
The Platypus is a semi-aquatic mammal endemic to eastern Australia and Tasmania. Together with the four species of echidna, it is one of the five species of monotremes, the only mammals that lay eggs instead of giving birth to live young.
The bizarre appearance of this egg-laying, duck-billed mammal baffled naturalists when it was first discovered, with some considering it an elaborate fraud. It is one of the few venomous mammals; the male Platypus has a spur on the hind foot which delivers a poison capable of causing severe pain to humans. It is theorized that it is used as an offensive weapon to assert dominance during the breeding season.

Until the early 20th century it was hunted for its fur, but it is now protected throughout its range. Although captive breeding programs have had only limited success and the Platypus is vulnerable to the effects of pollution, it is not under any immediate threat.
The body and the broad, flat tail of the Platypus are covered with dense brown fur that traps a layer of insulating air to keep the animal warm. The Platypus uses its tail for storage of fat reserves, an adaptation it shares with the Tasmanian Devil. It has webbed feet and a large, rubbery snout; these are features that appear closer to those of a duck than to those of any known mammal. The webbing is more significant on the front feet and is folded back when walking on land. Unlike a bird's, the snout of the Platypus is a sensory organ with the mouth on the underside. The nostrils are located on the dorsal surface of the snout while the eyes and ears are located in a groove set just back from it; this groove is closed when swimming. Platypuses have been heard to emit a low growl when disturbed and a range of other vocalisations have been reported in captive specimens.

Monotremes are the only mammals known to have a sense of electroreception: they locate their prey in part by detecting electric fields generated by muscular contractions. The Platypus' electroreception is the most sensitive of any monotreme. The Platypus feeds by digging in the bottom of streams with its bill. The electroreceptors could be used to distinguish animate and inanimate objects in this situation. When disturbed, its prey would generate tiny electrical currents in their muscular contractions which the sensitive electroreceptors of the Platypus could detect.
After mating, the male takes no part in caring for its young, and retreats to its yearlong burrow. The female softens the ground in the burrow with dead, folded, wet leaves and she fills the nest at the end of the tunnel with fallen leaves and reeds for bedding material. This material is dragged to the nest by tucking it underneath her curled tail.
The female Platypus has a pair of ovaries but only the left one is functional. It lays one to three (usually two) small, leathery eggs (similar to those of reptiles), that are about 7/16 inches in diameter and slightly rounder than bird eggs. The eggs develop in utero for about 28 days with only about 10 days of external incubation. After laying her eggs, the female curls around them.
The newly hatched young are vulnerable, blind, and hairless, and are fed by the mother's milk. Although possessing mammary glands, the Platypus lacks teats. Instead, milk is released through pores in the skin. There are grooves on her abdomen that form pools of milk, allowing the young to lap it up. After they hatch, the offspring are suckled for three to four months. Around four months the young emerge from the burrow.
The oldest discovered fossil of the modern Platypus dates back to about 100,000 years ago, during the Quaternary period. . Monotrematum sudamericanum, another fossil relative of the Platypus has been found in Argentina, indicating that monotremes were present in the supercontinent of Gondwana when the continents of South America and Australia were joined via Antarctica