Menstruation
Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.
Monotreme
The key anatomical difference between monotremes and other mammals gives them their name; "monotreme" means “single opening” in Greek, referring to the single duct (the cloaca) for their urinary, defecatory, and reproductive systems. Monotremes, like reptiles, have a single cloaca; marsupials also have a separate genital tract; whereas most placental mammal females have separate openings for reproduction (the vagina), urination (the urethra), and defecation (the anus). In monotremes, only semen passes through the penis; urine is excreted through the cloaca. The monotreme penis is similar to that of turtles, and is covered by a preputial sac.