Metabolism and Thermoregulation in Crosses between Wild and Random-Bred Laboratory House Mice (Mus domesticus)

We conducted a "common garden" experiment to compare aspects of thermoregulation and metabolism in wild and random-bred laboratory house mice (Mus domesticus) and their reciprocal crosses. Analysis of covariance (ANCOVA) indicated that, after accounting for effects of body mass and other appropriate covariates (e.g., age), wild (N = 12) and hybrid (N = 25) mice together tended to have higher (+9.7%; P = .0904) maximal nonshivering thermogenesis (max-NST) in response to norepinephrine injection than did lab mice (N = 18). Wild (-17.2%; P = .0941) and hybrid (wild dam: -23.1%; P = .0001; lab dam: 11.2%; P = .0749) mice also tended to have lower basal metabolic rates (BMRs) than did lab mice. As a consequence of these trends, wild and hybrid mice, analyzed together, had signijicantly higher (+ 18.2%; P = .0263) regulatory NST (max-NST minus BMR) than did lab mice. Paralleling the dzfferences in regulatory NST, wild and hybrid mice together had signz3cantly more (+21.2%; P = ,0129) interscapular brown adipose tissue than did lab mice. However, plasma thyroid hormone levels (triiodothyronine [T,] and thyroxine [T*]) showed no signif icant dzfferences among groups, and, even for the traits that exhibited signzjicant dzfferences, the overlap of values for individual mice was substantial. Moreover, mean values for max-NST and BMR for all four groups fell well within the range of values reported in the literature for small mammals. We therefore concluded that random-bred laboratory mice are not necessarily "degenerate" with respect to their metabolic and thermoregulatory capacities. Finally, the two reciprocal cross groups dzffered in BMR (P = .0314), which thus indicated a maternal in .uence: wild-dam X lab-sire mice had signz$cantly lower BMRs than did lab-dam