The buccal buckle: the functional morphology of venom spitting in cobras

Abstract

Multiple radiations of Asiatic and African cobras have independently evolved the ability to expel their venom as a pressurized horizontal stream, a behavior commonly referred to as spitting. Though the unique fang morphology of spitting cobras is well known, the functional bases of venom spitting have received little attention. The combined results of gross and microscopic morphology, high-speed digital videography, experimental manipulations of anesthetized cobras and electromyography reveal a two-part mechanism for spitting venom. Contraction of the M. protractor pterygoideus (PP) causes displacement and deformation of the palato-maxillary arch and fang sheath; ultimately this displacement removes soft tissue barriers to venom flow that are normally present within the fang sheath. The M. adductor mandibulae externus superficialis (AMES) is activated simultaneously with the PP; the AMES increases venom pressure within the venom gland, propelling a stream of venom through the venom duct and out the fang. The displacements of the palato-maxillary arch, which form the first part of the spitting mechanism, are very similar to the motions of these bones during prey ingestion (the pterygoid walk), suggesting that venom spitting may have evolved from a specialization of prey ingestion, rather than prey capture.