Evaluation of swab and related tests as a bioassay for assessing responses by squamate reptiles to chemical stimuli

The ability of squamates to detect chemical cues from adaptively important sources including prey, predators, and conspecifics has been tested frequently by presenting stimuli on cotton-tipped swabs or ceramic tiles. In many such studies the primary response variable is tongue-flicking, which is widely interpreted to indicate sampling for vomerolfaction. I review the basic experimental method and consider limitations regarding its application and interpretation and ways to overcome them. Effects of experimenter proximity and the assumed invisibility of chemical stimuli are considered, as are use of cologne as a pungency control, senses used in making chemical discriminations, and interpretation of results when there are no significant response differences among stimulus classes. Although the assumption that tongue-flicking reveals vomerolfactory sampling and the necessity of an intact vomeronasal system for normal responses to pheromones have been demonstrated where tested, very few species have been examined. In some squamates for which these assumptions have not been examined experimentally, especially eublepharid geckos, attacks on swabs bearing prey chemicals and performance of antipredatory displays in response to predator chemicals occur with no prior tongue-flicking. Not only are assays based on tongue-flicking useless in such cases, but the discriminations are likely based on olfaction. Issues specific to the study of responses to prey chemicals, predator chemicals, and pheromones are discussed. For many purposes, swab tests provide rapid, conclusive assays of ability to respond differentially to biologically relevant stimuli. However, other methods may be superior for studying some responses, and swab tests are not always applicable.