RUI: Functional Convergence in the Venom Delivery System of Snakes

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Snakes have one of the most specialized feeding mechanisms among vertebrate animals. One of the key aspects of snake feeding is the presence of a venom delivery system in roughly one-fifth of the living snakes. Molecular and anatomical studies suggest that a venom delivery system has evolved independently in at least four snake lineages. The Viperidae (including pit-vipers and true vipers), the Elapidae (cobras and sea snakes), the Atractaspidae (Stiletto snakes from Africa), and several taxa - such as the boomslang - within the Colubridae have all evolved venom delivery systems. In recent years a suite of techniques were developed to study the mechanics of venom injection in one representative viper, the western diamondback rattlesnake (Crotalus atrox). The purpose of this grant is to apply these same techniques to the mechanics of venom delivery in representative elapids. This will be the first experimental analysis of venom expulsion in an elapid, and a comparison with the earlier studies will highlight the degree of functional convergence among snake venom delivery systems. The venom delivery system will be examined through four integrated projects on the kinematics, neuro-muscular control, functional morphology, and behavioral ecology of venom injection. The kinematics of venom expulsion will be studied using high-speed digital videography and computer-based image analysis techniques to document the degree to which the monocled cobra (Naja kaouthia) modulates its strike depending on such factors as the size of the target and the behavioral context of the strike. The kinematics of venom "spitting" by the red spitting cobra (N. pallida) will also be documented. In snakes the venom is expulsed through skeletal muscle contraction; by regulating the contractile state of the muscle the snake could control the amount of venom injected. The extrinsic venom gland musculature will be studied in N. kaouthia using anatomical techniques to document the gross and microscopic structure of the muscle and its innervation. Subsequent experimental analyses will combine muscle stimulation with measurements of venom flow and pressure to document any spatial functional specialization within the muscle, and the motor units within the muscle will be mapped using glycogen depletion. In cobras venom forced from the venom gland by muscle contraction flows through the venom duct into a series of soft tissue chambers which are surrounded by the fang sheath, a drape of connective tissue and epithelium. The functional morphology of this region will be studied in both N. kaouthia and N. pallida using gross and microscopic anatomy, radiopaque tracers (used to document venom barriers), cannulation experiments, and recordings of venom pressure. The behavioral ecology of venom injection will be studied by chronically implanting transonic flow probes onto the venom duct of N. kaouthia. Venom flow (measured with flow probes) will be synchronized with high-speed digital videography while the cobra strikes a variety of defensive and predatory targets. This will document the kinematics of venom flow during the strike and reveal the extent to which cobras inject differential volumes of venom depending on target size or behavioral context.