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Primate Menstrual Cycle

1. The term, *menstrual cycle* refers to the hormonal and reproductive tissue changes that occur in adult female mammals during their reproductive years. Depending upon the subject matter, the cycle is usually divided into three parts, the follicular phase (the time when the follicle(s) in the ovary are maturing and beginning to secrete estrogen) (hence the term, follicular phase), the midcycle phase (when estrogen levels are maximal and when ovulation occurs (the follicle ruptures and the ovum[egg] is released) and the luteal phase (when both progesterone and estrogen are secreted by the corpus luteum [which developed from the follicle], hence the term, luteal phase. Estrus is the condition of heightened female sexual motivation, it occurs during the midcycle phase (facilitated by estrogen, inhibited during the luteal phase as a result of rising progesterone concentrations) and is reflected by the female's sexual attraction and approach to the male (the female is also most attractive to the male at this time). Estrus is characteristic of most if not all mammals, depending on how estrus is defined. If estrus is defined broadly, it relates to all mammals, perhaps even humans (although this is not generally accepted). Some scientists limit the definition of estrus to only those mammals in which the female exhibits a reflexive response to the male, termed lordosis. If estrus is defined by behavioral evidence of heightened female sexual motivation, then all the great apes exhibit estrus.

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.

4. Estrus varies in duration, as noted above, and it also varies in terms of how the females of different species behave when they are in estrus. (Incidentally, *estrus* is the noun, *estrous* is the adjective.) Among gorillas, the females are very assertive when they are in estrus and they approach the male conspicuously and solicit copulation. Female orang-utans are somewhat similar to the gorillas in this respect; orang-utans do their courting in trees, however, gorillas on the ground. Female orang-utans and gorillas live in one-male mating systems, and this their behavior is thought to be an adaptation to their particular type of mating system. In the absence of intermale competition for estrous females (such as when there is only one adult male in the group who does the mating), the male is relatively passive as regards initiating copulation, and the female is more assertive. Among chimpanzees, which live in a multimale mating system, there is considerable intermale competition for estrous females and the males do most of the sexual soliciting. All this occurs primarily during the midcycle phase, about the time of ovulation, when the female is fertile. All these events, hormonal and behavioral, are described as subserving the requirements of reproduction. The sexual behavior is facilitated by the hormonal conditions at the time when impregnation may occur, i.e., when the ovum may be fertilized. The female's condition of estrus, therefore, is seen as the process by which the female and the male are brought together to copulate at the time when it will result in impregnation.

Ronald D. Nadler
Yerkes Regional Primate Research Center
Emory University
Atlanta, Ga 30322 USA
Fax: 1-404-727-7845
Reproductive Biologist