<P> A mare may be 4 to 10 days in heat and about 14 days in diestrus . Thus a cycle may be short, around 3 weeks . Horses mate in spring and summer, autumn is a transition time, and anestrus rules the winter . </P> <P> A feature of the fertility cycle of horses and other large herd animals is that it is 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 that activate the breeding cycle to be released . As it happens, this benefits these animals in that, given a gestation period of about eleven months, it prevents them from having young when the cold of winter would make their survival risky . </P> <P> Rats typically have rapid cycle times of 4 to 5 days . Although they ovulate spontaneously, they do not develop a fully functioning corpus luteum unless they receive coital stimulation . Fertile mating leads to pregnancy in this way, but infertile mating leads to a state of pseudopregnancy lasting about 10 days . Mice and hamsters have similar behaviour . The events of the cycle are strongly influenced by lighting periodicity . </P> <P> A set of follicles starts to develop near the end of proestrus and grows at a nearly constant rate until the beginning of the subsequent estrus when the growth rates accelerate eightfold . Ovulation occurs about 109 hours after the start of follicle growth . </P>

Where did the term in heat come from