The scaling of terrestrial striking performance in western ratsnakes (Pantherophis obsoletus)

In many organisms, juveniles have performance capabilities that partly offset their disadvantageous sizes. Using high-speed video recordings and imaging software, we measured the scaling of head morphology, axial morphology, and defensive strike performance of Pantherophis obsoletus across their ontogeny to understand how size and morphology affect performance. Head measurements were negatively allometric whereas the cross-sectional area (CSA) of epaxial muscles displayed positive allometry. The greater relative muscle CSA of larger ratsnakes allows them to produce higher forces relative to their mass, and those forces act on a relatively smaller head mass when it is thrust forward during striking. Maximum strike accelerations of 70-273.8 ms^-2 and velocities of 1.08-3.39 ms^-1 scaled positively with body mass but differed from the geometric predictions. Velocity scaled with mass^0.15 and acceleration scaled with mass^0.17 . Larger snakes struck from greater distances (range = 4.1-26 cm), but all snakes covered the strike distances with similarly short durations (84 ± 3 ms). The negatively allometric head size, isometry of anterior mass, and positively allometric muscle CSA enable larger P. obsoletus to strike with higher velocities and accelerations than smaller individuals. Our results contrast with the scaling of strike performance in an arboreal viper, whose strike distance and velocity were independent of body mass. When strike distance is modulated, all other performance capacities are affected because of the interdependence of acceleration, velocity, duration, and distance.