Estrous Cycles and Mating Behavior in a Free-Ranging Band of Rhesus Monkeys

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Abstract
The introduced rhesus monkeys on Cayo Santiago, Puerto Rico, have an annual reproductive cycle with estrous periods limited to the months from July to January. The most intensive breeding occurs in September and October. The periodicity of estrus in the 34 mature females of one social group was studied during the 1960 breeding season. Among the 28 which gave birth, 20 apparently conceived during the first estrous period, 6 did not conceive until the second and the time when 2 conceived was uncertain. Distinct estrous periods occurred during the early part of gestation. Thirteen of the 28 pregnant females had at least one such postconception estrous period. Higher ranking males were in general more active breeders than lower ranked animals, but breeding activity did not decrease with decreasing dominance in a regular and predictable manner. Aggression among males as a result of competition for estrous females was not seen.