Scaling of the axial morphology and gap-bridging ability of the brown tree snake,Boiga irregularis

Abstract

Networks of branches in arboreal environments create many functional challenges for animals, including traversing gaps between perches. Many snakes are arboreal and their elongate bodies are theoretically well suited for bridging gaps. However, only two studies have previously investigated gap bridging in snakes, and the effects of size are poorly understood. Thus, we videotaped and quantified maximal gap-bridging ability in a highly arboreal species of snake (Boiga irregularis), for which we were able to obtain a large range in snout-vent length (SVL=43-188 cm) and mass (10-1391 g). We expected smaller snakes to bridge relatively larger gaps than larger individuals because of their proportionately higher ratio of muscle cross-sectional area to mass. The maximal length of the gaps spanned by B. irregularis had negative allometry, indicating that smaller snakes could span a greater proportion of their length than larger snakes. The greatest relative gap distance spanned (64% SVL) was by the smallest individual. The majority of snakes (85%) simply crawled slowly to cross a gap. Most of the suspended portion of the body and the path traveled by the head were below the perch that supported the posterior body, which may decrease the tendency of the snake to roll. Some (15%) of the snakes used another behavior in which the neck inclined as much as 45 degrees and then rapidly lunged towards the anterior perch, and this enabled them to cross larger gaps than those using the crawling behavior. Perhaps the launching behavior of the gliding tree snakes (Chrysopelea sp.) evolved from an ancestral behavior of lunging to bridge gaps analogous to that of the brown tree snakes. An estimate of the muscle strain required to prevent the body of the snake from buckling suggests that, despite being light-bodied, brown tree snakes bridging a gap may be at the limit of the physiological capacity of their epaxial muscles.