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. I do not understand what you mean by *intensity* of the menstrual cycle.
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.
Source:
Ronald D. Nadler
Yerkes Regional Primate Research Center
Emory University
Atlanta, Ga 30322 USA
Fax: 1-404-727-7845
nadler@rmy.emory.edu
Reproductive Biologist