Do all mammals have belly buttons?
Not all mammals have belly buttons. Placental (or eutherian) mammals have belly buttons. They nourish their fetuses by a placenta that is connected to the umbilical cord. Non-placental mammals do not have belly buttons – this group of mammals includes monotremes (metatherian mammals), which lay eggs, and marsupials (prototherian mammals), which nurture their young in a pouch.