Intraspecific and interspecific variations in the swimming characteristics of snakes according to their lifestyle.

Snakes exhibit extraordinary ecological diversity and occupy a broad range of terrestrial, aquatic, and intermediate environments. Despite their widespread capacity for swimming, quantitative assessments of their aquatic locomotion remain scarce. Furthermore, the roles of intraspecific variation in shaping swimming performance and kinematics have been largely overlooked, hindering robust interspecific comparisons. This study systematically investigated intraspecific and interspecific variation in 287 individuals representing seven snake species along a terrestrial to aquatic continuum. Nine conventional swimming traits were quantified, including swimming speed and undulation characteristics such as frequency, wavelength, lateral velocities (head, body and tail) and amplitudes (head, body and tail). Undulation frequency and lateral velocity exhibited strong positive correlations with swimming speed, whereas wavelength showed a strong negative correlation. Semi-aquatic taxa attained the highest swimming speeds, while fully aquatic snakes displayed the lowest speeds. Intraspecific variation in swimming performance and kinematics was moderately reduced in aquatic species. Morphological traits, sex, and reproductive status significantly influenced both speed and kinematic profiles within species. Principal component analyses further revealed distinct kinematic domains among certain species (e.g., no overlap between aspic viper and sea snakes), while others, such as the green whip snake, exhibited broad overlap with all taxa examined. These findings demonstrate that ecological specialization to aquatic habitats does not unilaterally dictate swimming speed or kinematic patterns. Moreover, conventional kinematic parameters alone are insufficient to study the evolutionary trajectories of aquatic locomotion in snakes. Integrating hydrodynamic, endurance capacity, and diving performance will be essential to better understand how natural selection has shaped locomotion in aquatic snakes.