Estrous cycle

The estrus cycle, also known as the oestrous cycle (Latin oestrus, originally from Greek οἶστρος meaning sexual desire; slang In heat,) comprises the recurring physiologic changes that are induced by reproductive hormones in most mammalian therian females.

Unlike humans and other primates, whose constant sexually receptive menstrual ovulation is not readily apparent, nearly all placental mammals go thought this estrus cycle to show their readiness to breed (only) at specific (often annual) times.

Estrous cycle and furry
Being anthropomorphic in nature, female furries have the fantasy choice of sexually role-playing between behaviors characteristic of animals that go through either menstrual, estral, or both cycles, only limited by user imagination. (fan, artists, others).

In furry culture, as with in real life (with the exception of hermaphrodite or MPreg furs), males furries do not go in heat, only aroused in furry life by the the heat cycle of the female furry character, menstrual or estral.