AN EXPERIMENTAL CONFIRMATION OF MORPHOLOGICAL ADAPTATION: TOE FRINGES IN THE SAND-DWELLING LIZARD UMA SCOPARIA.

Abstract

Comparative studies that reveal close associations between a character and a common selective pressure (e.g., sexual dimorphism with polygamous mating systems [Campbell, 1972]; clutch size with foraging mode [Huey and Pianka, 198 1]) can provide strong evidence for their adaptive value (Clutton-Brock and Harvey, 1979; Ridley, 1983). For morphological characters, the demonstration of the performance advantage to organisms possessing the trait in question strengthens conclusions of adaptive convergence suggested by formenvironment correlations. Studies of differential performance among species varying in the expression of a character have been used to support adaptive explanations for the character's evolution (e.g., Alberch, 1981), but interspecific comparisons (e.g., Anderson and Karasov, 1981) are weaker than intraspecific ones because of possibly confounding differences in physiological or morphological features, especially if the species are not closely related. Intraspecific functional performance studies are difficult, as variation among conspecifics in the expression of a trait of purported adaptive significance is usually so slight that demonstrations of functional advantages are both difficult and rare (e.g., Herrera, 1978; Garland, 1984). Comparative studies of morphological traits suggest three main characters that affect running in lizards: relative limb proportions (Snyder, 1954, 1962; Jaksic et al., 1980; Moermond, 1979), toe pads with lamellae (Russell, 1979; Williams and Peterson, 1982), and toe fringes (Mosauer, 1932; Luke, 1986). Experimental studies of the functional significance of these characters can be difficult. The first two are complex functional units, and, as with any such morphological system, manipulations (altering limb length or removing toe pads) to demonstrate their significance in increasing locomotor performance are problematic, as it is difficult to establish appropriate controls. Numerous intraspecific studies have quantified differential running performance in lizards (e.g., Ballinger et al., 1973; Bennett, 1980; Huey, 1982; Punzo, 1982; Hertz, 1983; Huey and Hertz, 1984), but none have studied characters presumed to have evolved specifically to aid locomotion (Laerm [1973] anecdotally reported reduced locomotor effectiveness in uncontrolled fringe removal experiments with water-running Basiliscus). Laterally projecting toe fringes on sand-dwelling lizards have long been interpreted as an adaptation for running on sand (Stebbins, 1944; Norris, 1958) and are considered a classic case of convergent evolution (Mosauer, 1932; Bellairs, 1969). A comparative study shows that fringes have evolved at least 26 different times among five lizard families, being confined mainly to sand-dwelling species (Luke, 1986; they also occur in some lizards that run on water, e.g., Laerm, 1974), supporting an adaptive explanation for their evolution. In this study, I performed experimental removals of toe fringes of a sand-dwelling lizard, Uma scoparia, to assess whether they are indeed an adaptation that significantly increases locomotor performance on sand.