Aquatic Locomotor Kinematics of the Eastern Water Dragon (Intellagama lesueurii)
Quantitative studies of the axial undulatory swimming techniques used by secondarily aquatic vertebrates have been largely restricted to crocodilians. Numerous members of the suborder Lacertilia (lizards) are also known to swim using axial undulatory techniques, but how they do so has received minimal attention from the scientific community. We investigated the morphology and undulatory locomotor kinematics adopted by the Eastern Water Dragon (Intellagama lesueurii) through observation of natural swimming and filming of animals in a flume tank with a high speed camera. We found that morphological modifications associated with improved swimming ability and correlations between wave characteristics and swimming velocity are limited to the tail. The shape of dorsal spines and the reduction in the width of transverse processes of the caudal vertebrae result in a mediolaterally compressed tail instead of the typically rounded or dorsoventrally compressed tail seen in other Australian agamids. Axial undulatory swimming in I. lesueurii was found to be conceptually similar to that of crocodilians, but the relatively long and thin terminal part of the tail produces a different shaped undulatory wave. Unlike crocodilians and fishes, I. lesueurii does not use frequency moderated velocity control. Instead, changes in velocity are solely controlled by the phase speed of the propagating wave. The combined effect of these traits is comparable efficiency and performance in the water relative to that of crocodilians and an improvement relative to terrestrial lizards.Abstract
Lateral view of an adult male Intellagama lesueurii. A and B are areas shown in detail in Figures 3 and 4, respectively. The cranial tail part is described as the section of tail spanning from the pelvic girdle to the tail spine bifurcation. The section, which is typically one snout–vent length long supports the bulk of the muscle mass of the tail and is relatively stiff compared to the terminal tail part. The terminal tail part is the remaining span of the tail. It varies in relative length from individual to individual and can also be reduced in size from naturally accrued damage.
The left hind limb of Intellagama lesueurii, showing no sign of morphological adaptation to swimming.
Spines on the middorsum of an adult male Intellagama lesueurii (as seen in Fig. 2A). Cranial is shown by the arrow.
The section of tail from an adult male Intellagama lesueurii where the dorsal spine bifurcates (as seen in Fig. 2B). Cranial is shown by the arrow.
The differences in cross-sectional shape in the tail of four Australian agamids: Pogona barbata, Hypsilurus spinipes, Chlamydosaraus kingii, and Intellagama lesueurii. Tails are scaled so that the heights are even at their base/vent.
Differences between the first seven tail vertebrae of Intellagama lesueurii and Pogona barabata. The diagram is scaled such that the sections depicted are the same length for an equivalent number of vertebrae. A side view (below) and corresponding top view (above) are provided for both species. Cranial is shown by the arrow.
The undulatory cycle of Intellagama lesueurii at 1/15 second intervals swimming at a speed of approx 0.7m/sec. The locomotor technique is anguilliform in nature; however, the section terminal of the pelvic girdle shows the steepest increase in amplitude. Each series of colored bars depicts the propagation of a wave front.
The undulatory characteristics of a swimming Intellagama lesueurii. Amplitude range depicts how the width of lateral motion varies along the length of the animal. Peak amplitude was used to track wave propagation. Wavelength depicts the length of a complete wave, the differences between W1 and W2 show that wavelength varies along the length of the body. Attack angle is the offset of the tail's propulsive surface from perpendicular to the animal's trunk.
Change in wavelength with forward velocity at the tail spine bifurcation in Intellagama lesueurii (F1,19 = 9.899, P < 0.01). A Linear Mixed Model was used to account for the numerous animals used in the experiment. Data were collected from swimming observation from four individuals; letters indicate different individuals.
Change in whole body phase speed with forward velocity in Intellagama lesueurii at swimming speeds ranging from 0.403–1.067 m/sec (F1,24 = 19.690, P < 0.001). A Linear Mixed Model was used to account for the numerous animals used in the experiment. Data were collected swimming observations from eight individuals; letters indicate different individuals.
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