The quantitative genetics of maximal and basal rates of oxygen consumption in mice.

A positive genetic correlation between basal metabolic rate (BMR) and maximal (VO(2)max) rate of oxygen consumption is a key assumption of the aerobic capacity model for the evolution of endothermy. We estimated the genetic (V(A), additive, and V(D), dominance), prenatal (V(N)), and postnatal common environmental (V(C)) contributions to individual differences in metabolic rates and body mass for a genetically heterogeneous laboratory strain of house mice (Mus domesticus). Our breeding design did not allow the simultaneous estimation of V(D) and V(N). Regardless of whether V(D) or V(N) was assumed, estimates of V(A) were negative under the full models. Hence, we fitted reduced models (e.g., V(A) + V(N) + V(E) or V(A) + V(E)) and obtained new variance estimates. For reduced models, narrow-sense heritability (h(2)(N)) for BMR was <0.1, but estimates of h(2)(N) for VO(2)max were higher. When estimated with the V(A) + V(E) model, the additive genetic covariance between VO(2)max and BMR was positive and statistically different from zero. This result offers tentative support for the aerobic capacity model for the evolution of vertebrate energetics. However, constraints imposed on the genetic model may cause our estimates of additive variance and covariance to be biased, so our results should be interpreted with caution and tested via selection experiments.

Individual variation in locomotor behavior and maximal oxygen consumption in mice.

Individual differences in open-field activity and emotionality (number of defecations and urinations), voluntary wheel running, voluntary and forced maximal sprint running speed on a photocell-timed racetrack, swimming endurance, and maximal oxygen consumption (VO2max) were studied in 35 random bred male ICR mice. With the exception of latency in the open field and voluntary speed on a racetrack, all measurements were significantly repeatable on two successive trial days. Maximal oxygen consumption (corrected for body size) was positively correlated with amount of wheel running during each day of a 7-day test, and the correlation became stronger throughout the testing period; however, none of the daily correlations reached statistical significance. The first factor from a principal components (PC) analysis showed positive loadings (component correlations) for all measures of speed in the open field, for both voluntary and forced maximal speeds on the racetrack, and for VO2max, but a negative loading for emotionality. Wheel running and VO2max loaded positively on PC 2. Only swimming endurance loaded strongly on PC 3; this trait was uncorrelated with any other measure of physiology or behavior. These results suggest that measures of both voluntary and forced locomotor speed, as well as amount of voluntary wheel running, may be related to aerobic physiological capacities in untrained mice.

Maximal sprint speeds and muscle fiber composition of wild and laboratory house mice.

We compared males from four groups of house mice (Mus domesticus), all bred and raised under common conditions in the laboratory: randombred Hsd:ICR; a wild population from Wisconsin; hybrids from lab dams; hybrids from wild dams. Wild mice were much faster sprinters (maximal forced sprint speeds over 1.0 m ranged from 2.38 to 3.34 m/s) than were lab mice (range = 0.89-1.68 m/s). Hybrids exhibited intermediate speeds (range = 1.54-2.70 m/s) and body masses, indicating largely additive inheritance. Type-specific mean muscle fiber cross-sectional areas of the gastrocnemius muscle did not differ significantly among groups. Percentage cross-sectional areas occupied by each of the three identified fiber types also did not differ significantly among groups, nor did they covary with body mass. For their body mass, however, lab mice had smaller gastrocnemius muscles than did wild and hybrid mice, which had muscles of similar size. Although we cannot rule out the possibility that smaller gastrocnemius muscles or slight differences in fiber composition account for the lower sprint speeds of the lab mice, we suggest that differences in unmeasured physiological, behavioral or motivational factors are probably the primary cause. This interpretation is supported by a lack of correlation between individual differences in sprint speed and either relative gastrocnemius muscle mass or muscle fiber type composition.

Morphological and physiological responses to altitude in deer mice Peromyscus maniculatus.

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.

Exercise physiology of wild and random-bred laboratory house mice and their reciprocal hybrids.

We conducted a "common garden" experiment to compare aspects of exercise physiology and voluntary wheel-running behavior in wild and random-bred (i.e., non-inbred) laboratory house mice and their reciprocal crosses. Analysis of covariance indicated that, after effects of body mass and other appropriate covariates (e.g., age at testing) were accounted for, wild (range 2.46-3.30 m/s, n = 12) and hybrid (range 1.69-3.30 m/s, n = 24) mice exhibited forced maximal sprint running speeds that averaged approximately 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 (VO2max) 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 7-day test on voluntary activity wheels, both wild and hybrid mice ran significantly more total revolutions (+101%), 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 organisms 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.

Genetic variation and correlations between genotype and locomotor physiology in outbred laboratory house mice (Mus domesticus).

Laboratory strains of house mice (Mus domesticus) are increasingly used as model organisms in evolutionary physiology, so information on levels of genetic variation is important. For example, are levels of genetic variation comparable to those found in populations of wild house mice? We studied allozymes to estimate genetic variation in outbred Hsd:ICR mice, which have been used in several studies with evolutionary emphasis. The physiological significance of allozyme variation remains obscure. Several workers have reported relationships between multi-locus heterozygosity and metabolic traits, but endotherms have not been studied. Therefore, we also measured mice for basal metabolic rate (BMR), maximal oxygen consumption during forced treadmill exercise (VO2max), and 12 other traits related to locomotor physiology, before genotyping them for 10 allozyme loci. Four of these loci were polymorphic, all were in Hardy-Weinberg equilibrium, and inbreeding coefficients were not significantly different from zero. Average heterozygosities were 11%, similar to values reported for wild populations of house mice. Fourteen percent of the associations between single-locus genotype and physiological traits were statistically significant. Multi-locus heterozygosity was not significantly related to VO2max, but was positively correlated with BMR, a result opposite to the negative correlation between standard metabolic rate and heterozygosity reported in many ectotherms. Therefore, the proposed mechanisms for the effect of multi-locus heterozygosity on metabolic rate in ectotherms may not apply to endotherms.

Effects of ozone on evaporative water loss and thermoregulatory behavior of marine toads (Bufo marinus).

Ozone (O(3)) is a strong pulmonary irritant and causes a suite of respiratory tract inflammatory responses in humans and other mammals. In addition to lung injury, rodents exposed to O(3) exhibit a pronounced decrease in core body temperature at rest, which may offer a protective effect against O(3) damage. The effects of O(3) on other vertebrates have not been studied. Compared to individuals exposed to air (N=34), Bufo marinus toads exposed to O(3) (N=32) for 4 h lost 3.78 g body mass (adjusted mean from analysis of covariance, body mass mean+/-SD, 90.1+/-21.90 g). We tested the thermoregulatory responses of 22 toads in a thermal gradient 1, 24, and 48 h after 4-h exposure to air (N=11) or 0.8 ppm O(3) (N=11). Individual toad thermal preferences were also significantly repeatable across all trials (intraclass correlation=0.66, P <0.001). We did not observe a direct effect of O(3) exposure on the preferred body temperatures (PBT) of toads. However, O(3) exposure did have an indirect effect on selected temperatures. Ozone-exposed toads with higher evaporative water loss rates, in turn, also selected lower PBT, voluntary minimum, and voluntary maximum temperatures 24 h post-exposure. Ozone exposure may thus alter both water balance and thermal preferences in anuran amphibians.