Estrous cycle in domestic animals

An estrous cycle is a period of time in a female, between two subsequent heats or estrus. They occur during her reproductive cyclicity, after reaching puberty. Females are receptive to copulation with males and are able to facilitate the transport of spermatozoa through the uterus only during heat. At this moment, their reproductive system is in optimal environmental conditions, close to ovulation and able to become pregnant. Estrous cycles vary between species; there are some differences according to the moment in which non pregnant females present heats or anestrus (period of time of reproductive quiescence during which femalesdo not display estrus). In relation to this, females are classified as:

• Polyestrous continuous, such as cows and sows, present heats between regular periods of time during all the year. They can become pregnant without regard to the season of the year.

• Seasonally polyestrous, including mares, queens, ewes and nanny-goats, present several heats during their reproductive season and remain in anestrus during their non-reproductive season. They are classified as “short day breeders” when they present estrous cycles during autumn when day length is decreasing (goats and ewes) and “long day breeders” (mares and queens) when they show estrous mainly during spring, when day length increases.

• Monoestrus, such as bitches, show one or at the most two heats per year. If they do not become pregnant after the heat, they present a long luteal phase and then remain in anestrous until the following year.

The length of the estrous cycle and the duration of the heat also vary between species (Table 1). Ovulations occur in most domestic species during the last third part of the heat. Exceptions are cows, which ovulate around 12 hours after finishing sexual receptivity, and queens, which are induced ovulators.

quiz

1 An estrous cycle is a period of time in a female, between two subsequent ________ or estrus.

	heats
	pregnancies
	ovulations
	menses

2 Females are receptive to copulation with males and are able to facilitate the transport of spermatozoa through the uterus only during ________.

	pregnancy
	ovulation
	heat
	menstruation

3 Polyestrous continuous animals, such as cows and sows, can become pregnant ________.

	only during a specific season
	without regard to the season of the year
	only during heat
	only after ovulation

4 Seasonally polyestrous animals, like mares and queens, are classified as “long day breeders” when they show estrous mainly during ________.

	winter
	autumn
	spring
	summer

5 Monoestrus animals, such as bitches, show ________ heats per year.

	multiple
	one or at the most two
	three to four
	continuous

References

Johnston S., Root Kustritz M. and Olson P. (2001). The feline estrous cycle in: Canine and feline Theriogenology. Saunders Company. Chapter 25 pp 396-403.

Senger, P., 2005. Pathways to pregnancy and parturition, 2° revised Edition. Current Conceptions, Inc., Washington, USA.