The duck-billed platypus is a semi-aquatic mammal. It is found in Australia, where its dependence on permanent freshwater for survival limits its distribution to the wetter, eastern side of the continent.

The most distinctive characteristic of this animal is its leathery bill, shaped like that of a duck and from which it gets its name. Platypuses have electro-receptors in the bill that allow them to detect the weak electric fields generated by the bottom-dwelling invertebrates that are their principal prey. Thus, platypuses can hunt successfully in very murky water.

The adult male duck-billed platypus is about 65 cm (2 ft.) long, including its 12 to 15 cm (4.5 to 6 in.) tail. Its head, trunk and tail are completely covered with brown fur. Their five-toed feet are webbed. The male has a spur on each heel. The spur is directed backwards and inwards, and connects with a poison-secreting gland. The spur is used in self-defence, and also when fighting other males over access to the females.

In a combination that is unusual among mammals, females lay eggs, but suckle their young with milk. The females have no teats; the young feed through numerous tiny openings in the skin of the mother's belly.