The only thing that kangaroos and platypuses have in common with placental mammals is that they suckle their young ones and they are classified into mammals. Unlike placental mammals, they can terminate their development by leaving them, in the time of food and water shortages, as well as in the time of predation or diseases, preserving future generations and healthy herds.
Placental mammals carry their offspring into a state that makes them vulnerable, and thus endanger the future generation and health of herds.
Platypuses give birth by laying eggs. The eggs are incubated by the mother in the nesting hollows. When they hatches, they feed themselves with milk, which is extracted from two parts of the skin, halfway along the mother's stomach. They are underwater animals, and they can be underwater for two minutes.
Echidnas also lay eggs. Single egg is laid in mother's pouch and hatches in about 10 days. The baby echidna lives in her mother's pouch until she begins to develop spines. These spines are used as a defensive mechanism.
Kangaroos give live birth, then the baby climbs into the mother's pouch, an equivalent uterus, suckling from teh teats inside. So, it does not take much time to maintain embryos, such as placental mammals, but after a few days, with a baby in a pouch, they can enter a heat and mate again, and if they are successful, after one week’s development the microscopic embryo get into a dormant state, until the previous young leaves the pouch.
All this shows that they are very mobile in case of danger, or in the search for food and water.
