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Characterization of the oestrous cycle and mating season of squirrel monkeys from copulatory behaviour

Summary. Observations of the behaviour of squirrel monkeys, including 8 opposite-sex pairs during daily 30-min social encounters and 2 mixed-sex permanent groups during daily 30-min observation sessions, over a 14-month period were used to determine the periodicity of the oestrous cycle and annual mating season. The median and modal length of the oestrous cycle was 8 days, within which copulations were limited to a 1–2 day period. In a cyclic female, plasma progesterone levels over a 24-day period dropped from 85–151 ng/ml to 25 ng/ml 2 days before oestrus. In non-cyclic females plasma progesterone values were < 15·4 ng/ml. Males exhibited a 6·8–19·7 week 'season' of copulation and ejaculation. The onset of this 'mating season' in August coincided with the annual peak in male body weight (the 'fatted male' phenomenon).