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

Snakes are a hyper diverse group that occupy a wide range of habitats. Irrespective of their lifestyle, all snakes can swim. Yet, information on swimming kinematics and performance is scarce. Furthermore, the influence of intraspecific variation in swimming performance and kinematics has not been considered, limiting comparisons between species. We investigated intraspecific and interspecific variation in seven snake species along a continuum from terrestrial to aquatic lifestyles. We recorded nine conventional swimming traits of 287 individuals: swimming speed, frequency, wavelength, lateral velocity (head, body and tail) and amplitudes (head, body and tail) of undulations. The results show that: 1) Undulation frequency, lateral velocities and wavelength are strongly correlated with swimming speed. 2) The highest swimming speed was observed in semi-aquatic snakes, while the slowest was found in fully aquatic snakes. 3) Intraspecific variation in speed and kinematics is slightly lower in aquatic species. 4) Morphology, sex and reproductive status have significant effects on speed and kinematic variables within the species. 5) Thanks to the large number of individuals tested, PCA analyses showed that the kinematic domains of certain species do not overlap (e.g. aspic viper versus sea snakes), while the kinematic domain of other species overlaps strongly with all others (e.g. whip snake). Adaptation to an aquatic life is therefore not a clear determinant of the way snakes swim, particularly in terms of speed. Moreover, conventional kinematic parameters are not sufficient to study the evolution of swimming in snakes. It seems essential to consider hydrodynamics, endurance, and diving performance to better understand how natural selection has shaped locomotion in aquatic snakes.