Published online by Cambridge University Press: 27 September 2011
Our ancestors were unique in having the lowest rate of reproduction of any living mammal. This was achieved by the postponement of puberty until well into the second decade of life, a maximal probability of conception of only about 25% per menstrual cycle even when ovulation had commenced, a 4-year birth interval as a result of the contraceptive effects of breast-feeding, and sharply declining fertility during the fourth decade of life, leading to complete sterility at the menopause (Short, 1976, 1983).