* They have fur: Platypuses have a dense, velvety fur that helps insulate them in the water.
* They give birth to live young: Although they lay eggs, they nurse their offspring with milk produced by mammary glands.
* They are warm-blooded: Like other mammals, platypuses maintain a constant body temperature.
* They have three middle ear bones: This is a unique feature of mammals.
* They have a four-chambered heart: This is another defining characteristic of mammals.
While the platypus possesses some unique features like laying eggs and having a duck-like bill, the overall combination of characteristics place it firmly within the class Mammalia.