A Case of Hybrid Vigor in the Albino Mus Musculus

Abstract

Two strains of mice, both albino Mus musculus and here designated as H T M and E L T M, are known to differ in the number of tailrings as previously reported (Fortuyn 1 ). They also show differences in the number of young per litter which the mother, in the absence of the father, is able to raise to an age of one month. In each strain this has been tested in the offspring of a single pair so as to reduce variability. Table I gives the size of the litters. In 45 first litters of H T M the average number of young which lived with the mother to an age of one month is 4.75 ± 0.48 (mean error). For 28 second litters this figure is 5.60 ± 0.48. In 32 first litters of E L T M the average number of young, one month after birth, is only 1.53 ± 0.37, in 39 second litters it is 1.74 ± 0.37. The differences with their mean errors amount to 3.22 ± 0.53 for the first litters and 3.86 ± 0.60 for the second, and are therefore statistically significant in both cases. This difference in fecundity is explained by the fact that in H T M only 5 first litters and 2 second ones are completely destroyed by the mother, whereas in E L T M these figures are 18 and 19. Moreover, in those cases where the litter is not completely destroyed, more young survive in H T M, which may have litters of 10 and 11 young than in E L T M which never had litters of more than 7 young. No observations have been made on the question of whether death occurs shortly after birth through the inability of the young to suck, or because the mother has no milk, or kills the young independently from either condition.