<Li> Long - day breeders, such as horses, hamsters and ferrets are sexually active in spring and summer . </Li> <P> Species that go into heat twice per year are diestrous . </P> <P> Monoestrous species, such as bears, foxes, and wolves, have only one breeding season per year, typically in spring to allow growth of the offspring during the warm season to aid survival during the next winter . </P> <P> A few mammalian species, such as rabbits, do not have an estrous cycle and are able to conceive at almost any arbitrary moment (comparable with humans, who, however, have a menstrual cycle in place of an estrous cycle). </P>

Where does the term in heat come from