The comparative biomechanics of an ophidian defensive behaviour: head triangulation in hognose snake (Heterodon) and an egg-eating snake (Dasypeltis)

Head triangulation is a regular component of the defensive behaviour of bot h Heterodon and Dasypeltis. In Heterodon platyrhinos, the triangulation is achieved primarily through rotation of the quadrate in the transverse plane with the distal end of the quadrate moving dorsad. The quadrate is pulled by the m. cervicomandibularis acting in conjunction with one of the jaw add uctors. Head triangulation in Dasypeltis scabra is produced by rotation of the quadrate mainly in the frontal plane with the distal end of the quadrat e moving craniad. Experimental analysis documents that contraction of the m . adductor mandibulae externus superficialis is temporally linked to this d isplacement of the quadrate. The presence of two different mechanical bases for head triangulation in these distantly related colubrids suggests that this behavioural display has evolved independently in these ophidian lineag es.