Monotremes are among the most enigmatic - and weird - creatures in the animal kingdom, bridging the gap between mammals, birds and reptiles.
Join us as we explore the remarkable world of egg-laying mammals, where ancient lineage meets extraordinary adaptation.
What are monotremes?
Monotremes are egg-laying mammals. Monotremes aren’t viviparous (live-bearing) but oviparous, so young are sustained by yolk until they hatch. Hatchlings (nicknamed puggles) must lick the milk secreted from a mother’s skin, as females don’t have nipples. There are only five living species: four kinds of echidna plus the platypus – a semi-aquatic creature that looks like a beaver and a duck’s love-child.
Weirdest animals: meet the weird freaks and oddballs of the natural world
Monotremes have just one orifice or ‘cloaca’ for urination, intercourse and defaecation, hence the name – monotreme comes from the Greek for ‘single opening’. It’s one of several reptilian or bird-like traits that were probably present in the common ancestor of all living mammals.
But while monotremes are considered primitive, they have some sophisticated features too. Their snout (or beak/bill) can sense electricity to detect prey, for example, and platypus males have venomous leg spurs.
Where are monotremes found?
Monotremes are found in Australia and New Guinea. This distribution of monotremes reflects the fact that Australia and South America were once connected as a supercontinent, Gondwana.
Why do monotremes produce eggs?
A complex placenta and long pregnancy has benefits, such as protecting a baby from predators and extreme temperatures. But carrying a foetus for several months has costs: it’s exhausting for females and makes mother and child vulnerable to attack or injury.
Pregnancy also causes conflicts over resources and triggers immune responses because one individual is a parasite within the other’s body. So in certain environments, like Australia’s harsh habitats, natural selection may have favoured mammals that can abandon an egg to survive, until conditions are better for their reproductive success.
How many monotremes - egg laying mammal - species are there?
There only five species of monotremes today, four species of echidna and the duck-billed platypus
