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 im peded or slowed down by several constraints or trade-offs between conflicti ng functions. This is frequently observed by ecomorphological studies focus ing on lower taxonomic levels: form-function relationships get blurred beca use subtle adaptive traits remain hidden or simply do not exist. Therefore, a rigorous analytic approach is required. (ideally) assessing the links be tween the four stages of the adaptive process simultaneously (i.e., from ge neric variation to variation in design, to variation in performance, to dif ferential fitness), taking into consideration all potential factors hinderi ng the normal progression of this process. Lizard locomotion is a good mode l for such an analysis. Locomotion is essential in many ecologically releva nt functions (feeding, predator avoidance, etc.). It consists of several co mponents (speed. acceleration, endurance, manoeuvrability, etc.) and modes (level running, climbing, etc.) with conflicting demands, leading to potent ial trade-offs. Moreover, several of its components proved to be heritable and obvious relations between habitat use and locomotor design are often ab sent (e.g., in lacertid lizards). Two cases, focusing on the potential trad e-off between climbing and level-running, are presented to illustrate the s ubtle interplay between variation in ecology, performance and design in liz ard locomotion. (1) For two gekkotans (a climber and aground dwelling speci es) 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 basi s 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 des ign traits' as an intermediate stage between variation in design and perfor mance.