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.