Studies on Sterility and Prenatal Mortality in Wild Rabbits

Abstract

1. The technical problems underlying the estimation and analysis of the total prenatal mortality in a mammal are reviewed, with particular reference to the rabbit. 2. The material consisted of 7137 wild rabbits, of which 5089 were females. These comprise eleven series, according to the year, locality and treatment, particulars of which are given. 3. The experimental error in counts of corpora lutea was investigated and the technique critically examined. The changes in size of the corpus luteum during pregnancy were measured and a growth curve constructed. The mean number of corpora lutea in a litter, as determined from macroscopic counts, was found to be significantly lower at the beginning and end of pregnancy than in the middle. If this is due to omissions in the counts it represents a maximum error of 6.8% of corpora lutea. Microscopic counts of the corpora lutea in freehand sections of 180 pairs of ovaries of pregnant animals were made as a control. These revealed a total error in the original counts of 6.4% of the corpora lutea, affecting 25.6% of the litters, but this was distributed evenly throughout gestation. 4. Corpora lutea atretica formed from unruptured follicles are present in 16% of the pairs of ovaries, and comprise less than 2% of all corpora lutea. They are formed at the same time as the normal corpora lutea, and there is no evidence that any are formed subsequently during pregnancy in the rabbit. 5. It is estimated that 0.23% of the follicles which ovulate produce two embryos through the liberation of two ova. 6. Transperitoneal migration of 0.37% of ova, affecting 2% of the litters, was found to occur. The probability of migration either from left to right or from right to left appeared to be equal. 7. Significant changes in the mean number of implantation sites in the uteri counted at successive stages of pregnancy were observed, and particularly a decline at the end of gestation. They are of the same order as those in the mean number of corpora lutea, which they will tend to compensate so far as estimates of loss of ova before implantation are concerned. It is by no means clear that these are due to experimental error in the counts. 8. Estimates of the loss of embryos before implantation are based on the difference between the number of corpora lutea in the ovaries and of implantation sites in the uteri, and estimates of the loss after implantation on the difference between the number of implantation sites and of developing embryos. No significant relation has been found between the loss before and after implantation respectively, and it is concluded that the loss suffered after implantation is distributed independently of that which occurs before implantation.