Aquatic Bipedalism in the Basilisk Lizard: The Analysis of an Adaptive Strategy

Abstract

The strategy of running on water by the iguanid genus Basiliscus is analyzed and discussed in view of the functional interrelationship between morphology, behavior and the physical nature of the habitat. High-speed cinematographic analysis of the locomotor behavior on water and on a solid substrate shows the aquatic gait to be bipedal and similar to the terrestrial bipedal gait. When Basiliscus runs on water, vertebral flexion and rotation of the pelvic girdle are, however, especially noticeable. A geometric model is presented which permits description of the functional significance of vertebral flexion, girdle rotation and limb retraction in the stride. Differences in locomotor behavior when running on water and on land are shown to be a function of the mechanical properties of the substrate. Hydrodynamic drag provides the forces necessary for support and propulsion in the water. Drag forces result from the elongate pes, rotated orientation of the toes, the presence of lateral toe fringes and the rapidity with which the lizard moves through the water. The functional significance of the toe fringes as a locomotor adaptation is discussed. Toe fringes are shown not to be necessary to permit the lizard to run on water as had been suspected by previous authors. They do, however, increase the efficiency of running on water. Aquatic bipedalism is as it permits the lizard to avoid both terrestrial and aquatic predators. Furthermore, both interand intraspecific differences in habitat preference and differential niche utilization are shown to correlate with the functional ability to run on the water. INTRODUCTION Exploitation of a specific habitat or parameter of that habitat in a particular fashion -often referred to as adaptive strategy-implies a functional interdependence between morphology, behavior and the physical nature of the habitat. The ability to utilize a particular parameter of a habitat is often reflected in habitat preference and niche utilization. It depends largely upon the functional requirements that the habitat demands from the morphological and behavioral complexes of the animal as well as the effectiveness with which the complexes of the animal respond to these requirements. Animals employ many odd and unique methods, by human standards, to optimize their opportunities for reproductive success. In many cases it is perhaps the novelty of a particular strategy which confers its selective advantage, for a new and novel strategy may often provide opportunities for the utilization of unused or unoccupied habitat parameters. The genus Basiliscus (Family Iguanidae) occurs commonly from Tamaulipas and Michoacan in Mexico S to Ecuador and Colombia. The five recognized species have a very distinct preference for an aquatic habitat (Maturana, 1962). They are primarily distributed inland around rivers and streams as well as ponds and small lakes