10 Fascinating Facts about Monotremes

Aside from laying eggs, there are many other unusual facts about monotremes, some of the oldest and most primitive mammals on Earth.

Monotremes, such as platypuses and echidnas, are among the oldest, rarest and most unusual animals on the planet. These basal mammals, which are only found in Australia and New Guinea, are well-known for laying eggs rather than giving birth to live young, but that’s not where the weirdness ends. In fact, one may even ask what makes them mammals in the first place. However, in contrast to reptiles and birds, monotremes do have several defining mammalian characteristics, the most notable being that they have mammary glands. Like almost all other mammals, they are also warm-blooded and have hair but, for the most part, that’s where the similarities end.

#1. Only 5 Species Left

Platypus John Gould

The duck-billed platypus is one of Australia’s most iconic and unusual animals.

There are at least 5,500 different mammalian species on Earth, but only five of them are monotremes. The five extant species fall into two families. Ornithorhynchids only have one species, the platypus, which is found in eastern Australia and Tasmania. The other four are echidnas, which are also known as spiny anteaters, despite not being related to true anteaters in any way. Of these four species, three are found only in New Guinea, while the other lives both there and in Australia. Sadly, the western long-beaked echidna, which is found only in western Indonesia, is currently listed by the IUCN as critically endangered.

#2. The Oldest Mammals on Earth?

The duck-billed platypus is one of Australia’s most iconic and unusual animals.

The fact that monotremes lay eggs is partly what earned them their subclass name prototheria. In Greek, this means ‘first wild animal’, owing to their evolutionarily primitive characteristics. Prototherians first appeared at around the same time as the first dinosaurs, approximately 210 million years ago during the Late Triassic. However, fossil monotremes are unusually rare, and the story of their evolution certainly contains a lot of holes. Nonetheless, anatomical evidence suggests that they diverged from the mammalian lineage long before true marsupials and placental animals evolved.[/caption]

#3. Some Are Poisonous

Though neither lethal nor necrotising, platypus venom can cause extreme pain in humans.

When people think of poisonous animals, it’s usually things like snakes and insects that come to mind. Nonetheless, platypuses are one of the very few venomous mammals in existence. Although both male and female platypuses develop the venom-delivering spurs on their hind limbs, females lose them before they reach sexual maturity. The venom itself can paralyse small animals and, while not lethal to people, it can cause extreme, incapacitating pain. The fact that platypuses are venomous is another testament to their ancientness; venomous spurs are a lot more common in the fossil record than they are in extant mammals.

#4. Only One Posterior Orifice

All reptiles, birds and amphibians have a cloaca, a single posterior orifice for both digestion and reproduction. By contrast, almost all mammals have two or three separate specialized orifices for reproduction, urination and defection. Monotremes, however, are different. They are equipped with a cloaca, and the urinary, defecatory and reproductive systems all share the same tract. In fact, that’s where the word monotreme comes from – in Greek, it means ‘single opening’. Only a handful of other mammals, including marsupials, have this opening, although marsupials still have separate digestive and reproductive tracts.

#5. Extremely Low Metabolic Rates


A common mammalian characteristic is to have a high metabolic rate but, in the case of monotremes, it’s remarkably low. Their body temperatures are also much lower than other mammals at about 31°C in the case of platypuses. Nonetheless, they’re still endothermic, meaning that they can maintain a constant body temperature, regardless of ambient temperatures. However, their lower metabolic rates may be an adaptation to the relatively harsh environmental conditions in which they evolved, rather than being inherited from their more ancient ancestors. All species typically live around 10 years in the wild.

#6. Prehistoric Monotremes

Taxidermied EchidnaAnagoria

Monotremes are not exactly known for being large creatures, but this was not always the case. The largest monotreme ever discovered was defined about a hundred years ago and has since been estimated to have weighed around 66 lb (30 kg). Around three feet (1 metre) long, this ancient species of long-beaked echidna lived in Western Australia during the Pleistocene era. Like many of Australia’s prehistoric megafauna, zaglossus hacketti disappeared around 12,000 years ago, perhaps because of human encroachment. In fact, aboriginal rock paintings in Australia may even depict this unusual prehistoric beast.

#7. Cretaceous Monotremes

Although monotremes have probably been around since the first dinosaurs, only two species from the Mesozoic Era (248 to 66 million years ago) have so far been discovered. Both lived during the Early Cretaceous, which leaves an enormous gap in the fossil record lasting tens of millions of years. Nonetheless, these fossil monotremes provide definitive evidence that the unusual creatures have been around for at least 120 million years. The fossil animals, named steropodon and teinolophos, were discovered in Australia and both had a similar appearance to modern platypuses.

#8. South American Origins

Until the discovery of an unusual fossil in Argentina in 1992, the only fossil monotremes, like their extant relatives, were known from Australia and New Guinea. However, it now seems like monotremes may have a South American origin. Marsupials, for example, likely came from South America between 34 and 50 million years ago when the continent was still joined to Australia through Antarctica. The discovery of the 61 million-year-old South American Monotreme suggests that these animals were once far more widespread. To date, however, this is the only incidence where a monotreme fossil has been found outside of Oceania.

#9. Ancestral Insights

Duckbill Platypus skeletonWikimedia Commons

Although a monotreme is indisputably a mammal, its bizarre mix of mammal, bird and reptile characteristics may give us some fascinating insights into our distant origins. It’s believed that monotremes branched off from other mammals sometime between 160 and 210 million years ago according to biochemical studies. After all, first mammals were almost certainly egg-laying creatures, just like their synapsid predecessors. However, we have no direct evidence of this yet, since these fragile creatures and their eggs do not leave much of an imprint on the fossil record. In short, we may never know.

#10. Bizarre Mating Rituals

Monotremes just have to be weird no matter what they’re doing, and mating rituals are certainly no exception. Echidnas, which breed during the winter, form ‘mating trains’ that can last up to six weeks. During this time, up to 11 male echidnas line up nose to tail with a female waiting for her to signal that she’s ready for mating. If all goes well, an egg containing a puggle (yes, there’s a word for a baby echidna!) will be born. Platypuses, by contrast, have a slightly less refined approach to mating, whereby males use their poison-inducing spurs to attack other males as they compete for a mate.

Monotremes certainly are some of the most unusual mammals on our planet, and there’s a whole lot we still don’t know about them given the 75-million year gap in the fossil record. What do you think makes these creatures so fascinating? Let us know in the comments below!

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