鴨嘴獸是很特別的卵生哺乳動物，牠們的消化道、排泄道與生殖道均開口於身體後方的洩殖腔內。科學家認為牠們產卵方式和鳥類及爬蟲類相似，可說是演化同源的證據。鴨嘴獸外觀獨特，嘴巴似鴨嘴，沒有外耳殼，眼睛小，尾巴扁平，腳掌帶蹼，四肢分布在身體兩旁，行進像爬行動物。雄性鴨嘴獸後肢有刺，會分泌毒素，稱為毒距骨，用於擇偶期搶奪伴侶。

鴨嘴獸是夜行性獨居動物，早晨和黃昏時最為活躍，潛入水中，靠敏感的扁嘴探測獵物，捉食甲殼類、軟體動物、蠕蟲或昆蟲。喜歡穴居，會在河岸挖掘地道作為棲所，在地洞中產卵，蛋殼層似橡皮，母獸把蛋抱在身上保暖，卵大約10天後孵化，母鴨嘴獸沒有乳頭，小鴨嘴獸靠舔食母親腹部毛下乳腺分泌的乳汁，4個月後出洞。

鴨嘴獸已成為澳洲代表性動物，澳洲的20分錢硬幣上刻畫的就是游水中的鴨嘴獸。以往因毛皮珍貴而遭獵捕，加上易受環境污染影響，致使數量遞減，目前已列為國際保護動物。

The platypus is a unique oviparous mammal. It is a monotreme, referring to the cloaca, which is the exit for digestive, urinary and genital organs. Scientists believe that the way a platypus lays eggs is similar to birds and reptiles, which is considered to be the evidence of common ancestor. The platypus has a unique appearance — a bill like a duck, no visible ears, small eyes, flat tail, webbed feet and all four limbs protruding from the side of the body, making it look like a reptile when moving on land. The spur on the male's hind limb, called calcaneus spur, produces venom, which is primarily used as a weapon to assert dominance during the breeding season.

The platypus is generally thought to be nocturnal and solitary, and is most active at dawn and dusk. While swimming, it uses its sensitive electroreceptors and mechanoreceptors in its bill to detect prey and objects in the water. Platypus likes living in burrows on riverside banks, where females lay their leathery eggs and keep them warm by embracing them. After around 10 days of external incubation, the eggs hatched. Females do not have nipples, so offspring must lick secretions from mammary glands under the mother's stomach. The platypus is an iconic symbol of Australia and is featured on its 20-cent coin.

In the past, the species was widely hunted for its fur, and pollution in habitats also contributed to a steep decline in populations. It is currently listed as an internationally protected species.