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The ovarian cycles of four adult female spider monkeys (Ateles geoffroyi) were followed daily throughout 30 days by means of vaginal swabs and blood samplings. Cytological analyses of the vaginal swabs and radioimmunoassay determination of the daily levels of estradiol‐17β (E2) and progesterone (P4) were done in order to classify the kind of ovarian cycle of this species. Our results show that Ateles geoffroyi females display menstrual cycles of about 24 days on average. By comparison with the well‐known menstrual cycles of women, apes, and Old World monkeys, the four distinctive cytological phases (bleeding, follicular, periovulatory, and luteal) could be recognized; mid‐cycle E2 peaks followed by mid‐luteal increases of the same hormone were present in all four females. P4 levels were higher after the E2 peak, although both hormones were present throughout the cycles. Also, age‐dependent features, hormone profiles, and changes in menstrual phases lengths were detected. Am. J. Primatol. 44:183–195, 1998. © 1998 Wiley‐Liss, Inc.

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