Hereditary Variations in Litter-Size of Rabbits

Abstract

An analysis of 569 pregnancies occurring in our breeding colony during the 5-year period from 1929 to 1933 has indicated the presence of wide variations in the mean gestation periods of different breeds. 1 These differences were attributed to hereditary factors. The present report is concerned with an analysis∗ of the size of the litters resulting from these 569 pregnancies, the particular purpose being to ascertain whether breed had any influence on litter-size.

The pregnancies were the result of matings made in all months with the exception of July and August. Eleven breeds consisting of 10 standard bred strains and one intensely inbred line of albinos which may be considered as a breed or family are represented. The mean litter-size ranged from 3.92 for Polish rabbits to 7.14 for the Beveren and Flemish breeds.

It was found that the variance between breeds was significantly greater than the variance within breeds (Var. between means of breeds = 40.43; Var. within breeds = 4.67; F = 8.65; P = 0.01—, significant). The demonstration of heterogeneity between breeds is interpreted as indicating that with respect to litter-size, each breed represented a homogeneous, normally distributed population. Certain environmental factors which might account for the breed differences were considered.

All animals received the same diet and were housed indoors under uniform conditions. Seasonal factors at the time of mating did not influence the litter-size, since a significant difference was not noted between the mean values calculated from all matings in the 2 intervals from March to October, and from November to February (March to October: n = 295, M_n = 5.18 ± 0.14; November to February: n = 274, M_n = 5.47 ± 0.14, D = 0.29 ± 0.19, t = 1.5, not significant). Moreover, the variance within 2 months intervals was larger than the variance between 2 months intervals, although the difference was not significant (Var. between means of 2 months classes = 11.53; Var. within 2 months classes =5.30; F = 2.18, not significant). With respect to season therefore, the population appeared to be homogeneous.