Individual variation and repeatability of maximum cold-induced energy assimilation in house mice

Maximum cold-induced rate of food consumption in cold-acclimated laboratory house mice Mus domesticus averaged 10.4 g/day in females (body mass before cold exposure = 25.2 g) and 10.7 g/day in males (body mass = 31.7 g); corr esponding maximum rates of energy assimilation were 139 kJ/day in females a nd 144 kJ/day in males. The traits were highly repeatable: the intraclass c orrelation coefficient for three trials at -10 degreesC was greater than 0. 7 for absolute values and greater than 0.6 for values independent of the in itial body mass (residuals from ANCOVA). The estimate of repeatability of t he mass-independent traits depends on which estimate of body mass (initial or after cold exposure) is used as a covariate in the regression or ANCOVA model. We conclude that the values of maximum cold-induced food consumption and energy assimilation are reliable estimates of the performance of indiv iduals under a specific set of environmental conditions (low ambient temper ature, but not limited access to food) and hence may be subject to phenotyp ic natural or artificial selection.