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.
Maternal transfer in aquatic mammals
Marine mammals are exposed to a variety of chemicals throughout their life, mostly through their diet. Once the chemicals are accumulated in the body tissues of the mammals, a portion of these chemicals in the female mammals are transferred to their offspring during gestation and lactation.