prey chemical discrimination, adding plant food to the ancestrally animal diet set up the potential for natural selection to increase the sensitivity or responsiveness to plant food chemicals. This hypothesis must be tested comparatively; it cannot be tested using a single species. Among scleroglossan lizards, actively foraging insectivores can discriminate between prey and control stimuli using only chemical cues sampled by tongueflicking (Cooper, 1995, 1997, 1999, 2000a). This generalization applies to all species yet tested, but few species have been studied in most families. One of the most thoroughly studied groups is the Scincidae, one of three families in the Scincoidea. Prey chemical discrimination has been demonstrated in several actively foraging species of skinks (Loop and Scoville, 1972; Burghardt, 1973; Nicoletto, 1985; Cooper and Vitt, 1989; Cooper and Hartdegen, 1999; Cooper, 2000a) and was found to be absent in a single ambush forager (Cooper, 2000a). In the primarily herbivorous lygosomine skink Corucia zebrata (Cooper, 2000b), chemical cues from both plant and animal cues induced increased tongue-flick rates and higher frequency of biting than did control chemicals. Much less is known about chemosensory responses to food in the remaining families of Scincoidea. Gerrhosauridae and Cordylidae are the two families of the Cordyliformes, the sister taxon of the Scincidae. In the Cordylidae, prey and plant chemical discriminations are absent in an ambushing, insectivorous species, HAO, E.-M., AND K. ADLER. 1993. Herpetology of hina. Soc. Stud. Amph. Rept. Contrib. Herpetol. Cordylus cordylus (Cooper and Van Wyk, 1994; Cooper and Steele, 1999). The sole gerrhosaurid studied, Gerrhosaurus nigrolineatus, an insectivore (Branch, 1998) which is believed to be an active forager, exhibited prey chemical discrimination (Cooper, 1992). In this paper I present the first test of the hypothesis that an omnivorous species (Rogner, 1997) of gerrhosaurid lizard, Gerrhosaurus validus, discriminates plant food chemical cues from control chemicals. This species appears to be an active forager on the basis of limited field data (Cooper et al., 1997). In addition to providing a test of the specific prediction, the results add to the data base needed to test the hypothesis that correlated evolution has occurred between plant diet and chemosensory responsiveness to plant chemicals. Tests of responses to animal prey chemicals provide an additional test of the established evolutionary relationship between prey chemical discrimination and foraging mode in lizards (Cooper, 1995, 1997). Nine adult Gerrhosaurus validus were obtained from a commercial dealer and were housed in an accredited