Crawling without wiggling: muscular mechanisms and kinematics of rectilinear locomotion in boa constrictors.

A central issue for understanding locomotion of vertebrates is how muscle activity and movements of their segmented axial structures are coordinated, and snakes have a longitudinal uniformity of body segments and diverse locomotor behaviors that are well suited for studying the neural control of rhythmic axial movements. Unlike all other major modes of snake locomotion, rectilinear locomotion does not involve axial bending, and the mechanisms of propulsion and modulating speed are not well understood. We integrated electromyograms and kinematics of boa constrictors to test Lissmann's decades-old hypotheses of activity of the costocutaneous superior (CCS) and inferior (CCI) muscles and the intrinsic cutaneous interscutalis (IS) muscle during rectilinear locomotion. The CCI was active during static contact with the ground as it shortened and pulled the axial skeleton forward relative to both the ventral skin and the ground during the propulsive phase. The CCS was active during sliding contact with the ground as it shortened and pulled the skin forward relative to both the skeleton and the ground during the recovery phase. The IS shortened the ventral skin, and subsequent isometric activity kept the skin stiff and shortened during most of static contact while overlapping extensively with CCI activity. The concentric activity of the CCI and CCS supported Lissmann's predictions. Contrary to Lissmann, the IS had prolonged isometric activity, and the time when it shortened was not consistent with providing propulsive force. Decoupling propulsion from axial bending in rectilinear locomotion may have facilitated economical locomotion of early snakes in subterranean tunnels.

Why arboreal snakes should not be cylindrical: body shape, incline and surface roughness have interactive effects on locomotion.

Depending on animal size, shape, body plan and behaviour, variation in surface structure can affect the speed and ease of locomotion. The slope of branches and the roughness of bark both vary considerably, but their combined effects on the locomotion of arboreal animals are poorly understood. We used artificial branches with five inclines and five peg heights (≤40 mm) to test for interactive effects on the locomotion of three snake species with different body shapes. Unlike boa constrictors (Boa constrictor), corn snakes (Pantherophis guttatus) and brown tree snakes (Boiga irregularis) can both form ventrolateral keels, which are most pronounced in B. irregularis. Increasing peg height up to 10 mm elicited more of the lateral undulatory behaviour (sliding contact without gripping) rather than the concertina behaviour (periodic static gripping) and increased the speed of lateral undulation. Increased incline: (1) elicited more concertina locomotion, (2) decreased speed and (3) increased the threshold peg height that elicited lateral undulation. Boiga irregularis was the fastest species, and it used lateral undulation on the most surfaces, including a vertical cylinder with pegs only 1 mm high. Overall, B. constrictor was the slowest and used the most concertina locomotion, but this species climbed steep, smooth surfaces faster than P. guttatus. Our results illustrate how morphology and two different aspects of habitat structure can have interactive effects on organismal performance and behaviour. Notably, a sharper keel facilitated exploiting shorter protrusions to prevent slipping and provide propulsion, which became increasingly important as surface steepness increased.