Michael R. Dohm

QUANTITATIVE GENETICS OF SPRINT RUNNING SPEED AND SWIMMING ENDURANCE IN LABORATORY HOUSE MICE (MUS DOMESTICUS)

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 Shaws (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.

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.

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.

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.

Quantitative Genetics of Scale Counts in the Garter Snake Thamnophis sirtalis

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, supraand infralabials, preand 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, dams 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

PHYSIOLOGICAL VARIATION AND ALLOMETRY IN WESTERN WHIPTAIL LIZARDS (CNEMIDOPHORUS TIGRIS) FROM A TRANSECT ACROSS A PERSISTENT HYBRID ZONE

A hybrid zone involving Cnemidophorus tigris punctilinealis (formerly gracilis) and C. tigris marmoratus in southwestern New Mexico and adjacent Arizona is narrow and characterized by abrupt and concordant change in both morphological characters and allele frequencies studied by protein electrophoresis. We compared adult C. tigris sampled from three locations that span the hybrid zone. Body mass was positively associated with both treadmill endurance at 1.0 km/h and maximal sprint running speed on a high-speed treadmill, although the largest individuals were not the fastest sprinters. Males and females differed significantly for maximal sprint ruloning speed, liver mass, and kidney mass (ANCOVA with body mass as covariate). We found no statistically significant population differences for body mass, maximal sprint running speed, standard metabolic rate at 40C, blood hematocrit levels, or heart mass. Hybrids tended to have lower treadmill endurance rlnning capacities as compared with the pure forms, but the difference was not statistically significant. Cnemidophorus tigris punctilinealis and the hybrids both had significantly heavier kidneys, relative to body mass, than did C. tigris marmoratus. Hybrid individuals also had significantly heavier livers as compared with either pure population. However, the present data cannot rule out the possibility that the observed differences in organ masses were related to reproductive status as opposed to being genetically based population differences. Thus, our results do not suggest that hybrid individuals differ from nonhybrids with respect to Darwinian fitness.