Tongue-flick Rates Research Articles

Chemosensory responses to chemical and visual stimuli in five species of colubrid snakes

Snakes utilize chemical and visual stimuli during predation, however the emphasis on these cues and which cues are used to initiate predation varies among species. For example, rattlesnakes using the ambush strategy rely on chemical cues to locate an ambush station, then visual and thermal cues to initiate envenomating strikes, then chemical cues again to track prey. By contrast, many natricine snakes use chemical cues to initiate predation, increasing the rate of tongue flicking regardless of whether visual cues are present. The present study examined the individual and interactive effects of chemical and visual stimuli of prey on the predatory behavior of five snake taxa representing three feeding guilds. Bull snakes ( Pituophis catenifer ), Eastern Corn snakes ( Pantherophis guttatus ), and Midland Rat snakes ( Scotophis spiloides ) have a diet primarily consisting of mammals; Western Fox snakes ( Mintonius vulpina ) prey primarily on bird eggs; and Common Kingsnakes ( Lampropeltis getula ) prey equally on mammals and reptiles. Three patterns of response to chemical and visual stimuli of the test prey ( Mus musculus ) were observed. Mammal specialists responded to chemical cues. Fox snakes responded to visual cues, but not to chemical cues. Kingsnakes exhibited increased rates of tongue flicking in response to both chemical and visual stimuli. This study suggests correlations between the evolution of prey preference, foraging ecology and the utilization of chemical or visual stimuli by snakes.

Interpopulational variation in chemosensory responses to selected steroids from femoral secretions of male lizards, Podarcis hispanica, mirrors population differences in chemical signals

Chemical signals are important for mate and species recognition. If variation in chemical signals occurs between populations of the same species, these differences could later preclude mating between populations and lead to speciation. In the Iberian wall lizard, Podarcis hispanica, the lipophilic fraction of femoral secretions of males is mainly a mix of steroids and fatty acids. Among steroids, the most abundant compounds are cholesterol and cholesta-5,7-dien-3-ol, which are implicated in intraspecific communication and sexual selection. Interpopulational differences in chemical signals of males, and in response to these chemicals, could contribute to reproductive isolation between populations, which would explain the known genetic differences between these populations. Chemical analyses indicated that five distinct populations of this lizard from Madrid (Central Spain) differed in the proportions of two steroids (cholesterol and cholesta-5,7-dien-3-ol) in femoral secretions. Moreover, lizards discriminated and had high chemosensory responses (i.e., high tongue-flick rates) to these steroids, but showed interpopulational differences. Lizards from populations with cold temperatures and high relative humidity (i.e., northern Madrid) elicited higher responses to these steroids, whereas the converse occurred for lizards from populations occupying dry and hot habitats (i.e., southern Madrid). Interestingly, the magnitude of the chemosensory responses to cholesta-5,7-dien-3-ol in each population mirrored the abundance of this compound in secretions of males of that population. These results suggest that the importance of cholesta-5,7-dien-3-ol in males’ secretions might be lower for lizards from the southern populations. These differences in the relative importance of chemical signals could explain reproductive isolation and cryptic speciation between populations of this lizard.

Chemosensory Exploration of Male Scent by Female Rock Lizards Result from Multiple Chemical Signals of Males

Multiple sexual signals may convey information on males' quality. However, most research focused on visual signals, ignoring chemical signals. In vertebrates, chemical signals are probably often a multicomponent mixture of several active compounds, but they are not well known. We examined the potential additive and interactive effects of 2 compounds (oleic acid and ergosterol) naturally found in femoral gland secretions of male rock lizards on chemosensory exploration behavior of females. Tongue-flick (TF) rates of females to male secretions may result from the combination of responses to multiple compounds. There may be an additive or synergetic effect because male secretions with the highest proportions of both compounds received the highest TF rates, suggesting that their scents were more intriguing. However, there might be an interactive effect too; female TF rates were higher to males' scent with high proportions of ergosterol alone, even if proportions of oleic acid were low, than to high proportions of oleic acid but with low proportions of ergosterol. Further bioassays testing TF behavior of females to standard compounds, presented alone or combined in different concentrations confirmed these findings. Variations in female TF behavior might be explained because different compounds signal different male traits of different importance for females. Our study suggested that femoral secretion of male rock lizards may act as a multicomponent chemical signal.

Post-bite Elevation in Tongue-flick Rate by Neonatal Garter Snakes (Thamnophis radix)

A post-biting elevation in tongue-flicking rate was demonstrated experimentally in neonatal, ingestively naive garter snakes (Thamnophis radix). That the snakes also exhibited apparent searching movements suggests that strike-induced chemosensory searching occurs in nonvenomous snakes lacking previous experience with food or prey chemicals. Two litters of neonates differed in numbers of tongue-flicks emitted, but had similar relative magnitudes of response across experimental conditions. The existence of post-bite elevation in tongue-flick rate (and presumably strike-induced chemosensory searching) argues for a genetic basis for these chemosensory behaviors in a nonvenomous species of snake, extending the recent finding that strike-induced chemosensory searching is fully developed in ingestively naive neonatal rattlesnakes. Possible patterns of evolution of post-bite elevation in tongue-flick rate, and the strike-release-trail strategy of highly venomous snakes are discussed.

Response of Western Diamondback Rattlesnakes (Crotalus Atrox) to Chemical Cues of Mice (Mus Musculus) of Different Genders and Reproductive Status

Eight western diamondback rattlesnakes (Crotalus atrox) were exposed to 6 stimuli: (1) clean, unused bedding; (2) an adult male mouse; (3) an adult lactating female mouse; (4) an adult lactating female mouse with a litter; (5) 2 adult nonlactating female mice, to control for the extra surface area in Condition 4; and (6) a litter of newborn mice. All stimuli were presented in opaque plastic boxes with 18 perforations in each side; hence, chemical but not visual cues were available to the snakes. Our goal was to examine if chemical cues from rodents of different genders, lactating female rodents, and rodent litters influence predatory behavior in rattlesnakes. Results indicated that C. atrox responded to odors arising from adult mice (Mus musculus) with higher rates of tongue flicking than were seen in the control condition, but the 4 conditions involving adult mice did not differ significantly from each other. Neonatal mice did not produce consistently higher rates of tongue flicking than did the control condition.

Behavioral Responses to Potential Prey Through Chemoreception by the Sharp-Tailed Snake (Contia tenuis)

The Sharp-tailed Snake (Contia tenuis) is a small (usually <30 cm total length), cryptic species found along the west coast of the United States and north into southwestern British Columbia. Because of its secretive nature, little is known about its behavioral ecology. We tested behavioral responses of 13 adult C. tenuis collected from a site in eastern Washington to potential invertebrate prey odors. We presented snakes with 2 control odors (water, cologne) and 2 possible invertebrate prey odors (earthworm, slug). Overall, there was a significant difference in both the time-to-first-tongue flick (latency) and mean tongue flick rate (number of tongue flicks/60 s trial) for the odors tested. The mean latency period was 6.0 ± 1.87 s for earthworm and 4.1 ± 1.57 s for slug. The mean tongue flick rates for earthworm and slug were 13.8 ± 4.09 flicks/s and 39.7 ± 15.79 flicks/s, respectively. These results support prior claims of a preference for slugs by C. tenuis. This preference for slugs may also explain the presence of C. tenuis in areas of anthropogenic disturbances with an abundance of slugs.

Chemical Polymorphism and Chemosensory Recognition between Iberolacerta monticola Lizard Color Morphs

In the lizard Iberolacerta monticola, there are 2 discrete color morphs ("blue" vs. "green") that seem to have alternative reproductive strategies. Because scent marking of territories and chemoreception are important in social organization of these lizards, we explored whether there is also chemical polymorphism and chemosensory recognition between color morphs. Analyses by gas chromatography-mass spectrometry showed that adult males of different morphs had similar chemical compounds in femoral gland secretions, but the proportions of some shared chemicals were different; blue morph males had higher proportions of steroids and lower proportions of fatty acids than green males. Differential tongue-flick rates to scents from femoral secretions of males indicated that males were able to detect the scent of any male from a baseline odorless control (water) but did not discriminate between the scent of males of different morphs. In contrast, females detected and discriminated between scents of the 2 male morphs by chemical cues alone. We suggest that differences in males' chemical signals between morphs and the ability of females of discriminating chemically between male morphs might be required to maintain spatial behavior, social organization, and mate choice decisions and, thus, be important for the maintenance of alternative reproductive strategies and polymorphism in lizards.

Response of hatchling Komodo Dragons (Varanus komodoensis) at Denver Zoo to visual and chemical cues arising from prey

Five hatchling Komodo Dragons (Varanus komodoensis) at Denver Zoo were observed in two experiments that studied the effects of visual and chemical cues arising from prey. Rate of tongue flicking was recorded in Experiment 1, and amount of time the lizards spent interacting with stimuli was recorded in Experiment 2. Our hypothesis was that young V. komodoensis would be more dependent upon vision than chemoreception, especially when dealing with live, moving, prey. Although visual cues, including prey motion, had a significant effect, chemical cues had a far stronger effect. Implications of this falsification of our initial hypothesis are discussed.

Discrimination of conspecific faecal chemicals and spatial decisions in juvenile Iberian rock lizards (Lacerta monticola)

Faeces play a role in intraspecific chemical communication in many vertebrates, including lizards. Here, we hypothesised that juvenile Iberian rock lizards (Lacerta (=Iberolacerta) monticola) use substrate-borne conspecific faecal chemicals to assess the social environment and base their spatial decisions. To test this hypothesis, we prepared chemical stimuli by dissolving faeces in an organic solvent (dichloromethane, DCM) and conducted chemosensory trials where we quantified latency to the first tongue flick (TF) and TF rates when young captive-borne juveniles were placed in an arena compartment whose substrate was labelled by (1) a control (DCM), (2) their own faecal chemicals, (3) faecal chemicals of another juvenile, (4) faecal chemicals of an adult female or (5) faecal chemicals of an adult male. Following TF observations, we removed a partition that separated the labelled compartment from a chemically unlabelled one, and recorded for 5 min when juveniles first crossed to the unlabelled compartment and total time spent in the labelled compartment. Each juvenile was tested with all stimuli in a randomised order. In addition, juveniles belonged to different families and were unrelated to and unfamiliar with faeces donors. Taken together, TF and spatial responses toward stimuli indicate that juveniles discriminated between faecal chemicals of conspecific juveniles, adult females and males, and that they avoided remaining in substrates labelled by adult male faecal chemicals. We suggest that juveniles assess the social environment based on conspecific faecal chemicals thus avoiding aggression and cannibalistic risks undertaken from encounters with adult males.

Chemosensory cues associated with snake predators affect locomotor activity and tongue flick rate in the whiptail lizard, Aspidoscelis dixoni Scudday 1973 (Squamata Teiidae)

The unisexual teiid lizard Aspidoscelis dixoni Scudday 1973 is shown experimentally to be capable of detecting and responding to chemosensory stimuli from three species of naturally-occurring sympatric predatory snakes (Masticophis flagellum, M. taeniatus, Arizona elegans) and to discriminate these stimuli from those associated with a non-predatory snake (Leptotyphlops humilis segregus), as well as controls (water and cologne). Each lizard was exposed separately to the following: chemical stimuli from each of the predatory snakes, non-predatory, snake, water control, or pungency control (cologne). Lizards were placed in a test chamber whose floor was divided into 4 square grids of equal size, and for each trial, the number of times that a grid was crossed (locomotor activity), the number of tongue flicks observed (tongue flick rate, TFR), and time from beginning of trial to first tongue flick (latency), were recorded. Lizards crossed grid lines at significantly higher rates when exposed to chemosensory cues associated with all predatory snake species as compared to activity in the presence of nonpredator snake cues, water or cologne. There were no differences in number of grid crossings elicited by any of the three predatory snakes. TFR differed significantly among all treatments, but no significant difference was found among lizards exposed to water, cologne, or cues associated with L. h. segregus. TFRs were significantly higher, and latencies significantly lower, when lizards were exposed to chemosensory cues associated with predatory snake. Results suggest that because all lizards had no prior experience with any of the snakes, their ability to detect and respond to chemosensory cues of predators must be innate.

Strong response to anuran chemical cues by an extreme dietary specialist, the eastern hog-nosed snake (Heterodon platirhinos)

Squamate reptiles use the lingual–vomeronasal system to identify food using only chemical cues. In lizards, most of which are dietary generalists that consume a wide variety of arthropods and other small animals, correlated evolution has occurred between addition of plants to the diet and responsiveness to plant chemical cues. In snakes, many of which are dietary specialists, several studies have detected differences in response strength among populations that correspond to the importance of prey types in different geographic locations. In one species of Thamnophis Fitzinger, 1843, such variation in responsiveness has been demonstrated to have a genetic basis. We studied tongue-flicking and biting responses to chemical cues from a range of potential prey types by nine ingestively naive hatchlings of the eastern hog-nosed snake ( Heterodon platirhinos Latreille in Sonnini and Latreille, 1801), which is an extreme toad specialist that less frequently eats other anurans. The snakes responded most strongly to chemical cues from the southern toad ( Bufo terrestris (Bonnaterre, 1789)), as indicated by significantly greater tongue-flick rate. Only two individuals bit in response to chemicals cues, both to the toad cues. Elevated tongue-flick rates were also elicited by chemical cues from the green frog ( Rana clamitans Latreille in Sonnini de Manoncourt and Latreille, 1801), but the mean rate for frog cues was less than for toad cues. Responses to other potential prey types did not differ from those of the control stimuli. Our findings are consistent with those of several other investigators in showing close correspondence between the inclusion and importance of dietary items and the intensity of chemosensory investigation in snakes. Studies of diverse dietary specialists are needed to establish the generality of this relationship in snakes and to demonstrate that diet and chemosensory responses to food cues coevolve.

Effects of testosterone supplementation on chemical signals of male Iberian wall lizards: consequences for female mate choice

In some lizards, female mate choice is influenced by chemicals secreted by males, e.g., via the femoral glands. Secretions of the femoral glands are under direct androgenic control and vary seasonally with androgen production. However, whether increased testosterone (T) levels affect the concentration and chemical composition of secretions or their attractiveness to females is unknown. We manipulated T levels of male Iberian wall lizards, Podarcis hispanica, with silastic implants. Differential tongue-flick rates indicated that females detected and discriminated between femoral secretions of control and T-implanted males based on chemical cues alone. Females showed greater responses to secretions of T-males, which might suggest that T increased the concentration of chemical signals. Further analyses by gas chromatography–mass spectrometry indicated that T supplementation induced qualitative changes in lipid composition of femoral secretions. T-males had decreased relative proportions of cholesta-5,7-dien-3-ol in secretions, which might be related to the negative effects of T on the immune system and on lipid metabolism. However, experiments of choice of males’ scent showed that females neither preferred nor avoided the scent marks of T-males but preferred males that maintained higher proportions of cholesta-5,7-dien-3-ol in secretions, independently of the experimental manipulation. Thus, because this steroid is negatively affected by T, there might be trade-offs among increasing T levels to increase the production of chemical secretions, maintain metabolism, and attract females.

Relationships between chemosensory behaviour and foraging mode within lacertid lizards

We assess associations between chemosensory capacity and foraging mode within lacertid lizards. Species of Lacertidae differ considerably in indices of foraging mode, and therefore seem well suited to test the adaptive nature of a coupling between foraging and sensory ecology. We observed tongue-flick (TF) rates of members from eight species in the field and in experimental conditions with no prey stimuli, with chemical stimuli, with visual stimuli and with both chemical and visual stimuli. All species increased TF rates in response to both purely visual and purely chemical prey cues, and the increase was most pronounced when both types of stimuli were offered simultaneously. Absolute TF rates in experimental conditions differed considerably among species, and species means obtained in experimental situations correlated closely with those observed in the field. Species that spend a relatively large amount of their time budget actively searching for food tend to use their vomeronasal system more frequently. Although all species in our study retained the capacity of recognizing prey chemicals, our data corroborate the idea of a functional link between an active foraging style and the usage of chemical information.

Elevated tongue-flicking rate to cricket surface chemicals by the arthropodivorous rough green snake Opheodrys aestivus

AbstractThe rough green snake Opheodrys aestivus is a dietary specialist on insects and other arthropods. In a laboratory experiment, I tested chemosensory responses to chemical cues from several taxa palatable to other snakes and two control stimuli presented on cotton swabs. The snakes tongue-flicked at significantly higher rates in response to cues from crickets than from any of the other stimuli, and none of the other potential prey types elicited more tongue-flicks than cologne, the control for response to an odorous substance irrelevant to feeding. These findings are consistent with existing data for a few other species of snakes that are prey specialists, suggesting that correlated evolution may occur between diet specialization and chemosensory responsiveness to cues from the specialized prey. Tests of additional species of prey specialists are needed to establish whether this hypothesis is correct.

Chemosensory Responses by Female Iberian Wall Lizards, Podarcis Hispanica to Selected Lipids Found in Femoral Gland Secretions of Males

In many lizards, secretions of males are used in social behaviors, but it is almost unknown which specific chemicals are relevant in mediating these behaviors. Moreover, only a few studies have examined whether lizards can actually discriminate between different chemical compounds. We tested for differential chemosensory responses of female Iberian Wall Lizards, Podarcis hispanica, to two lipids (cholesterol and cholesta-5,7-dien-3-ol) found in femoral secretion of males, and which previous behavioral studies suggested may be relevant in female mate choice. The higher tongue-flick (TF) rates directed at cotton swabs containing these two lipids indicated that females detected and discriminated these chemicals from a control. There were no significant differences between responses to cholesterol and to the whole natural femoral secretion from males, probably because cholesterol is the major predominant component in secretions of males. The higher TF rates to cholesta-5,7-dien-3-ol than to cholesterol indicated that females discriminated these two steroids. Moreover, females seemed able to assess changes in concentration of cholesta-5,7-dien-3-ol and responded more strongly to higher concentrations. These results, together with the previous finding of female preference for areas scent marked by males with higher proportions of this steroid in their secretions, might support that cholesta-5,7-dien-3-ol was a “key” chemical that might be used by females to choose potential mates.

A Noninvasive Technique for Blocking Vomeronasal Chemoreception in Rattlesnakes

To examine the effects of vomeronasal deprivation on strike-induced chemosensory searching (SICS) in rattlesnakes we used a newly developed technique to anesthetize the vomeronasal organs. We compared rate of tongue flicking after striking prey in avomic rattlesnakes to vomic controls. Avomic rattlesnakes exhibited significantly fewer tongue flicks after striking prey than did vomic controls. However, avomic subjects still exhibited a significantly higher rate of tongue flicking (RTF) than did subjects that were presented with prey but not allowed to strike indicating that the increase in RTF seen in avomic snakes reflects a component of predatory behavior that is released by the strike and not dependent upon further vomeronasal activation. A second experiment was conducted to examine the role of the vomeronasal system (VNS) in location of envenomed prey. Disruption of the VNS chemoreception significantly reduced the number of poststrike tongue flicks, significantly increased the latency to locate the prey item and significantly increased the latency to commence swallowing the prey item, indicating that normal expression of these behaviors is dependent upon a functioning vomeronasal system.

Interpopulational differences in chemical composition and chemosensory recognition of femoral gland secretions of male lizards Podarcis hispanica: implications for sexual isolation in a species complex

Molecular studies suggest that the Iberian wall lizard, Podarcis hispanica, forms a species complex with several monophyletic types. In Central Spain two of these types are spatially not isolated and may interact. Sex pheromones are important for species recognition and, thus, differences between lizards’ types in chemicals used in intraspecific communication could lead to reproductive isolation. Analyses by GC-MS showed that the femoral gland secretions of adult males of different types were different. Males of one type had twelve exclusive compounds, and proportions of some shared compounds differed. This presumably would reflect selection for the persistency and efficiency of chemical signals in different environments; less volatile compounds and with a higher chemical stability being favoured in the type of lizards inhabiting more humid climatic conditions. Differential tongue-flick rates to scents from femoral secretions indicated that males were able to detect and discriminate between males of different types based on chemical cues alone. In contrast, females detected but did not seem able to discriminate between scents of the two types of males. Thus, multiple factors might be simultaneously acting either against or in favor of speciation, leading, on the one hand, to genetic differences between types, but, on the other hand, probably precluding an effective reproductive isolation in areas where both types of lizards may interact.

Does foraging mode influence sensory modalities for prey detection in male and female filesnakes, Acrochordus arafurae?

Theoretical models predict that modalities of prey detection depend upon foraging modes: ambush foragers will rely on visual cues to launch a strike whereas active searchers will use chemical cues to locate prey. Testing this prediction is hampered by phylogenetic conservatism; ideally, we need to compare closely related animals that differ in foraging mode. Aquatic filesnakes from tropical Australia offer a unique opportunity of this kind, because female filesnakes ambush large fish in deep water whereas male filesnakes search actively for smaller fish in shallow water. We exposed freshly captured filesnakes to artificial prey items providing visual and/or chemical cues and measured tongue-flick rates and feeding responses of the snakes. Males responded most intensely to fish scent, regardless of movement, whereas females responded strongly to movement. Thus, our data provide the first intraspecific (sex-based) evidence for a functional relationship between foraging mode and the types of cues used for prey detection.

Thermal dependence of chemical assessment of predation risk affects the ability of wall lizards, Podarcis muralis, to avoid unsafe refuges

Prey often respond to predators by increasing refuge use, but some refuges may expose prey to other types of predators. Because predators are not always visible inside refuges, their chemical stimuli may provide early warning of their presence. However, in ectotherms, chemosensory detection of predators may be thermally dependent. Lizards often hide in cold refuges, where their body temperature (Tb) may decrease, and this might affect their ability to detect chemicals of saurophagous snakes that ambush inside refuges. We examined the effects of Tb of wall lizards, Podarcis muralis, on their ability to detect chemicals of smooth snakes, Coronella austriaca. Differential tongue flick (TF) rates and behavioral patterns of lizards in response to scent of smooth snakes showed that lizards with optimal Tb discriminated and avoided the snake's scent, whereas lizards with suboptimal Tb did not. We also examined the importance of this effect on movement patterns and refuge use by lizards in terraria with safe (odorless) or unsafe refuges (snake-scented). While results of this experiment were variable, there was evidence of snake avoidance in that lizards at optimal Tb spent less time in snake-scented refuges relative to odorless refuges. Therefore, this study provides evidence that chemosensory discrimination of snake chemicals is thermally dependent, and, thus, suboptimal Tb impedes a lizard's ability to avoid snake-scented refuges.

Chemosensory Recognition of Its Lizard Prey by the Ambush Smooth Snake, Coronella austriaca

The Smooth Snake, Coronella austriaca, is an ambush predator that waits for its main prey, the Wall Lizard, Podarcis muralis, inside dark rock crevices where lizards retreat. Pheromonal secretions of lizards could be used by snakes to select foraging sites but also during predatory episodes when identifying lizards under conditions of low visibility is beneficial. We used cotton applicators labeled with lizard scent to determine whether Smooth Snakes can discriminate the chemical cues of Wall Lizards. We also asked whether snakes could discriminate between male and female lizards, or detect male scents before female ones, which could indicate differential susceptibility of the sexes to predation. The greater tongue-flick rate in response to Wall Lizard scent than to deionized water or cologne indicated that C. austriaca is able to discriminate the chemical cues of Wall Lizards, but it did not discriminate between the sexes of lizard prey.