Scaling relationships of maximal gape and prey size of snakes for an egg-eating specialist (Dasypeltis gansi) and a dietary generalist (Pantherophis obsoletus)

Abstract

The size of both predators and prey can profoundly affect the foraging ecology of animals. This is especially true for snakes which consume prey whole. Although their gape imposes an anatomical limit on prey size, different snake species have a wide variety of prey types, prey sizes, and dietary breadth. To test whether a dietary specialist that only consumes bird eggs (Dasypeltis gansi) differed from a generalist that occasionally eats eggs (Pantherophis obsoletus), I quantified and compared the scaling relationships between overall size, cranial dimensions, and maximal gape and the orientation and mobility of relevant bones. Compared to P. obsoletus with a given overall size, D. gansi had maximal gape areas approximately 3–4 times greater and a significantly larger relative contribution to gape by the intermandibular (IM) soft tissues (50% vs. 19%). The greater distensibility of the IM soft tissues also allowed much greater lateral displacement of the distal end of the lower jaw in D. gansi versus P. obsoletus, but at maximal gape, the bones of these species had unexpectedly similar orientations in lateral view. The large gape of D. gansi was attained despite having a significantly shorter head than P obsoletus for a given snout–vent length. As a result of their large gape and more slender build, the predicted values of relative prey masses (RPM) for D. gansi at maximal gape vastly exceeded those of P. obsoletus for a given prey type. Although eggs are much stouter than rats, values of RPM at maximal gape for D. gansi eating eggs greatly exceeded those for P. obsoletus eating rats. Perhaps, Dasypeltis has the greatest size-corrected gape and potential values of RPM (for a given prey shape) of any snake taxon as these values of D. gansi exceed those from all previously studied species with similar data.