Aquatic Locomotor Kinematics of the Eastern Water Dragon (Intellagama lesueurii)

Abstract

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. Although no extant members of the suborder Lacertilia (lizards) are known to be fully aquatic, some species are known to have close associations with freshwater, brackish and marine habitats (Bartholomew et al., 1976; Bauer and Jackman, 2008). These associations range from dependence on riparian vegeta- tion to active foraging in the water (Dawson et al., 1977; Shine, 1986; Mayes et al., 2005). Aquatic dependence is found with increased frequency among varanids, iguanids, scincids, and agamids from tropical latitudes, and it is among these species that improved aquatic locomotor capabilities relative to more terrestrial species are likely to occur. When presented with the challenge, most terrestrial lizards have some capacity to swim (Braun and Reif, 1985). The action of walking can often be coopted for swimming (Snyder, 1962; Gray, 1968; Ritter, 1996); however, in the absence of morphol- ogies associated with aquatic locomotion, this technique is highly inefficient (Braun and Reif, 1985). Morphological traits such as streamlining help to reduce drag (Lighthill, 1969, 1970) and webbed limbs can increase propulsive force. Animals may further adapt to the aquatic environment by changing their locomotor technique altogether. Key to this study is the tendency for secondarily aquatic tetrapods to posses flattened tails (Bedford and Christian, 1996) that are used as an aquatic locomotor organ. Braun and Reif's (1985) review of vertebrate swimming grouped aquatic locomotor techniques into two broad catego- ries: axial and paraxial. Axial techniques use the cyclic beating of the trunk to propagate a travelling sinusoidal wave while paraxial techniques use appendages such as arms and legs. Axial undulatory techniques are considered to provide the most efficient method for aquatic propulsion (Braun and Reif, 1985), and animals that use them usually rely on the tail as the main locomotor organ. Typically, axial undulation incorporates a travelling sinusoidal wave, generated rostrally and travelling caudally. Tails or fins are used to amplify the effect of the undulation. The shape of the sinusoidal wave can be used to further define the type of axial undulation (Breder, 1926; Braun and Reif, 1985). The propulsive wave type is categorized by two factors: the relative length of the sinusoidal wave, and to which part of the body the wave is confined. Propulsive techniques that use wavelengths shorter than the animal's body length are defined as undulatory, and wavelengths longer than the animal's body length are said to be oscillatory. Hence, a complete wave is observed within the length of an animal in undulatory swimming where as an incomplete wave is observed in oscillatory swimming. Where pronounced changes in wave amplitude are observed between the animal's trunk and caudal section, they are said to be refined to a subset of the animal's body. Based on these observations Braun and Reif's (1985) proposed the following categories: undulatory, subundu- latory, oscillatory, and suboscillatory.