According to the venom-metering hypothesis, snakes have the cognitive (decision-making) capacity to control, or meter, how much venom is ejected from the fangs. Critics of venom metering have argued, largely from absence of evidence, that differential venom gland contraction in snakes is trivial or nonexistent. To address this criticism, we videotaped the defensive bites of Naja nigricollis nigricollis during routine venom extractions. Mean duration of venom flow during a single pulse from a fang, when biting (0.35 sec) was significantly longer than that reported previously for spitting (0.066 sec). Moreover, mean mass of venom expended per pulse from a fang during biting (juveniles: 14.2 mg; adults: 188 mg) significantly exceeded that reported for spitting (1.85 mg). During a single bite, both juveniles and adults delivered venom via pulses that were single, multiple (each associated with a jaw contraction), unilateral (from one fang), or bilateral (from both fangs more or less simultaneously). Although juveniles and adults exhibited similar venom flow duration, adults delivered significantly more venom during biting at significantly greater rates of venom flow through the fang. Because venom gland contraction provides the only propulsive force for venom expulsion, our results confirm that N. n. nigricollis meters larger quantities of venom during biting than spitting via differential venom gland contraction. Because of the high degree of functional convergence between venom delivery systems of elapids (including Spitting Cobras) and viperids (the other large family of venomous snakes), the capacity for differential venom gland contraction may be widespread among snakes.
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