Kenneth V. Kardong

Rattlesnake Hunting Behavior: Correlations between Plasticity of Predatory Performance and Neuroanatomy

Rattlesnakes may shift between visual (eyes) and infrared (facial pits) stimuli without significant loss of predatory performance during an envenomating strike. The relative equivalency of these proximate stimuli is correlated with the organization of the associated neural pathways in the central nervous system. Visual and infrared information, although gathered by different sensory organs, converges within the optic tectum in an orderly spatiotopical representation where bimodal neurons respond to both stimuli. In turn, the tectum sends efferent pathways directly to premotor areas (brainstem) and indirectly to motor areas (spinal cord) where axial muscles involved in the strike might be activated. On the other hand, rattlesnakes do not maintain a high level of equivalent predatory performance when switching between chemosensory stimuli i.e., olfactory, and vomeronasal information. Deprived of vomeronasal input, strikes drop by about half, and poststrike trailing is lost entirely. Surprisingly, compensation by switching to information delivered via an intact olfactory input does not occur, despite the convergence of chemosensory information within the central nervous system. Finally, the launch of a targeted, envenomating strike involves both these modalities: radiation reception (visual, infrared) and chemoreception (olfactory, vomeronasal). However, in the absence of chemosensory information, the radiation modalities do not completely compensate, nor does the animal maintain a high level of predatory performance. Similarly, in the absence of radiation information, the chemosensory modalities do not completely compensate, nor does the animal maintain a high level of predatory performance. The absence of compensation in this multimodal system is also correlated with an absence of convergence of radiation and chemical information, at least at the level of first and second-order neurons, in the central nervous system.

Properties of duvernoy's secretions from opisthoglyphous and aglyphous colubrid snakes

Relatively little attention has been given to the biological properties of Duvernoys secretions produced by opisthoglyphous and some aglyphous colubrid snakes. A review is presented of literature pertaining to these secretions. Most detailed analyses of Duvernoys secretions and their biological properties have been performed since the late 1970s. The dispholidines, Dispholidus typus and Thelotornis sp., and the natricines, Rhabdophis tigrinus and R. subminiata, have received the most attention due to the high toxicity of their secretions and their medical importance. These species produce secretions with variably strong prothrombin-activating activity, defibrinating activity, and hemorrhagic potential. Boigines, and natricines other than Rhabdophis, produce secretions of low to moderate toxicity and are variably hemorrhagic and proteolytic. Xenodontines and homalopsines similarly show hemorrhagic potential with low to moderate toxicity. Neurotoxic activity has been reported only from secretions of the boigines, Boiga blandingi and B. irregularis and the xenodontine, Heterodon platyrhinos. These species produce secretions containing postsynaptically acting components. Analyses of some of these secretions have shown that enzymes common to many ophidian venoms such as phospholipases A and L-amino acid oxidase are uncommon in the colubrid secretions studied. This may be due to few studies assaying for multiple enzyme activities and/or the unavailability of many secretion samples for study. Methods of secretion extraction, storage, and assay are discussed. Projected future research and the adaptive implications of Duvernoys secretions are considered.

Biomechanics of the Hyolingual System in Squamata

The hyolingual system of Squamata is a highly versatile system used in different feeding, drinking, chemoreception, and social behaviors. In each of these activities, either the entire hyolingual system or one of its elements is used. For instance, in the majority of lizards, the tongue acts as the main element for liquid uptake, intraoral food and liquid transport, and in chemoreception, whereas the hyoid apparatus plays a major role during social interactions by acting on the ventral floor of the throat. In varanids, the hyoid apparatus is involved in both deglutition of foods and liquids, and during social displays.

The strike behavior of a congenitally blind rattlesnake

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COLUBRID SNAKES AND DUVERNOY'S ''VENOM'' GLANDS

One of the largest groups of snakes is the family Colubridae. This is a paraphyletic assemblage that includes a few venomous species, but most pose no special health risk to humans. Thirty to forty percent of colubrids possess a Duvernoys gland, a specialized oral gland located in the temporal region. Although it is a homologue to the venom glands of viperid and elapid snakes, the Duvernoys gland is anatomically and functionally distinct. Generally it lacks a large internal reservoir of secretion, emptying is under low-pressure flow, and the secretion is not delivered via hollow fangs. In contrast, true venom glands hold a large store of ready venom, expel the venom under direct action of striated muscles, and inject it as a high-pressure pulse via hollow fangs. Both the Duvernoys gland and the venom gland are part of a snakes trophic system, involved primarily in predatory behavior. True venoms are composed of potent toxins whose main biological role is to bring about rapid prey death. Although the secretion from the Duvernoys gland may include toxins, surprisingly only a few colubrids deploy it similarly to kill prey quickly. In fact, the biological role(s) of Duvernoys secretion remain today largely unknown. Therefore, it is misleading, in a functional and evolutionary context, automatically to call Duvernoys secretion a venom (biological role) when only its pharmacology (property) is known. Although Duvernoys secretion has some components in common with true venoms, some may be fundamentally different in chemical composition, likely because it is involved in different biological roles than a true venom. This means it likely includes novel chemical components with a promise of interest to human medicine.

Venom delivery of snakes as high-pressure and low-pressure systems

Two fundamentally different venom delivery systems are found among advanced snakes. One is a high-pressure system present in viperids and elapids wherein a pulse of venom is delivered quickly by a sudden pressure surge. The other is a low-pressure system found in some colubrids wherein release of oral secretions is more protracted. These differences help account for the great variation in medical signs and symptoms following bites of humans by colubrid snakes. Further, the high-pressure venom system of elapids and viperids represents an evolutionary innovation in snakes accompanied by a change from a mechanical to a chemical predatory strategy.

Kinesis of the Jaw Apparatus during Swallowing in the Cottonmouth Snake, Agkistrodon piscivorus

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Sensory Deprivation Effects on the Predatory Behavior of the Rattlesnake, Crotalus viridis oreganus

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Comparative Study of Changes in Prey Capture Behavior of the Cottonmouth (Agkistrodon piscivorus) and Egyptian Cobra (Naja haje)

Prey capture behavior between cottonmouths (Agkistrodon piscivorus) and Egyptian cobras (Naja haje) differ fundamentally in response to proximate factors. Cottonmouths, when presented with several mice in close succession, tended to release the first mice but hold on to later mice. Cottonmouths, too, were more deliberate in establishing coils from which to strike than the cobra. In the Egyptian cobra, there was no appreciable change in hold/release behavior through a sequence of up to 4 mice. More important was the retaliation of the mouse when struck. Egyptian cobras usually held a struck mouse, regardless of its position in the sequence, unless bitten by the mouse. Mice which bit the cobra were usually released. Cobra struck mice died more quickly if held in the jaws and the range of death rates was less than for mice released.

Dentitional surface features in snakes (Reptilia: Serpentes)

The complexity of snake tooth morphology is more varied than has been recognized in functional, evolutionary, or taxonomic studies. We surveyed a broad sample of species across taxonomic groups to document and summarize the variation present. Our survey included a scoring of dentitional features of 1169 specimens representing 611 species of snakes on four dentiferous bones (dentary, pterygoid, palatine, maxilla). Besides presence or absence of teeth on these bones, we ranked the tooth type on each bone on the basis of a four category system: basic, furrowed, grooved, or hollow tooth. Basic teeth, without surface recesses or grooves, were the most common tooth type. Hollow teeth (fangs) were found most commonly on the maxilla and grooved teeth were often adjacent. Grooved teeth, when present, were found only on the maxilla although teeth with furrows, shallow creases in the surface enamel, were found in low numbers on the dentary, pterygoid, and palatine. Teeth exhibited further specializations, including multiple grooves, basal reinforcing ridges, development of a blade-like design, variation in the degree to which the secondary groove in hollow teeth might be in evidence, and variation in the position of the groove along the shaft of the tooth.