Many mammalian species breed seasonally and alterations in the physical environment provide the necessary stimulation. Zuckerman (1932) stated the case for the existence of an uninterrupted sexual life in monkeys and apes, but evidence has since accumulated that establishes breeding with annual rhythmicity in several primate species (Lancaster & Lee, 1965). In the rhesus monkey, a peak of births has been reported between March and May both in North India (Southwick, Beg & Siddiqi, 1961; Prakash, 1962; Lindburg, 1967) and also in the islands near Puerto Rico (Altmann, 1962; Koford, 1965). A survey of the breeding activity of rhesus monkeys in the laboratory shows different findings in different colonies (Ponce de Lugo, 1964), but there is considerable agreement that a similar birth peak occurs in captivity between March and April (Hartman, 1931; Rowell, 1963; Wisconsin data, 1966, quoted by Eckstein & Kelly, 1966). The existence of an annual (here we avoid the term 'seasonal') rhythm in sexual performance and motivation has been difficult to establish with certainty because of the annual disturbance caused by
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