Virtually all mammals reproduce by giving birth to live young. However, there are a few which lay eggs. These are the monotremes. The most famous is the Australian duckbilled platypus, but there are also two types of echidna. The short-beaked echidna lives in Australia and New Guinea. Its larger, long-beaked relative is now known only from New Guinea, though fossil remains have been found in Australia. As with all mammals, monotreme mothers feed their babies with milk.

As well as laying eggs, these animals share further characteristics in common with each other, but which are different to all other living mammals. For example, the upper bone of the front leg sticks out sideways from the body, and even the inside of the ear is unusually built. Such details show that monotremes form a natural group of closely related creatures.

Discovery
One of the first Europeans to see an echidna was Captain Bligh. Following the famous Mutiny on the Bounty, this British naval officer was forced to leave the ship in a small boat and a big hurry. He came ashore in Australia with a few companions in 1789, when little was known of the land. While looking around, the party saw what appeared to be a large hedgehog, roughly the size of a football. When it felt threatened, it rolled itself up like a hedgehog too. Bligh could see this animal was odd, and so he made the first known drawing of an echidna. He also realized something else interesting. Rolling up into a ball is a very bad way of protecting yourself from hungry humans. Having finished his sketch, Bligh discovered that baked echidna is very tasty.

Soon, dead specimens of monotremes were sent to London for study. As scientists had never seen mammals anything like these before, they found it hard to understand them. A platypus is vaguely like an otter with a beak. This led to the suggestion that someone was playing a joke, and had made the specimen from parts of different animals. However, it was found to be real. In a platypus, the beak's supported on long jaw bones, and nothing like that is found in any birds. It's probably just as well that scientists didn't know about the egg-laying habit until the 1880s. Things must've were quite strange enough already.

Echidnas
At first glance, the echidna seemed a bit more familiar. As well as being hedgehoggy, it also looks something like a small anteater. The feet have claws for digging out nests, and the mouth contains a long, sticky tongue and no teeth. Not much was then known about those animals either. Perhaps that's why it was first described as a new species of anteater. However, it was soon clear that any similarities have everything to do with feeding habits, rather than with any close relationship.

As with most marsupials, female echidnas have pouches for the young (and the eggs). Again, this has nothing to do with a close relationship. How Mrs Echidna gets the eggs into her pouch is a mystery.

Platypus
If we forget about the beak, a platypus looks quite like a small, dark otter. It's got a streamlined body, webbed feet and waterproof fur. This is because both animals live in river banks, find their food in the water and need to be good swimmers. Look more closely and they're very different. Otters have lots of sharp teeth for catching and chewing fish. An adult platypus has hardly any teeth, and the ones it's got are difficult to recognize. They're low and wide and form crushing surfaces. The platypus eats shelled animals rather than fish, and doesn't have to do much chewing.

There are plenty more platypus peculiarities. When underwater, they use a kind of natural radar system to find their way around. For unclear reasons, the adult males produce poison. There's a special bit of bone on the back of the foot. The venom doesn't help with hunting, and so it may have something to do with defence. The difficulty is in knowing which natural enemy it might have reason to kick. A further possibility is that the poison might have some strange connection with breeding.

Threats
The biggest threat facing all monotremes is loss of habitat. As it's dependant upon a watery environment, living space for the platypus is more restricted. In Australia, echidnas are found in a wide variety of places; deserts, forests and on mountains up to 1,800 metres high. The long-beaked echidna of New Guinea is the most threatened. Habitat protection will hopefully ensure the survival of these weird mammals.

Fact File

Monotremes
Origins: The oldest known fossils are about 120 million years old.

Range: Australia and New Guinea. However, a few 60 million year old teeth are known from Argentina. These are more like the platypus than echidnas, (which are completely toothless).

Duckbilled platypus
Scientific name: Ornithorhynchus

Range: Freshwater habitats of eastern Australia and Tasmania.

Length and weight: About 50cm and up to 2 kilos, but females are smaller.

Diet: Shelled water creatures, insects and larvae.

Population: Common and protected.

Short-beaked echidna
Scientific name: Tachyglossus

Range: Australia and New Guinea, (both Papua New Guinea and Indonesia).

Length and weight: About 45cm and 4.5 kilos.

Diet: Ants, termites, bugs and worms.

Population: Sparse but widespread.

Long-beaked echidna
Scientific name: Zaglossus

Range: New Guinea, (both Papua New Guinea and Indonesia).

Length and weight: About 60cm and 8 kilos, though larger sizes have been reported.

Diet: Mainly worms, though insects may also be eaten.

Population: Rare and endangered.