Mammals have unique reproductive adaptations that have contributed to their evolutionary success. One of the distinguishing characteristics of mammals is their mammary glands, which allow for the production of milk. Even the more primitive mammals, such as the duck-billed platypus and the kangaroo, have some type of mammary glands. However, only the placental mammals (e.g. humans, horses, dogs) have a placenta, which supports the internal development of the embryo. The most primitive mammals have no placenta and can’t support the internal development of the embryo.