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SummaryWe examined heritabilities and correlations among measures of locomotor performance (speed, stamina) and among possible morphometric determinants of performance (hindlimb span, tail length) in families of hatchling lizards (Sceloporus occidentalis). We were particularly interested in determining whether these traits were heritable and thus might potentially respond genetically to selection. Moreover, we wished to determine whether speed and stamina are negatively genetically correlated, as suggested by a priori physiological and empirical considerations. All four traits appeared to be significantly heritable. Broadsense heritabilities were 0.33-0.36 for speed, 0.35-0.36 for stamina, 0.45-0.51 for hindlimb span, and 0.46--0.47 for tail length. Contrary to expectations, speed and stamina were not negatively genetically correlated. Hindlimb span and tail length, however, were negatively genetically correlated (but not phenotypically correlated). Hindlimb span and stamina were positively phenotypically correlated. Thus, for example, selection for longer hindlimb span could potentially result in shorter tails, contrary to evolutionary predictions based only on phenotypic correlations.

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I present evidence that the thermal sensitivity of sprint speed of Anolis lizards has evolved to match the activity body temperatures (T b ) experienced by local populations in nature. Anolis lizards from a range of altitudes in Costa Rica have limited thermoregulatory abilities and consequently have field T; that differ substantially in median and interquartile distance (a measure of variability). Experimentally determined maximal sprint temperatures (T; at which lizards run fastest) were positively correlated with median field Ti; and performance breadths (ranges of T b over which lizards run well) were correlated with the variability (interquartile distance) offield T; in the species I examined, Such correlations would be expected if the thermal sensitivity of sprint speed and field T; had evolved together to improve the sprint performance of lizards in nature, Integration of laboratory and field studies indicates that several species of Anolis regularly experience impaired sprint speeds in the field, despite apparent evolutionary modification of their thermal physiologies. However, this impairment would have been more severe if the thermal sensitivities of sprint speed had not evolved.Data from other groups of lizards indicate that the thermal sensitivity of sprint speed has not evolved to match T; of local populations (Hertz et al., 1983;Crowley, 1985). These lizards experience less variable T; and less impairment of sprint speeds in the field than do the anoles, Thus, selection for modification of the thermal sensitivity of sprint speed might have been stronger for anoles than for other groups of lizards.