Anestrus is a phase in the reproductive cycle of female mammals, characterized by the absence of estrus or heat. During this period, the ovaries are inactive, and there are no signs of sexual receptivity. Anestrus can occur in various species, including dogs, cats, and cattle, and its duration can vary based on factors like age, health, and environmental conditions. This phase is essential for allowing the reproductive system to rest and recover between breeding cycles. In some animals, such as horses, anestrus typically occurs during the winter months when daylight is shorter, while others may experience it due to hormonal changes or stress.