The word estrus comes from the gadfly Hera sent to torment Io
In Greek myth, Hera sent a stinging gadfly to chase Io, whom Zeus had turned into a heifer. The insect's name, oistros, came to mean frenzy, and biologists borrowed it for the hormonal cycle that makes female mammals periodically receptive to mating. Humans, unusually, cycle differently and give no outward signal of ovulation.
Greek writers stretched the word widely. Homer used it for panic, Euripides for madness, and Plato for a soul goaded by desire; Herodotus came closest to the modern sense, applying it to fish driven to spawn. In English it first meant frenzied passion, and only in 1900 did it come to mean an animal in heat. British spelling keeps the old diphthong as oestrus, and in every spelling the noun ends in -us while the adjective ends in -ous.
All mammals share the same hormonal machinery: pulses of a releasing hormone from the hypothalamus, two signalling hormones from the pituitary, and estrogen and progesterone from the ovaries. The difference lies in what happens to the uterine lining when no pregnancy follows. Species with estrous cycles reabsorb it, while humans, elephant shrews and a handful of others shed it through menstruation. Some animals with estrous cycles do bleed, which is easily mistaken for a period.
Biologists describe four phases. In proestrus, follicles grow and the lining builds, lasting a day or up to three weeks depending on species. Estrus is the receptive window, known as being in heat, sometimes flagged by a swollen vulva, mating calls or the lordosis reflex in which a four-legged female raises her hindquarters. Diestrus follows, dominated by progesterone from the corpus luteum; without pregnancy the lining is reorganised rather than lost.
Then comes anestrus, a rest phase that is often seasonal. Day length acts through melatonin from the pineal gland, which suppresses breeding in some species and encourages it in others. Pregnancy, nursing, serious illness and long-term energy shortage can also pause cycling. Farmers exploit this biology, using hormone devices to synchronise cattle and sheep. Males of many ruminants have their own seasonal counterpart, the rut.
Source: Estrous cycle
