Frequency
Some species, such as cats, cows and domestic pigs, are polyestrous and can go into heat several times a year. Seasonally polyestrous animals or seasonal breeders have more than one estrous cycle during a specific time of the year and can be divided into short-day and long-day breeders:
- Short-day breeders, such as sheep, goats, deer, elk—are sexually active in fall or winter.
- Long-day breeders, such as horses and hamsters, are sexually active in spring and summer.
Species that go into heat twice per year, such as most dogs, are diestrous.
Monoestrous species, such as bears, foxes, and wolves, have only one breeding season a year, typically in spring to allow growth of the offspring during the warm season to survive the next winter.
A few mammalian species, such as rabbits, do not have an estrous cycle and are able to conceive at almost any arbitrary moment.
Generally speaking, the timing of oestrus is coordinated with seasonal availability of food, and other circumstances such as migration, predation etc., the goal being to maximize the offspring's chances of survival. Some species are able to modify their oestral timing in response to external conditions.
Read more about this topic: Estrous Cycle
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