The possibility of functional relationships between energetics and life-history characteristics has been of considerable interest to evolutionary ecologists. Among species of mammals, life-history variables generally are not correlated with mass-independent basal metabolic rate, with the possible exceptions of maximal intrinsic rate of increase, litter size and reproductive effort during lactation. Whether this is generally true at the level of variation among individuals within a population (individual variation) is unclear. Therefore, we tested whether basal or maximal metabolic rates of random-bred female mice (Mus domesticus) were correlated with the size of their litters, litter mass, or mean offspring mass. The effects of variation in maternal mass, maternal age, experimental block and duration of fasting (for basal metabolic rate) were removed by calculating residuals from multiple regression equations. Basal and maximal metabolic rate were not significantly correlated with any of the life-history variables we studied. Thus, our results are generally consistent with those from interspecific comparisons of mammals: little evidence suggests necessary associations between metabolic rates and life history.

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Summary1. The concept of repeatability, the measurement of consistent individual differences, has become an increasingly important tool in evolutionary and ecological physiology. Significant repeatability facilitates the study of selection acting on natural populations and the concept has several practical implications for identifying traits. 2. When properly defined and measured, repeatability can set the upper limit to heritability. This is potentially a very useful interpretation of the repeatability of traits measured on natural populations because often, estimates of heritability cannot be obtained. Many recent reports of repeatability of individual differences for traits have made this interpretation. 3. However, repeatability estimates may not set an upper limit to heritability if: (a) measured traits are not genetically identical, (b) common environmental effects work in opposition to direct genetic effects, (c) the temporary environments for each trait are negatively correlated, (d) significant genotype-environment interaction is present, or (e) the traits are influenced by maternal effects. 4. The quantitative genetic theory that defines the concept of repeatability is reviewed and implications of violations of the five assumptions are discussed in the context of interpreting repeatability as an upper estimate to heritability.

m/s, n = 24) mice exhibited forced maximal sprint running speeds that averaged ~50% higher than those of random-bred laboratory mice (range 1.11-2.12 m/s, n = 19). Wild and hybrid mice also had significantly higher ( + 22%) mass-corrected maximal rates of oxygen consumption (VOzmax > during forced exercise and greater (+ 12%) relative ventricle masses than lab mice. Wild and hybrid mice also showed statistically higher swimming endurance times relative to body mass than lab mice, although these differences were insignificant when body mass was not used as a covariate. No significant differences were found for relative gastrocnemius muscle mass, liver mass, hematocrit, or blood hemoglobin content. During a 'I-day test on voluntary activity wheels, both wild and hybrid mice ran significantly more total revolutions (+ lOl%), ran at higher average velocities when they were active (+69%), and exhibited higher maximum revolutions in any single 1-min period (+41% on the 7th day of testing), but the total number of active 1-min intervals did not differ significantly among groups. In general, the behavioral and/or whole organism performance traits showed greater differences than the lower-level traits; thus, during the domestication of house mice, behavior may have evolved more rapidly than physiology. behavior; directional dominance; domestication; endurance; evolution; locomotion; maximal oxygen consumption; sprint speed; wheel running LABORATORY HOUSE MICE have served as model organisms in numerous physiological, behavioral, and quantitative genetic studies (12, 25, 35, and references therein). In exercise physiology, laboratory rats have been used more commonly than mice (4). For logistical reasons, however, mice are better candidates for study of the quantitative genetic basis of variation in physiological and associated behavioral traits. As a prelude to quantitative genetic analyses and artificial selection experiments with random-bred, genetically variable laboratory house mice, we conducted a series of studies to establish baseline information and to consider how appropriate a random-bred strain of mice may be for drawing evolutionary conclusions about metabolism and exercise physiology in small mammals (16, 22, 31, cf. 35). In this study we present information on the exercise capacities and associated behavioral traits of a strain of random-bred laboratory mice compared with a Wisconsin population of wild (commensal) house mice and with hybrids between the lab and wild mice.

We tested the hypothesis that locomotor speed and endurance show a negative genetic correlation using a genetically variable laboratory strain of house mice (Hsd:ICR: Mus domesticus). A negative genetic correlation would qualify as an evolutionary "constraint," because both aspects of locomotor performance are generally expected to be under positive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at testing, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaw's (1987) restricted maximum-likelihood programs, modified to account for our breeding design, which incorporated cross-fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We initially analyzed performances on each trial day and the maximal value. For endurance, the three estimates of narrow-sense heritabilities ranged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The heritability estimate for sprint speed measured on trial day 1 was 0.17, but negative for all other measures. Moreover, the additive genetic covariance between speeds measured on the two days was near zero, indicating that the two measures are to some extent different traits. The additive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistically significant. None of the measures of speed or endurance was significantly genetically correlated with body mass. Thus, we predict that artificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such experiments could lead to a better understanding of the physiological mechanisms leading to trade-offs in aspects of locomotor abilities.

Individuals within a species, living across a wide range of habitats, often display a great deal of phenotypic plasticity for organ mass and function. We investigated the extent to which changes in organ mass are variable, corresponding to environmental demand, across an altitudinal gradient. Are there changes in the mass of oxygen delivery organs (heart and lungs) and other central processing organs (gut, liver, kidney) associated with an increased sustainable metabolic rate that results from decreased ambient temperatures and decreased oxygen availability along an altitudinal gradient? We measured food intake, resting metabolic rate (RMR), and organ mass in captive deer mice (Peromyscus maniculatus bairdii) at three sites from 1,200 to 3,800 m above sea level to determine whether energy demand was correlated with organ mass. We found that food intake, gut mass, and cardiopulmonary organ mass increased in mice living at high altitudes. RMR was not correlated with organ mass differences along the altitudinal gradient. While the conditions in this study were by no means extreme, these results show that mice living at high altitudes have higher levels of energy demand and possess larger cardiopulmonary and digestive organs than mice living at lower altitudes.

This study addresses the quantitative genetic basis of phenotypic variation and covariation for a series of meristic traits in the garter snake Thamnophis sirtalis fitchi (six head scale counts: loreals, supra-and infralabials, pre-and postoculars, temporals; three body scale counts: ventrals, subcaudals, dorsal scale rows at midbody; two derived traits: umbilical scar size and position). Each trait was scored on approximately 540 offspring and their 47 dams captured in the wild while gravid. Correlations of the meristic traits with body mass at birth, dam's snout-vent length and body mass, litter size, and number of days each dam was held under laboratory conditions prior to giving birth were removed by computing residuals from multiple regression equations. Narrow-sense heritabilities (estimated by restricted maximum likelihood) of residuals were high for temporal scale counts (0.59), moderately large for ventral (0.29) and subcaudal scale counts (0.41), and low (in the range 0-0.12) for the other five traits. Probably as a consequence of the low statistical power of significance testing under restricted maximum likelihood, only the heritability for temporal scales was significantly different from zero. Phenotypic (rp = 0.25) and genetic (rg = 0.67) correlations between ventrals and subcaudals were positive and significant. Phenotypic correlations between the head scale counts were generally low; however, the genetic correlations were larger, suggesting relatively tight integration at the genetic level. Phenotypic correlations between the head and body scale counts were generally low, but several genetic correlations were large (e.g., rg =-0.59 for ventrals and infralabials, rg = 0.59 for subcaudals and supralabials). These data indicate that scale counts from different regions of the body are not evolutionarily independent characters, despite their different spatial and temporal relationships during development. Overall, genetic correlations were not strongly correlated with either phenotypic (r = 0.42) or environmental correlations (r = 0.16). EPIDERMAL scales are among the most distinctive features of living Reptilia. The functional significance of variation in reptilian scale counts is generally unclear (but see Hecht, 1952; Bennett and Licht, 1975; Regal, 1975), although numbers of ventral and subcaudal scales [which correspond directly to body and tail vertebrae (Alexander and Gans, 1966; Voris, 1975)] or their ratio may correlate with locomotor performance in garter snakes (Arnold and Bennett, 1988; Jayne and Bennett, 1989; unpubl.), and the number of head scales may affect cranial kinesis during ingestion of prey (S. J. Arnold, pers. comm.). Several early (Dunn,

We tested the hypothesis that locomotor speed and endurance show a negative genetic correlation using a genetically variable laboratory strain of house mice (Hsd:ICR: Mus domesticus). A negative genetic correlation would qualify as an evolutionary "constraint," because both aspects of locomotor performance are generally expected to be under positive directional selection in wild populations. We also tested whether speed or endurance showed any genetic correlation with body mass. For all traits, residuals from multiple regression equations were computed to remove effects of possible confounding variables such as age at testing, measurement block, observer, and sex. Estimates of quantitative genetic parameters were then obtained using Shaw's (1987) restricted maximum-likelihood programs, modified to account for our breeding design, which incorporated cross-fostering. Both speed and endurance were measured on two consecutive trial days, and both were repeatable. We initially analyzed performances on each trial day and the maximal value. For endurance, the three estimates of narrow-sense heritabilities ranged from 0.17 to 0.33 (full ADCE model), and some were statistically significantly different from zero using likelihood ratio tests. The heritability estimate for sprint speed measured on trial day 1 was 0.17, but negative for all other measures. Moreover, the additive genetic covariance between speeds measured on the two days was near zero, indicating that the two measures are to some extent different traits. The additive genetic covariance between speed on trial day 1 and any of the four measures of endurance was negative, large, and always statistically significant. None of the measures of speed or endurance was significantly genetically correlated with body mass. Thus, we predict that artificial selection for increased locomotor speed in these mice would result in a decrease in endurance, but no change in body mass. Such experiments could lead to a better understanding of the physiological mechanisms leading to trade-offs in aspects of locomotor abilities.