Studies on Sterility and Prenatal Mortality in Wild Rabbits

Abstract

1. The relation of the proportion of litters showing loss to the proportion of embryos lost is discussed and is shown to vary according to the way in which the mortality is distributed. 2. The proportion of ova lost before implantation in litters that survive varies according to the number of ova ovulated as the linear regression Y= 0.01395x+0.00895, where x = the initial number of ova in the litter. The loss of ova before implantation in the whole sample of 12,551 ova was 1188 or 9.5%. 3. The proportion of litters suffering loss before implantation in which some embryos survive implantation varies according to the number of ova ovulated as the linear regression Y=0.0858x-0.1289, where x=the initial number of ova in the litter. The number of litters suffering loss before implantation in which some embryos survive implantation in the whole sample of 2179 litters was 796 or 36.5 %. 4. Fewer litters suffered more loss than would be expected if the loss were distributed at random amongst the ova. The sample of litters showing loss is not homogenous, and the probability of loss of ova is greater in some of them than in others. 5. In litters that suffer a loss of two or more ova there is a tendency for the loss to be concentrated in one uterus of each pair. Some or all of this tendency may be accounted for by obstruction of the uterine tract on one side. The number of animals out of a total of 2073 which had lost all the ova on one side, but not on the other, was 92, an excess of 18, or 1.8%, over the expectation on a random basis. The occurrence of loss on one side does not affect the probability of loss on the other side. 6.Not more than 3.64% of litters are lost entirely before implantation and probably not less than 1.00%. The percentage of litters lost entirely before implantation is at a minimum in litters of an initial size of five or six, the most frequent sizes, and at a maximum for litters of the largest and smallest sizes. It is also at a minimum during the height of the breeding season, and is higher at the beginning and end. 7. Animals above the average body weight suffer less loss of ova before implantation in litters that survive than do those below the average body weight. There is no detectable seasonal variation in the loss of ova before implantation in litters that survive. 8. The loss of ova before implantation in litters that survive is heavier in animals with no milk in the mammary glands than in those with milk.