Absence of Discriminatory Tongue-Flicking Responses to Plant Chemicals by Helodermatid Lizards

Abstract

criminate between palatable plant food and control stimuli by tongue-flicking (Cooper and Alberts, 1990; Cooper, 2000a, 2000b, 2000c; Cooper and Flowers, 2000). Because tongueflicking serves to sample chemicals from environmental substrates for delivery to the vomeronasal organs, and because prey chemical discrimination in lizards requires vomerolfaction (Cooper and Alberts, 1991), plant chemical discriminations presumably are vomerolfactory. In snakes there is a close correspondence between diet and strength of chemosensory responses to potential foods (Burghardt, 1970; Arnold, 1981; Cooper et al., 2000a), but no such correspondence has been established in lizards. Because the large majority of lizards consume little or no plant matter (Iverson, 1982), herbivory may afford an opportunity to examine a similar correspondence between diet and chemosensory responsiveness in lizards. To assess whether such correspondence occurs, WEC has undertaken comparative studies to assess responses by herbivorous and omnivorous lizards to plant chemicals and prey chemicals. Responses to plant chemicals by species that do not eat plants must be determined to learn whether plant chemical discriminations are acquired in association with plant consumption. Actively foraging insectivorous and carnivorous lizards respond to chemical cues from animal prey by tongue-flicking and biting the stimulus sources, discriminating between them and control substances (Cooper, 1995, 1997, 1999). The ability to identify prey using chemical cues may be adaptive for location and recognition of prey (Cooper, 1994). If the ability to recognize prey is specific, actively foraging carnivores should not respond strongly to chemical cues from plants palatable to herbivorous lizar s. This prediction has been substantiated for scincine (Cooper et al., 2000b) and lygosomine skinks (Cooper and Hartdegen, 1999), a lacertid and a teiid (Cooper et al., 2000c); a eublepharid gecko (Cooper and Habegger, 2000), and a varanid (Cooper and Habegger, in press). For a phylogenetically based examination of correlated evolution between herbivory and plant chemical discrimination, information is needed on responses to plant chemicals for a wide taxonomic range of herbivores and insectivores. In this study we present data showing no indication of strong response to plant chemicals by the 2 extant helodermatid lizards, the gila monster, Heloderma suspectum, and the Mexican beaded lizard, H. horridum. The gila monster discriminates prey chemicals from control substances by tongue-flicking (Cooper, 1989) and exhibits prolonged strike-induced chemosensory searching, an increase in tongue-flick rate and scent-trailing to locate lost prey (Cooper et al., 1994; Cooper and Arnett, 1995; Garrett et al., 1996). Chemosensory behavior has not been studied in Mexican