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2. The menstrual cycle starts, i.e., menarche occurs, at different ages in different species. In Old World monkeys, such as the rhesus monkey, menarche occurs at about 4 years of age. In the great apes, it occurs at about 8-10 years if the animals are living in nature, at about 6-7 years if they live in captivity where they receive more nutritious food and mature more quickly. In humans, menarche occurs at about 11-14, but this can vary depending upon heredity, diet, and perhaps, climate.
3. The duration of the menstrual cycle varies with species; about 29 days in orang-utans, about 30 days in gorillas and about 37 days in chimpanzees. Incidentally, the duration of estrus also varies in these species; about 4-6 days in female orang-utans, about 2-3 days in gorillas and about 10-14 days in chimpanzees. Both the menstrual cycle and estrus vary in duration somewhat among females of the same species. The figures given here are approximations or averages.
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