Only human females and some other higher primates have a regular monthly menstrual cycle. Humans, however, are able to mate at any time. Females of almost all other mammal species will mate only at specific times during a year. These times are known as estrus or “heat.” It is only during estrus that egg cells are released for fertilization. Estrus occurs in wolves and deer once a year, in dogs usually twice a year, in cows and horses once a month, and in rats and mice every few days. Ovulation, mating, and fertilization occur on a specific timetable. In many animals, such as deer or sheep, estrus occurs.
