EVOLUTION OF SPRINT SPEED IN LACERTID LIZARDS - MORPHOLOGICAL, PHYSIOLOGICAL, AND BEHAVIORAL COVARIATION

Organismal performance abilities occupy a central position in phenotyp ic evolution; they are determined by suites of interacting lower-level traits (e.g., morphology and physiology) and they are a primary focus of natural selection. The mechanisms by which higher levels of organi smal performance are achieved during evolution are therefore fundament ally important for understanding correlated evolution in general and c oadaptation in particular. Here we address correlated evolution of mor phological, physiological, and behavioral characteristics that influen ce interspecific variation in sprint speed in a clade of lacertid liza rds. Phylogenetic analyses using independent contrasts indicate that t he evolution of high maximum sprinting abilities (measured on a photoc ell-timed racetrack) has occurred via the evolution of (1) longer hind limbs relative to body size, and (2) a higher physiologically optimum temperature for sprinting. For ectotherms, which experience variable body temperatures while active, sprinting abilities in nature depend o n both maximum capacities and relative performance levels (i.e., perce nt of maximum) that can be attained. With respect to temperature effec ts, relative performance levels are determined by the interaction betw een thermal physiology and thermoregulatory behavior. Among the 13 spe cies or subspecies of lizards in the present study, differences in the optimal temperature for sprinting (body temperature at which lizards run fastest) closely matched interspecific variation in median preferr ed body temperature (measured in a laboratory photothermal gradient), indicating correlated evolution of thermal physiology and thermal pref erences. Variability of the preferred body temperatures maintained by each species is, across species, negatively correlated with the therma l-performance breadth (range of body temperatures over which lizards c an run relatively fast). This pattern leads to interspecific differenc es in the levels of relative sprint speed that lizards are predicted t o attain while active at their preferred temperatures. The highest lev els of predicted relative performance are achieved by species that com bine a narrow, precise distribution of preferred temperatures with the ability to sprint at near-maximum speeds over a wide range of body te mperatures. The observed among-species differences in predicted relati ve speed were positively correlated with the interspecific variation i n maximum sprinting capacities. Thus, species that attain the highest maximum speeds are (1) also able to run at near-maximum levels over a wide range of temperatures and (2) also maintain body temperatures wit hin a narrow zone near the optimal temperature for sprinting. The obse rved pattern of correlated evolution therefore has involved traits at distinct levels of biological organization, that is, morphology, physi ology, and behavior; and tradeoffs are not evident. We hypothesize tha t this particular trait combination has evolved in response to coadapt ational selection pressures. We also discuss our results in the contex t of possible evolutionary responses to global climatic change.