Swimming performance in semiaquatic and terrestrial Oryzomyine rodents

Semiaquatic and terrestrial mammals frequently have to cross or move along water bodies, both trying to remain on the water surface using one or two pairs of limbs, combining different gaits and stride lengths and frequencies. This is the case of the semiaquatic water rats Nectomys and the cursorial Cerradomys, sister genera of the Oryzomyini tribe, capable of swimming using similar gaits. They provide an opportunity to investigate performance specializations involving the semiaquatic habitat, our objective in this study. Rodents were filmed at 30 frames s−1 in lateral view, swimming in a glass aquarium. Video sequences were analyzed dividing the swimming cycle into power and recovery phases. Differences in swimming performance were detected between species of Nectomys and Cerradomys, but not between species of the same genus. Absolute mean speed did not differ between the semiaquatic and terrestrial groups, but the semiaquatic Nectomys had longer stride lengths with lower stride frequency, whereas the terrestrial Cerradomys had higher stride frequency and relative swimming speed. The widest behavior repertoire of Nectomys allowed more efficient, but not necessarily faster swimming than the terrestrial Cerradomys. Efficient aquatic locomotion in Nectomys is ultimately a result of improved buoyancy by hydrophobic fur and subtle morphological specializations, which allow this genus to perform more efficiently in water than the terrestrial Cerradomys without compromising locomotion in the terrestrial environment.