Estrus is a phase in the reproductive cycle of many female mammals, during which they are fertile and receptive to mating. This period is often referred to as "heat." During estrus, hormonal changes trigger physical and behavioral changes, making the female more attractive to males. The length and frequency of estrus can vary among species. For example, dogs typically experience estrus twice a year, while cows may have a cycle every 21 days. The timing of estrus is crucial for reproduction, as it indicates the optimal time for mating to occur.