Morphology and fibre-type distribution in the tongue of the Pogona vitticeps lizard (Iguania, Agamidae)

Agamid lizards use tongue prehension for capturing all types of prey. The purpose of this study was to investigate the functional relationship between tongue structure, both surface and musculature, and function during prey capture in Pogona vitticeps. The lack of a detailed description of the distribution of fibre-types in the tongue muscles in some iguanian lizards has hindered the understanding of the functional morphology of the lizard tongue. Three methodological approaches were used to fill this gap. First, morphological analyses were performed (i) on the tongue surface through scanning electron microscopy, and (ii) on the lingual muscle by histological coloration and histochemistry to identify fibre-typing. Secondly, kinematics of prey capture was quantified by using high-speed video recordings to determine the movement capabilities of the tongue. Finally, electromyography (EMG) was used to identify the motor pattern tongue muscles during prey capture. Morphological and functional data were combined to discuss the functional morphology of the tongue in agamid lizards, in relation to their diet. During tongue protraction, M. genioglossus contracts 420 ± 96 ms before tongue-prey contact. Subsequently, Mm. verticalis and hyoglossus contract throughout tongue protraction and retraction. Significant differences are found between the timing of activity of the protractor muscles between omnivorous agamids (Pogona sp., this study) and insectivorous species (Agama sp.), despite similar tongue and jaw kinematics. The data confirm that specialisation toward a diet which includes more vegetal materials is associated with significant changes in tongue morphology and function. Histoenzymology demonstrates that protractor and retractor muscles differ in fibre composition. The proportion of fast glycolytic fibres is significantly higher in the M. hyoglossus (retractor muscle) than in the M. genioglossus (protractor muscle), and this difference is proposed to be associated with differences in the velocity of tongue protrusion and retraction (5 ± 5 and 40 ± 13 cm s(-1) , respectively), similar to Chamaeleonidae. This study provides a way to compare fibre-types and composition in all iguanian and scleroglossan lizards that use tongue prehension to catch prey.

Behavioral responses to plant toxins by two omnivorous lizard species

An ability to detect plant toxins and thereby avoid eating chemically defended plants would be very beneficial for omnivorous and herbivorous lizards. We studied the ability of the omnivorous Podarcis lilfordi to detect compounds belonging to three classes of common plant toxins, as well as responses indicating aversion. Solutions of the alkaloid quinine, saponin, and the phenolic coumarin, as well as distilled water (odorless control), were presented to lizards on cotton swabs. The lizards detected all three toxins as indicated by significantly decreased tongue-flick rates and tongue-flick attack scores in comparison with distilled water. Several other variables revealed aversion to saponin, including a low number of individuals that bit swabs, avoidance of swabs after tongue-flicking, performance after tongue-flicking the swab of repeated short-excursion tongue-flicks that were directed away from the swab and did not contact any substrate, failure to respond at all in the next trial, and wiping the snout on the floor of the terrarium. Reasons for apparent differences in tongue-flicking behavior between P. lilfordi and two other omnivorous lizard species are discussed. We also showed experimentally that saponin depresses the tongue-flick rate in the omnivorous Bonaire whiptail lizard, Cnemidophorus murinus. Tongue-flicking enables at least one lizard species to detect specific chemicals representing three major classes of plant toxins. It is hypothesized that this ability is a widespread adaptation to reduce ingestion of plant toxins.

Lingual and biting responses to selected lipids by the lizard Podarcis lilfordi

Many lizards can identify food using chemical cues, but very little is known about the chemical constituents used for this purpose. We experimentally investigated responses to several lipid stimuli by the omnivorous lacertid lizard Podarcis lilfordi, which had been shown previously to be capable of identifying prey using only chemical cues and to respond to pork fat by tongue-flicking and biting. In 60-s trials in which stimuli were presented on cotton swabs, the lizards responded very strongly to pure pork fat and to oleic acid, but not to cholesterol or glycerol. Latency to bite swabs, the number of individuals that bit swabs, and the tongue-flick attack score, TFAS(R), which combines effects of tongue-flicks and bites, showed stronger responses to fat than to cholesterol, glycerol, and distilled water but did not differ significantly from responses to oleic acid. Several lines of evidence show that oleic acid elicited strong chemosensory and feeding responses. For individuals that did not bite, the number of tongue-flicks was significantly greater for oleic acid than for distilled water or glycerol, and nearly so for cholesterol. Latency to bite was significantly shorter for oleic acid than for distilled water, and TFAS(R) was significantly greater for oleic acid than for distilled water and glycerol. In combination with pilot data indicating no strong response to the waxy, saturated palmitic acid, these findings suggest that oleic acid in particular and probably other unsaturated fatty acids found in animal fat contribute strongly to the food-related responses to lipids.

Chemosensory responses to sugar and fat by the omnivorous lizard Gallotia caesaris: with behavioral evidence suggesting a role for gustation

Many lizards can identify food using only chemical cues, as indicated by tongue-flicking for chemical sampling and biting, but the effectiveness of the chemical components of food are unknown, as is the relationship between response strength and concentration. We investigated responses by the omnivorous lizard Gallotia caesaris to representatives of two major categories of organic food chemicals, lipids and carbohydrates. The stimuli, pork fat and sucrose solutions of varying concentration, were presented to lizards on cotton swabs and their lingual and biting behaviors were observed during 60-s tests. In the first experiment, fat elicited more tongue-flicks and bites than saturated sucrose or water (odorless control), biting being limited to the fat condition. Lizards licked at high rates, but exclusively in response to sucrose. A lick was a lingual protrusion in which the dorsal surface of the tongue contacted the swab, in contrast to the anteroventral contact made during tongue-flicks. In a second experiment, the number of licks, but not the number of tongue-flicks, increased with the concentration of sucrose. The results indicate that lipids contribute to prey chemical discrimination and are adequate to release some attacks, but are not as effective as releasers of attack as mixtures of prey chemicals obtained from prey surfaces. The findings with respect to licking are novel, and suggest that licking may be a response to gustatory stimulation by sugar, in contrast to previously observed prey chemical discriminations shown to require vomerolfaction.