PRENATAL MORTALITY IN MAMMALS

Summary 1. The intention of this article is to provide a critical examination of the methods of estimating the amount, and of analysing the distribution, of prenatal mortality in mammals, in the light of researches on wild rabbits. 2. The reasons why the rabbit provides particularly favourable material for such studies are considered. A summary of the oestrous cycle and embryology provides an essential basis for study of the prenatal mortality. 3. The size of litter at conception is limited by the number of ova ovulated at oestrus, which varies from species to species. The number ovulated at the preceding oestrus in any individual can be estimated from the number of corpora lutea in the ovaries. The magnitudes of, and factors responsible for, errors in such estimates are examined. The frequency distribution and the relation to body weight of the number of ova ovulated are examined. Transmigration of ova from one ovary to the uterus of the opposite side may occur. 4. Several indirect methods of estimating the total prenatal mortality are available, and both the conditions under which each is applicable and the limitations of the results which each can yield are analysed. 5. Estimates based on counts made at autopsy of corpora lutea and of surviving embryos are of little value unless counts of implantation sites are included. It is then possible to separate data of loss before implantation from those of loss after implantation. The nature and limitations of these two sets of data are different. 6. The data available in the literature of loss before implantation in various species are considered. The loss of ova before implantation in wild rabbits is between 10.2 and 13.0 % of those ovulated, of which between 1.0 and 3 . 6% are lost in litters in which none of the ova survive to become implanted. The distribution of the loss and its significance are examined. 7. The data available in the literature of loss after implantation in various species are summarized. It is shown that estimates based on such data may be very misleading. Analysis of the loss after implantation in wild rabbits shows that not less than 35 % of the litters surviving implantation are lost in toto , mainly between the nth and 15th days post‐coitum. Between 1.0 and 3.5% of the embryos which have implanted are lost in the surviving litters. The distribution of the mortality, which is complex, is analysed and the bearing of the statistical, histological and experimental results obtained on the problem of the causes of the loss is discussed. 8. The total loss of ova ovulated in wild rabbits both before and after implantation combined, but excluding loss at parturition, is 43.3 % on the lowest estimate, of which total 10.2% are lost before implantation and most of the remainder before mid‐term. The loss of ova in litters that do not survive is 35.7%, and in litters that do survive is 7.6%. 9. The method of removal of dead embryos from the uterus is described and the processes of reabsorption and abortion are compared. The significance of these processes in relation to studies on prenatal mortality is discussed.