KINEMATICS AND EFFICIENCY OF STEADY SWIMMING IN ADULT AXOLOTLS (AMBYSTOMA-MEXICANUM)

The kinematics of steady swimming at a wide range of velocities was an alysed using high-speed video recordings (500 frames s(-1)) of eight i ndividuals of Ambystoma mexicanum swimming through a tunnel containing stationary water, Animals in the observed size range (0.135-0.238 m t otal body length) prefer to swim at similar absolute speeds, irrespect ive of their body size, The swimming mechanism is of the anguilliform type, The measured kinematic variables - the speed, length, frequency and amplitude (along the entire body) of the propulsive wave - are mor e similar to those of anguilliform swimming fish than to those of tadp oles, in spite of common morphological features with the latter, such as limbs, external gills and a tapering tail, The swimming speed for a given animal size correlates linearly with the tailbeat frequency (r( 2)=0.71), whereas the wavelength and tail-tip amplitude do not correla te with this variable, The shape of the amplitude profile along the bo dy, however, is very variable between the different swimming bouts, ev en at similar speeds, It is suggested that, for a given frequency, the amplitude profile along the body is adjusted in a variable way to yie ld the resulting swimming speed rather than maintaining a fixed-amplit ude profile, The swimming efficiency was estimated by calculating two kinematic variables (the stride length and the propeller efficiency) a nd by applying two hydrodynamic theories, the elongated-body theory an d an extension of this theory accounting for the slope at the tail tip , The latter theory was found to be the most appropriate for the axolo tl's swimming mode and yields a hydromechanical efficiency of 0.75 +/- 0.04 (mean +/- S.D.,), indicating that Ambystoma mexicanum swims less efficiently than do anuran tadpoles and most fishes, This can be unde rstood given its natural habitat in vegetation at the bottom of lakes, which would favour manoeuvrability and fast escape.