The seasonal appearance of mating activities and births of rhesus monkeys in a free-ranging colony on two small islands near La Parguera, Puerto Rico, were recorded from the founding of the colony in 1962 to August 1966. Mating activities began in late August or September, reached a peak during November and December, and ceased in March or April. Except for a wide spread early in the colony's history, births were confined to the period from March to July, with 74% occurring during May and June. Lactating females began mating activities later than non-lactating females. The onset of mating typically followed vegetation changes associated with a period of high rainfall in late summer. The correlation between rainfall and mating activities, perhaps resulting from vegetational changes, also appears to be present at Cayo Santiago, another island colony of monkeys, and in Northern India, the natural home of the rhesus monkey.
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