Is a Platypus a Mammal? Egg-Laying Mammals Explained

What makes an animal a mammal?

Scientists do not decide whether an animal is a mammal by how it gives birth. Mammals are defined by a set of features they inherited from a common ancestor, and the platypus has all of the key ones:

• Hair or fur. The platypus has one of the densest coats of any mammal.
• Mammary glands and milk. Females feed their young on milk for about three to four months.
• Three middle-ear bones. Only mammals have the hammer, anvil and stirrup bones that carry sound to the inner ear.
• A lower jaw made of a single bone on each side, the dentary.
• Warm-bloodedness. The platypus makes its own body heat and keeps a body temperature of about 32 °C (90 °F).

Laying eggs does not cancel any of this. Plenty of animal groups contain species that break the usual rule for their group, and egg laying turns out to be the original way all early mammals reproduced.

The three groups of living mammals

Living mammals fall into three main groups:

Monotremes branched off from the other mammals very early, more than 160 million years ago according to genetic estimates. Marsupials and placental mammals split from each other later. That is why the platypus is not a marsupial either; for a marsupial that is often misnamed, see Are koalas bears?.

The five living monotremes

Only five egg-laying mammals survive today, all in Australia and New Guinea:

1. Platypus (Ornithorhynchus anatinus): eastern Australia and Tasmania.
2. Short-beaked echidna (Tachyglossus aculeatus): across Australia and in southern New Guinea.
3. Western long-beaked echidna (Zaglossus bruijnii): New Guinea.
4. Eastern long-beaked echidna (Zaglossus bartoni): New Guinea.
5. Sir David’s long-beaked echidna (Zaglossus attenboroughi): one mountain range in western New Guinea.

The platypus and the echidnas look very different, one a swimmer with a soft bill and the other a spiny ant-eater, but they share the same basic biology. Fossils show that monotremes were once more varied, with platypus relatives in South America about 60–70 million years ago.

How platypuses lay eggs and make milk

A female platypus mates in late winter or spring and then digs a long nesting burrow in a riverbank. She usually lays two small eggs, sometimes one or three. Each is about 17 mm (0.7 in) long with a soft, leathery shell like a reptile’s egg, not the hard shell of a bird’s.

The eggs have already developed inside her for about 28 days. She curls around them in the nest, holding them against her belly with her tail, and they hatch after about 10 more days. The hatchlings are tiny, blind and hairless.

This is where the mammal side takes over. The mother has mammary glands but no nipples. Milk seeps through pores in two patches of skin on her belly, collects in her fur, and the young lap or suck it up. They feed on milk for about three to four months before leaving the burrow.

Echidnas do it a little differently. A female short-beaked echidna lays a single egg directly into a temporary pouch on her belly, where it hatches after about 10 days. The young, called a puggle, then drinks milk inside the pouch until its spines start to grow. Read more in What is a baby echidna called?.

Reptile-like features of the platypus

Because monotremes split off so early, they kept several features that other mammals lost:

• Egg laying with a leathery shell and a large yolk.
• A cloaca, a single rear opening for waste and reproduction. The word monotreme means “one hole”.
• Extra shoulder bones, including an interclavicle, similar to those of reptiles.
• A sprawling walk, with the legs held out to the sides rather than under the body.
• A lower body temperature than most other mammals.

Its genes are a mixture too. The platypus genome, first published in 2008, contains some genes otherwise known from birds and reptiles, and the animal has ten sex chromosomes instead of the usual two. One of its X chromosomes closely resembles the Z sex chromosome of birds.

Why people once doubted it

When the first preserved platypus reached Britain in 1799, some naturalists thought it was a fake made from parts of different animals. Once its reality was accepted, a long argument followed about what it was. Reports from Aboriginal people that platypuses laid eggs were long dismissed by European scientists, and the question was only settled in 1884, when the naturalist William Caldwell found a platypus with eggs in Australia. Around the same time, an echidna was found carrying an egg in its pouch. Together these discoveries confirmed that some mammals really do lay eggs.

For other animals that reproduce this way, see animals that lay eggs.