ON THE RELATION BETWEEN BIRTH WEIGHT AND LITTER SIZE, IN MICE

For mice, as for various other mammals, the relation between number N of young in a litter and the weight W of the litter can be expressed as W = aN(K). For adequately homogeneous data K has the nonspecific value 0.83. With data not homogeneous with respect to certain conditions the equation may still be descriptive, but with K higher than 0.83. Two kinds of mice obeying this formulation, with the same K, are an albino strain (AA) and a flex-tail foetal anemic (aa). Their ideal weights of a litter of 1 (W (1), free from effects of intrauterine competition) are quite different. Their F (1) offspring (from AA mothers) give W (1) precisely intermediate. To test the partition theory for the basis of the parabolic equation, backcross and F (2) litters were obtained in which for a span of litter sizes there occurred various proportions of anemic to non-anemic young. For equal numbers of each in the same litters the relation of weight of aa to weight of Aa young is again described by W(a) = aW(A)(K), and as before K = 0.83. Examination of the weights of anemic and of non-anemic young, for various proportions of the two in litters of different total numbers, shows that the partition theory can account for a number of the curious relations, including the fact that aa young and Aa young if in mixed litters increase in weight more for an increment of 1 in the litter than if in unmixed litters of the same N. This mechanical result of partitioning can be regarded as a kind of model for heterosis resulting from developmental disharmony.