REPRODUCTION AND SURVIVAL OF MICE AT 23° AND 32°C

Abstract

Summary. Details of the fertility of two inbred strains of mice, the reciprocal F1 hybrids between the strains and some F2 mice under temperate and high environmental temperature conditions are presented. Infertility was rare in the control mice but common in those exposed to heat. Genotypic differences were greater in the heat and chiefly of male origin, being related to the duration of exposure to the heat. Prolonged heat exposure caused an impairment of fertility which persisted even when the temperature stress was alleviated. There was some prolongation of the oestrous cycle at high temperatures but it was not closely related to the greatly reduced mating frequency which was the prime determinant of the failure to bear litters. The postimplantation and early postnatal mortality indicate a heterotic effect on survival. Prenatal mortality may have been greater in the mice exposed to heat than in the controls but the effect was too small to account for the environmental difference in the sizes of litters at birth. Mortality in the first 3 weeks of life was also greater in the heat where survival seemed to be a function of the inbred mother. The F2 litters had low mortality rates compared with inbred and F1 litters. Parental heterosis was apparent in both environments. The postnatal survival of litters was influenced by the litter's size at birth and its parity and the survival of individual animals was affected by the interval between birth and feeding and the body weight at birth. Both genotype and the temperature of the environment influenced the extent to which these variables operated.