In their most basic form, comparisons are used simply to identify which characteristics differ among species. Sometimes the goal is to identify alternative physiological or biomechanical mechanisms (multiple solutions; see, e.g., Bartholomew 1987) that have achieved a similar functional endpoint (e.g., longer legs vs. faster muscles, either of which may cause higher maximal sprinting abilities), or perhaps to identify new "models" in which to study particular phenomena (see, e.g., Faraci, Kilgore, and Fedde 1984; Kellogg and Shaffer 1993). Interspecific comparisons are also used frequently to elucidate the endpoint and/or the process of evolutionary adaptation, that is, genetic changes in response to natural selection (reviews in Harvey and Page1 1991; Miles and Dunham 1993; Losos and Miles 1994). Specifically, interspecific correlations between some aspect(s) of the phenotype (e.g., low rates of evaporative water loss) and some aspect(s) of the environment (e.g., heat and aridity) are taken to indicate that past and/or present natural selection acting

Lizard life-history characteristics vary widely among species and populations. Most authors seek adaptive or phylogenetic explanations for life-history patterns, which are usually presumed to reflect genetic differences. However, lizard life histories are often phenotypically plastic, varying in response to temperature, food availability, and other environmental factors. Despite the importance of temperature to lizard ecology and physiology, its effects on life histories have received relatively little attention. We present a theoretical model predicting the proximate consequences of the thermal environment for lizard life histories. Temperature, by affecting activity times, can cause variation in annual survival rate and fecundity, leading to a negative correlation between survival rate and fecundity among populations in different thermal environments. Thus, physiological and evolutionary models predict the same qualitative pattern of life-history variation in lizards. We tested our model with published life-history data from field studies of the lizard Sceloporus undulatus, using climate and geographical data to reconstruct estimated annual activity seasons. Among populations, annual activity times were negatively correlated with annual survival rate and positively correlated with annual fecundity. Proximate effects of temperature may confound comparative analyses of lizard life-history variation and should be included in future evolutionary models.

I investigated the relationship between microhabitat use and behavioral thermoregulation in the lizards Sceloporus occidentalis and S. graciosus. I studied two populations of each species along a 1300—m elevational gradient in the San Gabriel Mountains of southern California. These species occupy different, but overlapping, elevational ranges, with S. graciosus occurring at higher elevations. Because their ranges differ, the species experience different thermal environments; however, they are nearly identical in several measures of thermal physiology. Body temperatures of active lizards varied little with altitude, although air temperatures decrease substantially over this range. Both species compensated behaviorally for variation in thermal environments by varying basking frequency and microhabitat use. By thermoregulating, lizards maintained body temperatures that favor high levels of locomotor performance and other physiological functions. Microhabitat use varied markedly with elevation. Sceloporus occidentalis was almost completely arboreal at low elevation; lizards of both species were partly arboreal, partly terrestrial at intermediate elevation; and S. graciosus was mainly terrestrial at high elevations. Thus, the species converged in microhabitat use where sympatric. These shifts in microhabitat use paralleled altitudinal changes in (i) the spatial location of thermally suitable microhabitats and (ii) habitat structure. Behavioral thermoregulation, then, appears to be an important determinant of habitat use in these lizards. Results of this study may have important implications for lizard community ecology. Microhabitats that are otherwise appropriate for lizards can be thermally unsuitable depending on the local thermal environment. Hence, the thermal environment and lizard thermal biology constrain patterns of structural habitat use. As a result, physiological considerations could limit the behavioral and evolutionary options for species experiencing interspecific competition. For example, competitors in sympatry may be unable to evolve microhabitat resource partitioning because only a subset of microhabitats are thermally suitable.

No abstract