The Online Encyclopedia and Dictionary







Estrus (also spelled œstrus) or heat in female mammals is the period of greatest female sexual responsiveness usually coinciding with ovulation. Most female mammals will only seek Censored page and are considered most fertile during their heat, also known as the mating season or breeding season. Animals in which this is a prominent feature are usually said to have estrous (adjectival form) cycles.

In human females estrus is often associated with menstrual cycles, but some evidence suggests human females also undergo semi-annual heat cycles. Although sexual responsiveness is influenced by social and cultural factors, there is evidence that human females undergo emotional and physiological changes associated with estrus cycles and may be more likely to be irritable and/or sexualy aroused during the peaks of heat cycles. Further, just as males of other species respond in differing ways to females in heat, human males respond to the "call" of human females in estrus.

The term heat is also applied to the males of some species, e.g, elephant bulls, which is a period of secretion of testosterone-rich fluids.

The period of estrus in the female is preceded by an increase in estrogen (also spelled œstrogen). Evidence of the animal being in heat is signalled by leakage of blood from the vagina, or other changes in secretions and urinary habits.

The phases of the estrous cycle are proestrus, estrous, metestrus, and diestrus. Anestrus refers to a state where the normal cycle stops.

Estrous cycles are usually affected by the seasons. The number of hours daily that light enters the eye of the animal affects the brain, which governs the release of certain precursors and hormones. When daylight hours are few, these animals "shut down," become anestrous, and do not become fertile. As the days grow longer, the longer periods of daylight cause the hormones which activate the breeding cycle to be released. As it happens, this has a sort of utility for these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would militate against their survival.

Domestication of some animals, notably cattle, by man has stopped their seasonal breeding tendencies. The estrous cycles are also temporarily stopped by pregnancy and lactation.

Frequency of female estrous cycles in several mammals:

  • Cow - 21 days
  • Elephant - 16 weeks
  • Goat - 21 days
  • Rodents - 4–6 days
  • Human - 28 days


The Latin oestrus, meaning frenzy, or gadfly. This in turn comes from the Greek oistros, which has the same meaning.

See also

External links

  • Information on the estrous cycle

Last updated: 02-08-2005 16:36:24
Last updated: 04-25-2005 03:06:01