Scaling Relationships of Maximal Gape in Two Species of Large Invasive Snakes, Brown Treesnakes and Burmese Pythons, and Implications for Maximal Prey Size.

Abstract

Snakes are a phylogenetically diverse (> 3500 species) clade of gape-limited predators that consume diverse prey and have considerable ontogenetic and interspecific variation in size, but empirical data on maximal gape are very limited. To test how overall size predicts gape, we quantified the scaling relationships between maximal gape, overall size, and several cranial dimensions for a wide range of sizes (mass 8-64,100 g) for two large, invasive snake species: Burmese pythons (Python molorus bivittatus) and brown treesnakes (Boiga irregularis). Although skull size scaled with negative allometry relative to overall size, isometry and positive allometry commonly occurred for other measurements. For similar snout-vent lengths (SVL), the maximal gape areas of Burmese pythons were approximately 4-6 times greater than those of brown treesnakes, mainly as a result of having a significantly larger relative contribution to gape by the intermandibular soft tissues (43% vs. 17%). In both snake species and for all types of prey, the scaling relationships predicted that relative prey mass (RPM) at maximal gape decreased precipitously with increased overall snake size. For a given SVL or mass, the predicted maximal values of RPM of the Burmese pythons exceeded those of brown treesnakes for all prey types, and predicted values of RPM were usually least for chickens, greatest for limbed reptiles and intermediate for mammals. The pythons we studied are noteworthy for having large overall size and gape that is large even after correcting for overall size, both of which could facilitate some large individuals (SVL = 5 m) exploiting very large vertebrate prey (e.g., deer > 50 kg). Although brown treesnakes had longer quadrate bones, Burmese pythons had larger absolute and larger relative gape as a combined result of larger overall size, larger relative head size, and most importantly, greater stretch of the soft tissues.