Functional bases of the spatial dispersal of venom during cobra "spitting".

Spitting cobras expulse venom toward the face and/or eyes of potential predators as part of their defensive repertoire. Evaluating the accuracy of the cobras is difficult because the spit venom does not land as a point but rather is distributed, in some cases widely, in complex geometric patterns on the surface of the target. The purpose of this study was to explore the functional bases of the venom's spatial distribution. Using a combination of spatial analysis of "caught" venom, morphology, high-speed digital videography, and electromyography (EMG), three hypothesis were evaluated. Two of these hypotheses--that the spatial distribution was due to differential venom pressure produced by the contractile activity of the adductor mandibulae externus superficiali and that the spatial distribution was produced by the morphology of the venom canal within the fang-were both rejected. The third hypothesis--that the spatial distribution was due to rapid rotational movements of the head about the vertebral column--was supported by analyses of EMG activity within the cervical axial muscles and by predictions of venom-distribution patterns based on these cephalic displacements. These results suggest that the ability to "spit" venom is a unique suite of specializations involving both the axial and the cephalic systems.