Do mammals other than humans have menstrual cycles?

ANSWER 1: Menstrual cycles vary in length from an average of 29-32 days in orangutans to an average of 25-37 days in chimpanzees.

Females of other mammalian species go through certain episodes called `estrus' or `heat' in each breeding season.

During these times, ovulation occurs and females become receptive to mating, a fact advertised to males in some way.

If no fertilisation takes place, the uterus reabsorbs the endometrium: no menstrual bleeding occurs. Significant differences exist between the estrus and the menstrual cycle.

Some animals, such as domestic cats and dogs, do produce a very short and mild menstural flow, however due to its small amount (and personal cleanliness in cats) it passes pet owners largely unnoticed.

Manasa Nagarajan
Chennai

ANSWER 2: Mammals other than primates do not menstruate and their sexual cycle is called an estrous cycle.

In mammals the sexual activity of the male is more or less continuous but in most species the sexual activity of the female is cyclic. Most of the time the female avoids the male and repulses his sexual advances.

Periodically, however, there is an abrupt change in behaviour and the female seeks out the male, attempting to mate.

These short episodes of heat or estrus are so characteristic that sexual cycle in mammalian species that do not menstruate, is named the estrous cycle.

The change in female sexual behaviour is brought on by a rise in the circulating estrogen level. Rabbit and ferret come into heat and remain estrous until pregnancy or pseudo pregnancy results. In these species ovulation is due to a neuroendocrine reflex.

Stimulation of the genitals and other external stimulation at the time of copulation provokes release from the pituitary of gonadotropin that makes the ovarian follicle to rupture.

In captivity, monkeys and apes mate at any time. But in the wild, females accept the male more frequently at the time of ovulation.

In rats the underlying endocrine events are essentially the same as those in the menstrual cycle, but the first day of the cycle is the first day of bleeding, while day one of an estrous cycle is the first day of heat.

M. Vaitheeswaran
Thoothukudi, Tamil Nadu