Period in the sexual cycle of female mammals , except the higher primates , during which they are in heat (ready to accept a male for mating).
Some animals (e.g., dogs ) have only one heat during a breeding season; others (e.g., ground squirrels ) will come into heat repeatedly during the breeding season until impregnated. During estrus the female secretes pheromones that signal her receptivity to the males; her genital area may become swollen, and she may also give behavioral signals.