Breeding periods may be evolutionarily designed for the animals. If gestation occurs in winter, offspring are born in spring when temperatures and climate are most advantageous to the offspring. The period of time associated with development of the follicle and ovulation is referred to as the estrous cycle. The estrous cycle is analogous to the menstrual cycle of the human female. Estrous Cycle Estrous refers to the entire ovarian cycle in the female.
Books can be very confusing on the use of estrous and estrus.
When they say estrus they probably mean when an animal is in heat or proestrus. Large numbers of epithelial cells. Reduced numbers of epithelial cells and presence of increasing numbers of leukocytes. Diestrus I and II and the beginnings of Proestrus are roughly analogous to follicular phase in humans. At end of proestrus and beginning of estrus ovulation occurs.