Do female dogs and cats really menstruate?
Menstruation is the building and sloughing of the endometrium (lining of the uterus). Dogs and cats do NOT menstruate; only primates menstruate. However, cats and dogs do experience “heat” periods, during which they bleed. “By contrast, in species that have estrous cycles rather than menstrual cycles, females are only receptive to copulation while they are in heat (dolphins are a notable exception). The other significant difference is that in an estrous cycle, if no fertilization takes place, the uterus reabsorbs the endometrium: no menstrual blood flow occurs. Some animals, such as domestic cats and dogs, experience small amounts of bleeding while in heat. This phase of the estrous cycle corresponds most closely to the follicular phase of the menstrual cycle and should not be confused with menstruation.” Bleeding is not caused by a male “poking” the female reproductive organs as previously stated in an answer above.
