Reproduction in mammals is a closely orchestrated process wich requires the coordinated preparation of many tissues in the body of the female. Not only must an ovum be liberated from the ovary, the tissues of the female reproductive tract must be ready to transport both eggs and sperm to a common site when fertilization can occur. In the event of fertilization, the early embryos must be carried to the portion of the uterus that is prepared both to receive an embryo and to meet its nutritional requirements throughtout the duration of pregnancy. In the behavioral realm, the female must signal to the male her readiness for copulation, and the male in turn must be ready to respond. Most of the preparations for reproduction are a cyclic nature. The changes in structural and functional characteristics of both male and female reproductive tissues are mediated by hprmones, often interacting in tightly controlled feedback loops.
Originally the term estrus referred merely to the existance of a period of strong sexual desire made evident through behaviour, but it has become evident that estrous occurs close to the time of ovulation and that the characteristic behaviour is simply and external indication that all the complicated internal mechanisms of reproduction are ready to become functional. If pregnancy does not occur at this time, regressive changes follow and another period of preparation must ensue before conditions are again favourable for reproduction. This repeated series of changes is known as the estrous or sexual cycle. In the absence of pregnancy its phases are (1) a short time of complete preparedness for reproduction occompanied by ovulation and increased sexual desire (estrous), (2) period during which the fruitless preparation for pregnancy undergo regression (postestrum or metestrum) , and (3) a period of rest (diestrum) followed by (4) a period of active preparatory changes (proestrum) leading up to the next estrous, when everything is readiness for reproduction.
The length of time occupied by the estrous cycle varies widely among animals. In some it occurs only once in an entire year, with the estrous being so placed seasonally that when the young are born, conditions are favourable for their rearing. (In such animal, the long race period between cycle is called an estrous.) Species having only breeding season in a year are said to be monestrous. Other animal axhibit several breeding periods in a year; they are said to be poliestrous. A poliestrous rhytem is the underlyng condition in mammals generally. Many animals (sheep, for example) that have only one breeding season a year when living in the wild state develop a polistrous rhythem during the breeding season when living under domestication. Other mammals with short period of gestation, such as the rabbit, maybe poliestrous except in the winter. In these animals, like is a critical initiating factor. Only when the average daily amount of light gets above a certain the threshold does the hypophysis become active in the production of enough follicle-stimulating hormone (FSH) to set the whole reproductive cycle into operation.
source : Carlson, Bruce M. 1958. "Patten's Foundations of Embryology". USA: McGraw-Hill