Western Fence Lizard (Sceloporus occidentalis) chemical signals. III. An experimental ethogram of conspecific body licking

Chemical discrimination by tongue-flicking in lizards: A review with hypotheses on its origin and its ecological and phylogenetic relationships

Tongue-flicking is a synapomorphy of squamate reptiles functioning to sample chemicals for vomerolfactory analysis, which became possible in primitive squamates when ducts opened from the vomeronasal organs to the roof of the mouth. Extant iguanian lizards in families that do not use the tongue to sample chemical prey cues prior to attack partially protrude it in two feeding contexts: during capture by lingual prehension and after oral contact with prey. These lizards do not exhibit strike-induced chemosensory searching. Lingual prey prehension is present in iguanian lizards and inSphenodon, the sister taxon of Squamata. During attempts to capture prey, the tongues of primitive squamates inevitably made incidental contact with environmental substrates bearing chemicals deposited by prey, conspecifics, and predators. Such contact presumably induced selection for tongue-flicking and ability to identify biologically important chemicals. Most iguanian lizards are ambush foragers that use immobility as a major antipredatory defense. Because tongue-flicking at an ambush post would not allow chemical search beyond the vicinity of the head and would render them easier for predators and prey to detect, typical iguanians tongue-flick neither while foraging nor to identify predators. They do detect pheromones by tongue-flicking. Scleroglossan lizards are typically active foragers that rely on speed to escape. Being freer to move the tongue, they have evolved lingual sampling allowing detection of chemical cues of conspecifics, predators, and prey, as well as strike-induced chemosensory searching, some can follow pheromone trails by tongue-flicking. Some families have lingual morphology and behavior specialized for chemosensory sampling. In varanids and snakes, the taxa showing the greatest lingual specialization, additional prey-related chemosensory behaviors have evolved. In iguanian and scleroglossan families that have secondarily adopted the foraging mode typical of the other taxon, prey chemical discrimination involving tongue-flicking and strike-induced chemosensory searching are typical for the foraging mode rather than the taxon. Because foraging mode and state of prey chemical discrimination are stable within squamate families and to a large extent in higher taxa, both features have been retained from the ancestral condition in most families. However, in three cases in which foraging mode has changed from its ancestral state, the state of prey chemical discrimination has also changed, indicating that prey chemical discrimination is adaptively adjusted to foraging mode. Indeed, acquisition of lingually mediated prey chemical discrimination may have made feasible the evolution of active foraging, which in turn appears to have profoundly influenced the further evolution of squamate chemosensory structures and behavior, placing a selective premium on features enhancing the tongue's efficiency as a chemical sampling device. The advent of tongue-flicking to sample prey chemicals and thus detect hidden prey may have allowed generalist (cruise) or ambush foragers, if early squamates were such, to become specialists in active foraging. Alternatively, if the common ancestors of squamates were active foragers, the adoption of ambush foraging would have selected against participation of the tongue in locating prey. Acting jointly, tongue-flicking and active foraging have had momentous consequences for squamate diversification. Specialization for active foraging would appear to have had ramifying effects on antipredatory defenses, body form, territoriality, mating systems, and reproductive physiology.

Chemical access to the vomeronasal organs of the lizard Chalcides ocellatus

A functional association between tongue flicking and vomeronasal organ chemoreception in lizards has been assumed frequently, but never tested. To test this hypothesis, a cotton swab soaked in prey extract mixed with [3H]proline was touched ten times to the snouts of normal and tongueless lizards (Chalcides ocellatus), as well as normal lizards whose vomeronasal organ (VNO) ducts had been sealed with a tissue adhesive. Additionally, such a swab was held approximately 0.5 cm in front of the snout of normal and tongueless lizards for 1 min, which allowed no contact by tongueless animals and tongue contact only by normal lizards. Using autoradiography, substantial and comparable concentrations of labeled molecules were detected in the mouths, VNO ducts, and VNOs of normal subjects in both conditions, as well as tongueless lizards whose snouts made physical contact with swabs. No label was evident in VNOs of tongueless animals that did not contact swabs or normal animals with sealed VN ducts. Labeling was not apparent generally in the olfactory epithelium. These results suggest that: 1) the tongue is not required for, nor does it increase, chemical delivery to VNOs when snout contact occurs; 2) the tongue transfers molecules from the external environment to the oral region; 3) it is unlikely that nonvolatile chemicals reach the olfactory epithelium via an oral route; and 4) tissue adhesive is an effective means of preventing chemical access to the VN organs.

Conspecific odor detection by the male broad-headed skink, Eumeces laticeps: effects of sex and site of odor source and of male reproductive condition

Olfactory stimuli are sufficient for detection and discrimination of sex of conspecific lizards by the male broad-headed skink, Eumeces laticeps, a member of a large group of lizards with pronounced chemosensory abilities, the Autarchoglossa. The capacity of male broad-headed skinks to detect conspecific odors was assessed by measuring tongue extrusion rates in response to odor stimuli presented on moist cotton applicators. Tongue-flick rates of postreproductive males were significantly higher for cloacal odors of postreproductive conspecifics of both sexes than to distilled water and higher to female than male odors over the initial 20- and 60-sec intervals. In a second experiment using testosterone-treated males and estrogen-injected females, testosterone-treated males emitted significantly more tongue flicks to female cloacal odors than to the other stimuli, and two males bit applicators bearing male odors. Testosterone did not affect reaction to male cloacal odors, but markedly increased tongue-flick rates in response to cloacal odors of estrogen-treated females. Postreproductive males also responded to female, but not male, skin odors at a significantly higher rate than to water. Possible sources and presumed adaptive significance of conspecific odors are discussed.