Lizard locomotion: How morphology meets ecology

Biological evolution often leads, through natural selection, to an optimal fit between 'design' and ecology. However, the adaptation process may be impeded or slowed down by several constraints or trade-offs between conflicting functions. This is frequently observed by ecomorphological studies focusing on lower taxonomic levels: form-function relationships get blurred because subtle adaptive traits remain hidden or simply do not exist. Therefore, a rigorous analytic approach is required. (ideally) assessing the links between the four stages of the adaptive process simultaneously (i.e., from generic variation to variation in design, to variation in performance, to differential fitness), taking into consideration all potential factors hindering the normal progression of this process. Lizard locomotion is a good model for such an analysis. Locomotion is essential in many ecologically relevant functions (feeding, predator avoidance, etc.). It consists of several components (speed. acceleration, endurance, manoeuvrability, etc.) and modes (level running, climbing, etc.) with conflicting demands, leading to potential trade-offs. Moreover, several of its components proved to be heritable and obvious relations between habitat use and locomotor design are often absent (e.g., in lacertid lizards). Two cases, focusing on the potential trade-off between climbing and level-running, are presented to illustrate the subtle interplay between variation in ecology, performance and design in lizard locomotion. (1) For two gekkotans (a climber and aground dwelling species) the moments exerted by several important leg muscles appear to be tuned to their primary mode of locomotion. (2) In two sibling lacertid species, the inverse trade-off between climbing and running, put forward on the basis of observed substrate use, does not exist. Instead, a drastic difference in running performance, likely related to different running styles, emerged. The latter ease illustrates the potential use of 'integrated, dynamic design traits' as an intermediate stage between variation in design and performance.