Unlike most terrestrial surfaces, small diameter branches require a snake to make a small radius of curvature to grip them. Greater numbers of vertebrae and shorter axial muscle segments with longer portions of contractile tissue relative to tendon seem likely to enhance the strength and flexibility of constricting snakes, but this may come with a cost of reducing the speed of terrestrial locomotion ( Ruben, 1977). For example, in the species used in this study, the anterior tendon of a major epaxial muscle, the semispinalis-spinalis in the colubrid, Pantherophis gutattatus, spans 13 vertebrae, which is nearly twice the span, of seven vertebrae, for the homologous tendon in the boid, Boa constrictor ( Jayne, 1982). In addition to considerable ecological and behavioral diversity of more than 2500 extant species of snakes ( Greene, 1997), their axial morphologies also vary with phylogeny and specializations in habits and habitat ( Gasc, 1974 Jayne, 1982 Mosauer, 1935 Ruben, 1977). Hundreds of species of snakes from several phylogenetically distant lineages are arboreal ( Lillywhite and Henderson, 1993 Pizzatto et al., 2007), but the effects of branch structure on kinematics, gait selection and locomotor performance are only well known for a single species of rat snake, Pantherophis guttatus ( Astley and Jayne, 2007 Astley and Jayne, 2009 Gerald et al., 2008). By contrast, most studies of the arboreal locomotion of mammals have focused on kinematics, gait selection and dynamics ( Lammers and Biknevicius, 2004 Schmitt, 1998 Schmitt et al., 2006). Several studies of anoles have quantified the effects of branch diameter on maximal running speeds (reviewed by Irschick and Garland, 2001), but only limited kinematic data are available ( Spezzano and Jayne, 2004). Many phylogenetically diverse animals are arboreal and have evolved a variety of behavioral and morphological specializations, but previous studies on the effects of arboreal habitat structure on vertebrate locomotor function and performance are mainly limited to anole lizards ( Irschick and Losos, 1999) and primates ( Strasser et al., 1998). Thus, variation in environmental structure had several highly significant effects on locomotor mode, performance and kinematics that were species dependent. The stouter and more muscular boa constrictors were usually faster than the rat snakes when using the gripping gait, whereas rat snakes were faster when using lateral undulation on the surfaces with pegs. The frictional resistances that we studied had small effects compared with those of cylinder diameter, incline and the presence of pegs. For both species maximal speeds decreased with increased incline and were greatest on cylinders with intermediate diameters that approximated the diameter of the snakes. For all snakes the presence of pegs elicited lateral undulation and enhanced speed.

On horizontal cylinders that were a small fraction of body diameter, the boa constrictors used a balancing form of lateral undulation that was not observed for rat snakes. On most cylindrical surfaces lacking pegs, both species used concertina locomotion, which always involved periodic stopping and gripping but was kinematically distinct in the two species.

We simulated diverse arboreal surfaces to test whether environmental structure had different effects on the locomotion of snakes belonging to two distantly related species with differences in axial musculature and stoutness. Snakes have considerable diversity of axial anatomy, but the functional consequences of this diversity for arboreal locomotion are poorly understood because of the lack of comparative data. Arboreal habitats create diverse challenges for animal locomotion, but the numerical and phylogenetic diversity of snakes that climb trees suggest that their overall body plan is well suited for this task.

Boa constrictor size compared