The effects of slope and branch structure on the locomotion of a specialized arboreal colubrid snake (Boiga irregularis).

The surfaces in arboreal habitats have variable diameters, slopes, and branching structure that pose functional challenges for animal locomotion. Nevertheless, many lineages of snakes have independently evolved arboreality. We tested the effects of arboreal habitat structure on the locomotion of a highly arboreal species, the brown tree snake (Boiga irregularis), moving on seven diameters (0.6-21 cm) of cylinders oriented at three slopes (0°, 45°, 90°) and with or without pegs. Intermediate diameters of horizontal cylinders maximized speed, and some of the large-diameter cylinders without pegs were impassable when they were inclined. With increased slope the snakes were slower, and they changed from using lateral undulation with sliding contact and balancing to concertina locomotion with periodic static gripping. The presence of pegs increased the speeds of the brown tree snakes and resulted in them only using lateral undulation. Surface diameter, slope, and the occurrence of pegs also had widespread significant effects on the kinematics of the brown tree snakes. Overall, compared to anatomically less specialized corn snakes, brown tree snakes use more lateral undulation, are usually much faster, and are able to move on a wider variety of surfaces. Unlike some of the trade-offs found previously between two less specialized species of snakes with different stoutness when they used modes of arboreal locomotion that involved either balancing or gripping, the slender-bodied brown tree snakes excel at both. Hence, this species may not only be a "jack of all trades" but also a master of many.