Samplings: April 2009

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Spitting cobra, Naja pallida, does its thing.

Photo by Guido Westhoff

Sway your head in front of a spitting cobra, and in no time, you will be sprayed with venom from its fangs. The poison, harmless to the skin, causes stinging pain and even blindness if it gets in your eyes—and it usually does. Cobras seem to have remarkable control over their venom. How do they manage it?

To find out, Bruce A. Young, now at the University of Massachusetts Lowell, swayed his head in front of eleven closely monitored cobras. He wore a visor equipped with an accelerometer to track his own head movements, while those of the snakes were filmed. Transparent sheets mounted on the visor captured the venom, and electrodes monitored the activity of the snakes’ mandibular muscles, which squeeze it out.

Young and his colleagues found that the squirts landed on the sheets in unique geometric patterns, which neither the activity of the mandibular muscles nor the inner structure of the fangs’ venom canals could explain.

Analysis of the cobras’ movements, however, was revealing. When threatened by a moving face, the snakes wobble their heads up and down and side to side, tracking wide movements of the target with economically small head motions. When they’re ready to spit, they switch from tracking the target to leading it, and deliver a stream of venom where they predict the target will be. It’s the wobbling of the cobra’s head during the squirt that produces the patterns—and maximizes the chances of hitting the eyes. (Physiological and Biochemical Zoology)