Platypuses are one of only five species of extant monotremes—just them and four species of echidna—which split from the rest of the mammals 166 million years ago. These egg-laying mammals get their name from the hole that serves as both an anus and a urino-genital opening. In 2008, scientists deciphered the entire DNA of the duck-billed platypus and determined that, in accordance with the animal’s somewhat bizarre appearance, the platypus shared genes with reptiles, birds, and mammals.
Although platypuses are born out of leathery eggs, the babies nurse from their mother. Female platypuses, however, don’t have nipples. Instead, their milk is released out of mammary gland ducts on their abdomen. The babies drink it up by sucking it out the folds of their mother's skin, or her fur.
Platypuses are one of just a few venomous mammals, which is one of their more reptilian characteristics. But unlike snakes, a platypus’s venom isn’t in his teeth. Instead, males have a hollow spur on each hind leg from which venom is dispensed—but only sometimes. Although the spur itself is always there, the venom gland to which it is connected is seasonally-activated and only produces venom during mating season, indicating that its use is for fending off competing males.