15. The Menstrual Cycle of the Primates.‐Part III. The Alleged Breeding‐season of Primates, with special reference to the Chacma Baboon (Papio porcarius)

Abstract

Summary. During March, April, and May of 1930 observations were made in South Africa on the Chacma Baboon (Papio porcarius). Baboon packs, in which were nursing females as well as females with sexual skin swelling, were seen in the Eastern and Western Provinces. On May 4th twelve females belonging to the same pack were killed near Grahamstown, in the Eastern Province. Four were non‐pregnant: one of these was menstruating, one was in the pre‐ovulation, and two in the post‐ovulation phase of the cycle. Five were pregnant: the first of these had an embryo 2·5 mm. in length, the second one of 16·5 mm., the third one of 19 mm., the fourth one of 65 mm., and the fifth an apparently full‐term male fœtus with a crown‐rump length of 230 mm. Three were lactating, and their babies were caught alive. Data are provided regarding the times of milk‐tooth eruption in baboons, indicating that two of the babies were about two months old and the third about four months. A complete reproductive cycle in the baboon—the shortest interval of time between two births by the same female—is approximately a year. The females shot on May 4th were representative of practically all stages of the cycle. This proves that the Chacma Baboon has no demarcated breeding season, but that it can give birth at any time of the year. The question of the breeding of other Old World primates is discussed. Records of monkey births in captivity indicate that most Old World primates can conceive at any time. The possibility that there exists a seasonal variation in the monkey's conceptility rate does not conflict with the underlying fact that, the baboon, and probably most other Old World primates, has no demarcated breeding‐season. There is a fundamental distinction between mammals with a fixed anœstrus, like the ferret, and mammals like the baboon that have no anœstrus.