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° 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.

Perch size and structure have species-dependent effects on the arboreal locomotion of rat snakes and boa constrictors

“…and dipsadinine snakes, which also have extraordinarily long tails (Lillywhite and Henderson, 1993). A greater cross-sectional area of axial muscles should enhance gripping ability, but the associated increase in the diameter and mass of the snakes may be detrimental for other demands such as moving on extremely thin branches or bridging gaps (Lillywhite and Henderson, 1993;Jayne and Riley, 2007).…”

The effects of three-dimensional gap orientation on bridging performance and behavior of brown tree snakes (Boiga irregularis)

“…However, as body size increases, some animals such as primates tend to rely less on leaping and more on reaching to cross gaps (Fleagle and Mittermeier, 1980;Cannon and Leighton, 1994;Thorpe et al, 2009). Many phylogenetically diverse species of snakes are arboreal, and they rely mainly on extending their bodies to reach across gaps (Lillywhite et al, 2000;Lin et al, 2003;Jayne and Riley, 2007), although the combination of leaping and gliding of snakes in the genus Chrysopelea provides a spectacular exception (Socha, 2006). Compared with limbed animals, the bodies of all snakes are greatly elongated, and within snakes many arboreal specialists have independently evolved a more attenuate body shape than their non-arboreal relatives (Lillywhite and Henderson, 1993;Pizzatto et al, 2007).…”

“…Such elongate bodies appear especially well suited for reaching across gaps and crawling on fine terminal branches without breaking them. Despite the complex threedimensional structure of arboreal habitats, all previous data for snakes bridging gaps are for destinations confined to the horizontal plane (Lillywhite et al, 2000;Lin et al, 2003;Jayne and Riley, 2007;Mansfield and Jayne, 2011) even though this type of gap represents only a tiny subset of all of the gap orientations that occur in nature.…”

“…Perhaps such dynamic movements preceded the aerial behavior observed in other species of arboreal snakes. Riley, 2007). Even if a body is perfectly rigid, another problem could arise when the torque resulting from the unsupported body exceeds that of the supported body.…”

“…At this point the unsupported portion of the body would pitch in a downward direction, lifting the supported body off the substrate. However, in reality, snakes counter this tendency by producing a counter-torque by gripping or placing a portion of the body or tail beneath the supporting surface (Jayne and Riley, 2007).…”