The platypus puzzle

The platypus (Ornithorhynchus anatinus) has a puzzling array of features. Not only does it have that iconic duck bill, it lays eggs like a bird or reptile but feeds milk to its young like a mammal.

Males also have a pair of venomous spurs on their hind feet, but they don't use them for traditional attack or defence.

Watch the Museum's venom evolution expert, Dr Ronald Jenner, explore the toxic tactics of the platypus.

Unusual features

Despite its odd look, the platypus is perfectly adapted to its environment. It has a furry, otter-like body, a tail the same shape as a beaver's, and a mouth reminiscent of a duck's.

In his 1802 book, An Account of the English Colony in New South Wales, Colonel David Collins wrote of the webbed and clawed feet that allowed the animal to swim and burrow with ease.

He was also fascinated by the platypus's bill, noting, 'the most extraordinary circumstance observed in its structure was, it having instead of the mouth of an animal, the upper and lower mandibles of a duck.'

Before this account, when a skin and illustration of the animal were first sent to Europe, some suspected the strange animal was a hoax - perhaps a taxidermy construction of a duck's bill attached to the body of a mole.

The nineteenth century saw a number of hoax animals on display, such as P T Barnum's Fiji (Feejee) Mermaid and Albert Koch's Missouri Leviathan. But the platypus, as it was soon realised, was not among these.

How the platypus got its name

George Shaw, keeper of the natural history collections at the British Museum (which were to later become the Natural History Museum), accepted the platypus as a real animal. In 1799 he was the first to scientifically describe it, assigning it the species name Platypus anatinus, meaning flat-footed duck.

However, Platypus was already in use as the name of a genus of wood-boring ambrosia beetles. So in 1803 Johann Friedrich Blumenbach published another description of the animal under the name Ornithorhynchus paradoxus - 'paradoxical bird-snout'.

The animal later became recognised as Ornithorhynchus anatinus, meaning bird-snouted flat-foot. This hybrid name was accepted in accordance with the rules of priority when classifying animals with scientific names.  

Is the platypus a mammal?

When it was discovered, the platypus was difficult to classify, bearing characteristics of mammals, reptiles and birds.

To simply separate the orders, mammals are warm-blooded, give birth to live young and feed them milk. Birds are also warm-blooded but lay eggs, and reptiles are cold-blooded egg-layers that rely on the Sun or another heat source to warm them up.

The platypus was initially a challenge to classify due to the number of specimens available in Europe that were either incomplete or preserved. This meant that certain distinguishing features, such as mammary glands, were not obvious.  

The platypus was ultimately placed into a new order called Monotremata, alongside the four living species of echidna. Monotremes are, notably, egg-laying mammals that produce milk for their young.

The overlapping features has led some scientists to consider whether the platypus actually represents a missing link between reptiles and mammals.

Amniotes are the common ancestor of all mammals, birds and reptiles. Mammals split from birds and reptiles around 315 million years ago.

Monotremes are not direct relatives of birds and reptiles, but they are often considered the most basal of all mammals. They split from the line leading to placental mammals - which incluides humans - around 166 million years ago. The platypus's overlapping characteristics with other classes are likely evolutionary leftovers.