Chemosensory response in stunted prairie rattlesnakes Crotalus viridis viridis

Rattlesnakes use chemical stimuli in ambush site selection and for relocation of envenomated prey through strike-induced chemosensory searching. Shifts in responsiveness to prey chemicals have been documented in many snakes, and often correlate with prey commonly taken as snakes increase in age and size as well as geographical locations of the species. For instance, neonate rattlesnakes that prey primarily on ectotherms responded most strongly to chemical cues of commonly taken lizard prey, whereas adult rattlesnakes that prey primarily on small mammals responded significantly to chemical cues of commonly taken rodents. In the current study, 11 Prairie Rattlesnakes Crotalus viridis viridis which were classified as large neonates based on measures of snout-vent length (SVL) and body mass, yet chronologically were at or near adulthood, were tested for their responsiveness to chemical extracts of natural and non-natural prey items. Although the snakes had eaten only neonate lab mice (Mus musculus), they responded significantly more to chemical cues of natural prey items and particularly to chemical cues of prey normally taken by subadults (Peromyscus mice and Sceloporus lizard). These results suggest that ontogenetic shifts in responsiveness to natural prey chemical cues are innately programmed and are not based on body size or feeding experience in C. v. viridis. This does not imply, however, that growth and experience are without effects, especially with novel prey or rare prey that have experienced recent population expansion.

Prey chemical discrimination by a diploglossine lizard, the giant Hispaniolan galliwasp (Celestus warreni)

Prey chemical discrimination, the ability to respond differentially to prey chemicals and control stimuli, enables many squamate reptiles to locate and identify prey using chemical cues sampled by tongue-flicking and analyzed by vomerolfaction. Among lizards this ability is limited to species that are active foragers having insectivorous/carnivorous diets and to omnivores and herbivores, even those derived from ancestral ambush foragers. We experimentally studied responses by hatchlings of giant Hispaniolan galliwasps, Celestus warreni, which appear to have a strict animal diet and are putatively active foragers, to prey chemicals and control substances. More individuals tongue-flicked in the cricket condition than the water condition. Response strength indicated by the tongue-flick attack score, a composite index of response strength based on number of tongue-flicks, biting (one lizard) and latency to bite, was greater in response to cricket stimuli than plant (lettuce) stimuli, cologne or distilled water. Thus, the galliwasps exhibited prey chemical discrimination. Celestus warreni, the first representative of Diploglossinae to be tested, exhibits chemosensory behavior similar to that of other anguids. Although no quantitative data on foraging mode are available, another diploglossine, Diploglossus vittatus, is an active forager. The limitation of prey chemical discrimination to active foragers among lizards with animal diets lend further support to the likelihood that C. warreni is an active forager. The galliwasps did not exhibit plant chemical discrimination.

Calcium Transients in the Garter Snake Vomeronasal Organ

The signaling cascade involved in chemosensory transduction in the VN organ is incompletely understood. In snakes, the response to nonvolatile prey chemicals is mediated by the vomeronasal (VN) system. Using optical techniques and fluorescent Ca2+ indicators, we found that prey-derived chemoattractants produce initially a transient cytosolic accumulation of [Ca2+]i in the dendritic regions of VN neurons via two pathways: Ca2+release from IP3-sensitive intracellular stores and, to a lesser extent, Ca2+ influx through the plasma membrane. Both components seem to be dependent on IP3 production. Chemoattractants evoke a short-latency Ca2+ elevation even in the absence of extracellular Ca2+, suggesting that in snake VN neurons, Ca2+ release from intracellular stores is independent of a preceding Ca2+ influx, and both components are activated in parallel during early stages of chemosensory transduction. Once the response develops in apical dendritic segments, other mechanisms can also contribute to the amplification and modulation of these chemoattractant-mediated cytosolic Ca2+ transients. In regions close to the cell bodies of the VN neurons, the activation of voltage-sensitive Ca2+ channels and a Ca2+-induced Ca2+ release from intracellular ryanodine-sensitive stores secondarily boost initial cytosolic Ca2+ elevations increasing their magnitude and durations. Return of intracellular Ca2+ to prestimulation levels appears to involve a Ca2+ extrusion mediated by a Na+/Ca2+ exchanger mechanism that probably plays an important role in limiting the magnitude and duration of the stimulation-induced Ca2+ transients.

Responses to prey and plant chemicals by three iguanian lizards: relationship to plants in the diet

We examined responses of three iguanian lizards, the phrynosomatid Sceloporus poinsettii and the tropidurids Tropidurus hispidus and Phymaturus punae, to prey chemicals and plant chemicals. When chemical stimuli were presented on cotton swabs or on ceramic tiles, neither S. poinsettii nor T. hispidus discriminated among prey, plant, and control stimuli. In contrast, an individual of P. punae discriminated both prey and plant chemicals from control stimuli in swab tests, typically biting swabs bearing prey or plant cues. These findings are consistent with the hypothesis that plant chemical discrimination evolves in herbivorous iguanians such as P. punae. Sceloporus poinsettii, which appears to be entirely insectivorous at some times, but eats substantial quantities of flowers at others, did not discriminate among the stimuli. Because all previously tested herbivores and omnivores responded strongly to prey and plant chemicals, the absence of such discriminations by S. poinsettii raises questions about the degree and regularity of herbivory that may be required for plant chemical discrimination to evolve. The results extend the absence of prey chemical discrimination in ambush foragers to T. hispidus.

Do lingual behaviors and locomotion by two gekkotan lizards after experimental loss of bitten prey indicate chemosensory search?

Poststrike elevation in tongue-flicking rate (PETF) and strike-induced chemosensory searching (SICS) were assessed experimentally in two species of gekkonoid lizards belonging to families differing in foraging mode. PETF is an increase in rate of lingual protrusions after a prey item has been bitten and escapes or is removed from the mouth of a squamate reptile, whereas SICS is PETF combined with locomotory searching behavior. Eublepharis mucularius, the leopard gecko, is an actively, albeit slowly, foraging eublepharid. This species exhibited PETF for a duration of about five minutes based on total lingual protrusions. Labial-licks were initially much more frequent than tongue-flicks. A substantial increase in movement occurred during minutes 5-8, hinting that SICS might be present, but was not quite significant. SICS is likely present, as in other actively foraging lizards, but was not conclusively demonstrated. Handling the lizards induced increased locomotion in both the experimental condition and a control condition, presumably accounting for the apparent delay in onset of increased movement. The tokay gecko, Gekko gecko, a gekkonid ambush forager, performed no tongue-flicks, but exhibited PETF based on labial-licks during the first minute. SICS was absent. These findings support the hypothesis that SICS is absent in ambush foraging lizards, which do not use the lingual-vomeronasal system to search for prey. They are suggestive, but equivocal regarding the hypothesis that SICS is present in actively foraging lizards that exhibit lingually mediated prey chemical discrimination. The finding of PETF in G. gecko suggests that this species and several iguanians previously found to increase rates of labial-licking after biting prey may be able to detect prey chemicals.