Studies on Sterility and Prenatal Mortality in Wild Rabbits

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Abstract
1. Data of prenatal mortality occurring after implantation are derived either from (a) comparison of the number of implantation sites with the number of surviving embryos in pregnant uteri, or (b) the proportion of animals with nothing but dead and reabsorbing embryos in the uteri. The theoretical limitations to which such data are subject are examined and are found to be very different from those applicable to data of mortality occurring before implantation. 2. The proportion of embryos lost in the whole sample of 1834 litters ranging in age from implantation to full term (7-32 days) varies according to the size of litter at implantation as the linear regression Y=0.00084x+0.0709, where x=the number of implantation sites. The proportion of litters showing loss varies similarly as Y=0.02093x+0.0866. It is shown that the relation of the proportion of embryos lost to the proportion of litters showing loss does not accord with the assumption that the whole loss is falling at random upon the embryos as units, but that part of the loss must be falling on the litters as units. Such a mortality theoretically would be distributed as E=½l+p(1-l), and L=lqx-qx+1, where E=the proportion of dead embryos, L=the proportion of litters showing loss, l=the proportion of litters in process of being lost as units, p=the proportion embryos lost as units, q=1-p, and x =the number of implantation sites. This distribution provides a good fit for the data. 3. Altogether 164 animals were obtained in which all the embryos had died and were reabsorbing. The age at which the last embryo died could be determined in 101 of these. These data show that the greater the number of embryos which become implanted the less is the probability of the litter being lost, and that the majority of these litters were lost between the 11th and 15th days of gestation inclusive. 4. There is a corresponding maximum in the proportion of living litters containing some reabsorbing embryos between the 10th and 15th days of gestation inclusive. Not all the mortality in the 16-2o-day age group of living litters in excess of that in later age groups can be accounted for by reabsorption of whole litters. Therefore, either some litters must be aborted at this stage or some old reabsorption sites must be overlooked in the counts at subsequent stages or both must occur. 5. The proportions of litters lost as units and of embryos lost as units in surviving litters have been estimated in successive age groups of living litters. The estimated maximum proportion of litters in process of being lost in toto was 0.56 on the 12th day of gestation. The estimated proportion of litters lost, based on the proportion of living to dead litters obtained and allowing for the rate of reabsorption, was 0.35, but this estimate excludes any loss by abortion. The estimated proportion of litters lost, based on the relative frequency of animals obtained in early and late stages of pregnancy, was 0.355. 6. The proportion of litters lost in toto declines steeply with increasing body weight of the mothers. 7. The mortality varies with the mammary activity of the mothers, being greatest in those animals that were certainly lactating. 8. The incidence of the loss of whole litters varies both (a) from one locality to another in Great Britain, and (b) from one year to another in the same locality. 9. It is probable that the incidence of mortality is greater at the beginning and end of the breeding season than during its height. 10. An attempt is made to estimate the total mortality suffered both before and after implantation in those litters that survive to near full term. Of such litters not less than 41% have suffered some loss and not less than 11% of the ova ovulated have been lost.