Explanation:

Researchers reported that the swamp wallaby, a marsupial related to the kangaroo, is pregnant throughout its adult life. The swamp wallaby is likely the only mammal pregnant and lactating all life-long. It typically conceives a new embryo days before delivering the newborn from its previous pregnancy. Female wallabies have two uteri and two separate ovaries. At the end of a pregnancy in one uterus, a new embryo develops in the other uterus. Wallabies regularly have an embryo in the uterus, a young joey in the pouch, and a third semi-dependent young at foot, still drinking its mother’s milk. In the swamp wallaby (Wallabia bicolor), the new conception happens one or two days before the previous joey is delivered.