MILK-PRODUCTION AND CONSUMPTION AND GROWTH OF YOUNG OF WILD MICE AFTER 10 GENERATIONS IN A COLD ENVIRONMENT

Three classes of wild house mice, M. musculus, were studied: mice of the 10th generation in captivity in an environment kept at 23.degree. C (controls); a similar stock kept at 3.degree. C (Eskimo); offspring of controls transferred at mating to 3.degree. C (immigrants). For analysis of chemical composition, samples of milk were drawn from lactating females after they had been injected with oxytocin. For estimates of milk yield, lactating females were injected with tritiated water, and yield was calculated from the tritium content of young aged 10 days, over a 24 h period. The milk of Eskimo females had a much higher proportion of both fat and protein than had milk of other classes. The milk of immigrant females had more fat but less protein than that of controls. Eskimo females were heavier than immigrant females; control females were the lightest. Eskimo young at 10 days were heavier than those of the other classes. Their body fat was much higher; but their fat-free body weight was also above that of the others. Young of both classes in the cold drank more milk than the controls, and the Eskimo more than the immigrant young. Maternal body weight, milk intake by the young and body weight of the young were intercorrelated, but milk intake was the principal determinant of the body weight of the young. There was no evidence of differences in the growth potential of the young of different classes. Higher milk intake in the cold environment was an ontogenetic adaptation to low temperature. Superior growth and growth and greater concentrations of milk solids evidently reflected a genetical change in the Eskimo population. The differences observed represented 2 ways in which a small population of house mice could adapt rapidly to an adverse environment.