Scaling and relations of morphology with locomotor kinematics in the sidewinder rattlesnake Crotalus cerastes

The movement of limbless terrestrial animals differs fundamentally from that of limbed animals, yet few scaling studies of their locomotor kinematics and morphology are available. We examined scaling and relations of morphology and locomotion in sidewinder rattlesnakes (Crotalus cerastes). During sidewinding locomotion, a snake lifts sections of its body up and forward while other sections maintain static ground contact. We used high-speed video to quantify whole-animal speed and acceleration; the height to which body sections are lifted; and the frequency, wavelength, amplitude, and skew angle (degree of tilting) of the body wave. Kinematic variables were not sexually dimorphic, and most did not deviate from isometry, except wave amplitude. Larger sidewinders were not faster, contrary to many results from limbed terrestrial animals. Free from the need to maintain dynamic similarity (because their locomotion is dominated by friction rather than inertia), limbless species may have greater freedom to modulate speed independently of body size. Path analysis supported: (1) a hypothesized relationship between body width and wavelength, indicating that stouter sidewinders form looser curves; (2) a strong relationship between cycle frequency and whole-animal speed; and (3) weaker effects of wavelength (positive) and amplitude (negative) on speed. We suggest that sidewinding snakes may face a limit on stride length (to which amplitude and wavelength both contribute), beyond which they sacrifice stability. Thus, increasing frequency may be the best way to increase speed. Finally, frequency and skew angle were correlated, a result that deserves future study from the standpoint of both kinematics and physiology.

Methods
Full details on data collection and processing can be found in the Methods section of the published article.

Methods overview:

We measured morphometric and meristic traits for sidewinder rattlesnakes (Crotalus cerastes) captured at the Barry M. Goldwater Range near Yuma, Arizona, USA during June and July 2016. We then used two high-speed video cameras to record locomotor trials for for 66 individuals. We digitized a subset of videos using using the MATLAB programs DLTcal5 and DLTdv5 (Hedrick, 2008), choosing that subset based on trial quality (whether the snake performed multiple sidewinding cycles without stopping and turning), video and calibration quality, and representation of snakes from the entire size range of both sexes. We smoothed the data and extracted kinematic variables using custom MATLAB programs written by co-author BMS. The final sample for kinematic analysis included one representative trial for each of 26 individuals.