The Reproduction of the Wild Rabbit Oryctolagus cuniculus (L.).

Summary. 1The material consisted of 957 males, 1,529 females, and 1 inter-sex of wild rabbits obtained in Caernarvonshire between February 1941 and June 1942, inclusive. 2No sexual dimorphism in cleaned body weight was observed. The 1150–1199-gm. weight group was that of greatest frequency in both sexes, and very few animals were obtained exceeding 1500 gm. in weight. Body weight does not provide a good criterion for distinguishing adults from young in either sex. The average weight of young at birth probably is between 40 and 45 gms. 3The testes of adults reach a maximum size in April. They then decline in weight by over 60 per cent., reaching a minimum in July. The testes begin to increase in weight again in August. The drop in weight is accompanied by a decline in spermatogenesis, and it begins during the height of the breeding season, one month before the decline in the pregnancy rate. 4Copulation occurs at least twice as frequently with non-pregnant females which are not in œstrus as with those that are. Copulation also occurs frequently with pregnant females at all stages, but especially during the first four days of gestation. These copulations with females that are not in heat are most frequent during November, December and January, that is, during the quarter preceding the breeding season. 5The breeding season in Caernarvonshire is sharply defined; it begins in January and ends in June. More than 50 per cent, of females were pregnant for 17 weeks in 1941 and for 15 weeks in 1942. The 1942 breeding season was slightly earlier than that in 1941. 6Practically all females become pregnant again at each post-partumœstrus during the height of the breeding season. An exception is provided by the period of severe frost in February and March 1942, during which many animals failed to become pregnant at the post-partumœstrus. During March April and May over 80 per cent, of animals suckling are pregnant. 7Young females born early in the breeding season may breed before the end of it, or in the late summer or autumn. 8True pseudopregnancy is very rare in the wild animals. 9It was concluded that the time of the onset of the breeding season is determined by the females, but that its close is determined by the males. 10The wild rabbit differs from the tame rabbit chiefly in (a) the sharp delimitation of the breeding season, (b) the intensity of breeding during the season, (c) the maintenance of pregnancy during lactation normally, and (d) the extreme rarity of true pseudopregnancy. 11The pre-natal mortality during 1941 has been dealt with in a preliminary paper (Brambell, 1942). The conclusions arrived at are confirmed and extended. The proportion of litters showing loss was slightly higher in 1942 than in 1941, but this discrepancy is accounted for by (a) the difference in litter size in the two samples, and (b) experimental error. 12It is estimated that 60 per cent, of litters conceived are lost owing to the death and reabsorption of all the embryos. 13The proportion of litters showing loss is related directly to the initial size of litter. It is not related to (a) the cleaned body weight of the mother, (b) whether the litter is the first of the season or a subsequent one, (c) whether the mother is or is not suckling. 14It is shown that animals which have borne a living litter are as likely to lose the next, and no more likely, than those that have lost a litter already. 15The majority of the embryos die on or about the twelfth day. 16The possible causes of this extraordinary mortality are discussed. 17The loss of ova in litters that survive to birth is between 9 and 10 per cent. 18The number of ova ovulated at œstrus, as shown by the number of corpora lutea in the ovaries, varied from one to nine, five being the most frequent. The mean number was 5·36 for the whole sample. There was a significant difference in the mean numbers for 1941 and 1942 respectively, the former being 4·89 and the latter 5·64. The mean number of ova ovulated is related directly to body weight. It is shown that for a given body weight the mean number of ova ovulated was greater in 1942 than in 1941, but the increase in the mean number of ova ovulated for a given increase in body weight was similar in the two seasons. 19It is calculated that the most productive initial litter sizes are five and six, the inflection in the curve of number of embryos expected to survive according to initial litter size being at 5·7. 20A few instances in which the number of embryos exceeded the number of corpora lutea are recorded. These must have been due either to poly-ovular follicles or poly-embryony. Six instances of transperitoneal migration of ova are recorded, five being from right to left and one from left to right. 21It is estimated that the mean number of young born to each adult female is between 10·35 and 11·70 per annum. 22The post-natal sex-ratio of the whole sample was 50·49 ±0·76 per cent. males. The fœtal sex-ratio was 48·65 ± 1·045 per cent, males, which does not differ significantly from the post-natal ratio. The frequency of sex combinations in litters in utero shows an excess of observed over expected values in the central classes which may be significant. 23One definite intersex and some other abnormalities were encountered during the course of the work.