REPEATABILITY OF LOCOMOTOR PERFORMANCE IN NATURAL POPULATIONS OF THE LIZARD SCELOPORUS MERRIAMI

Field studies of natural selection are currently undergoing a resurgence (Price et aI., 1984; Endler, 1986). Recent theoretical and analytical advances (Lande and Arnold, 1983; Manly, 1985; Arnold and Wade, 1984a, 1984b) have made these new studies both rigorous and feasible, and criticisms of untested adaptive explanations (Gould and Lewontin, 1979)have made them necessary. One common approach involves an attempt to correlate interspecific or interpopulational differences in a given trait with environmental gradients, e.g., in predation rate or latitude (Lauder, 1981; Endler, 1986). A second, less common approach involves an attempt to detect the effects of individual differences on one or more components of fitness, e.g., fecundity or survivorship (Lande and Arnold, 1983; Arnold, 1986; Endler, 1986). These and related methods make a key assumption-that measurements of traits are repeatable (Arnold, 1986; Bennett, 1987). For example, the bill size ofan individual bird is assumed not to change significantly during the study. This assumption is often valid for morphological traits of adult animals (Falconer, 1981) (but see Smith and Zach, 1979; Price and Grant, 1984). Its validity for organismal "performance" traits, e.g., speed, stamina, or digestive efficiency (Huey and Stevenson, 1979; Arnold, 1983) has to date been established only in the laboratory (review in Bennett [1987]), not in the field. This is unfortunate, as such measures provide a crucial link between "lower-level" (physiological, morphological, biochemical) traits and fitness (Arnold, 1983). Showing that performance traits are repeatable in long-term field studies would encourage attempts to monitor selection on such traits, for only one or a few measurements of performance could adequately characterize an individual's performance. Alternatively, performance would need to be measured multiple times during the course ofthe study. Here, we investigate the repeatability ofsprint speed in natural populations of a lizard. Sprint speed is a performance trait that may influence fitness via its effectson feedingsuccess (Greenwald, 1974;Webb, 1986), predator avoidance (Shine, 1980; Christian and Tracy, 1981; Huey and Hertz, 1984b), and, possibly, social dominance. Sprint speed is significantly repeatable for up to two months in the laboratory (Bennett, 1980, 1987; Garland and Arnold, 1983; Garland, 1985; Garland and Else, 1987; Arnold and Bennett, pers. comm.). Nevertheless, the feasibility of studying natural selection on speed is unclear, because the sprint speed of a lizard may not be a fixed trait in long-term studies: an individual's speed may vary with body size and age (Huey and Hertz, 1982, 1984b; Garland, 1985), body temperature (Bennett, 1980; Huey and Hertz, 1984a), reproductive condition (Shine, 1980; Bauwens and Thoen, 198I), and injury history (Daniels, 1983), though perhaps not with season (Garland, 1985) or desiccation (Crowley, 1985b). These considerations raise an important issue: is sprint speed sufficiently repeatable in nature to render it convenient for field studies of natural selection? We address two specific questions. First, is the relative speed ofan individual repeatable across temperatures? In other words, is the rank order of an individual's speed independent of body temperature? Second, is absolute speed ofpopulations or individuals repeatable in time? If the answer to these questions is yes, then studies ofnatural selection will be facilitated, because an individual's (or population's) performance can be adequately assessed from a few measurements at a single body temperature. We studied the repeatability of sprint speed in Seeloporus merriami (Iguanidae), a small, diurnal, rockdwelling, insectivorous lizard that occurs in southwestern Texas and in adjacent areas of Mexico. Populations in Big Bend National Park, Texas are especially well suited for these analyses because their ecology is well known (Dunham, 1978, 1981, 1983). Sprint speed is probably relevant to Sceloporus merriami, as these lizards often sprint when capturing prey, escaping predators, and interacting socially. Adult lizards have an average cohort generation time of 1.5 years and a maximum measured lifespan of six years (Dunham, 1981).