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Monotremes
Temporal range: Early Cretaceous–Recent
Monotreme collage.jpg
Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right)
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Infraclass: Australosphenida
Order: Monotremata
C.L. Bonaparte, 1837
Subgroups

Monotremes are a special group of mammals. They belong to the order Monotremata. There are three main types of living mammals: monotremes, placental mammals (like humans and dogs), and marsupials (like kangaroos).

Monotremes are unique because their brains, jaws, and other body parts are different from other mammals. The biggest difference is that they lay eggs instead of giving birth to live young. But, just like all mammals, female monotremes feed their babies with milk.

Monotremes are part of a larger group called Australosphenida. This group also includes some extinct mammals that lived a long time ago in places like Madagascar, South America, and Australia.

Today, the only monotremes left live in Australia and New Guinea. However, we know from fossils that they used to live in more places, like South America. The living monotremes are the platypus and four types of echidnas. Scientists are still discussing how to best classify them.

The name monotreme comes from two Greek words: monos (meaning 'single') and trema (meaning 'hole'). This refers to their cloaca, which is a single opening for waste and reproduction.

What Makes Monotremes Special?

Like other mammals, monotremes are endothermic, meaning they can keep their bodies warm. They have hair on their bodies and produce milk to feed their young. They also have a single bone in their lower jaw and three tiny bones in their middle ear.

Brain and Senses

Monotremes, like reptiles and marsupials, don't have a main connection between the two halves of their brain called the corpus callosum. Other mammals, like humans, do. However, monotremes have another way for their brain halves to communicate.

Adult monotremes don't have teeth. But fossil monotremes and young platypuses did have a special type of molar tooth. Some scientists think monotremes lost their teeth because they developed a unique sense called electrolocation. This means they can sense electric fields, which helps them find food underwater or underground.

Monotreme jaws are built a bit differently from other mammals. The small bones that help them hear are fully inside their skull, just like in other mammals. However, scientists think these ear bones developed separately in monotremes compared to other mammals. The outside ear opening is still located at the base of their jaw.

Unique Genes and Body Features

Scientists have studied the platypus's genes. They found that monotremes have 5 pairs of sex chromosomes, which is very different from other mammals. One of these chromosomes looks like a bird's sex chromosome. This suggests that the sex chromosomes in marsupials and placental mammals developed after they split from the monotreme family tree. This also means that some features, like egg-laying, are shared with birds because they come from a very old common ancestor.

Monotremes also have extra bones in their shoulder girdle that other mammals don't have. They walk with their legs on the sides of their bodies, similar to reptiles. Monotremes have a spur on their ankle. In echidnas, this spur doesn't do anything. But in male platypuses, it contains a powerful venom. This venom is made of proteins also found in some reptile venoms. Scientists think having venomous spurs might be an ancient trait for mammals.

How Monotremes Reproduce

Monotreme eggs stay inside the mother for a while. They get nutrients directly from her. The eggs usually hatch within 10 days after being laid, which is much faster than reptile eggs.

Baby monotremes are called "puggles." They are tiny and look a bit like fetuses. They have strong front limbs that help them crawl around. This is important because monotremes don't have nipples. Instead, the mother's mammary glands release milk through openings in her skin. The puggles crawl around to find and lick the milk.

All five living monotreme species take care of their young for a long time. They have few babies but live for many years.

Monotreme eggs also develop differently. Most mammal eggs divide completely into new cells. But monotreme eggs, like those of birds and reptiles, only divide partially. This allows the yolk and the growing baby to share nutrients and get rid of waste.

Monotreme Body Functions

Monotremes have a lower body temperature than most other mammals. For example, a platypus's average body temperature is about 31°C (88°F). This is lower than marsupials (around 35°C or 95°F) and placental mammals (around 37°C or 98.6°F). Scientists believe this lower temperature helps them survive in their specific environments.

Even with a lower average temperature, monotremes can keep their body temperature steady in different conditions. For example, platypuses can stay warm even in icy mountain streams. Early studies were a bit confused because echidnas can "switch off" their temperature control in cold weather to save energy.

Scientists once thought echidnas didn't have rapid eye movement sleep (REM sleep), which is a deep sleep stage. But newer studies show they do have REM sleep, especially when the temperature is just right.

Monotreme milk has a special protein that fights bacteria. This might help protect the babies since they lick milk directly from the mother's skin, which could be less clean than drinking from a nipple.

Both platypuses and echidnas have spurs on their back legs. Echidna spurs don't have a known use, but male platypus spurs contain venom. This suggests that the common ancestor of platypuses and echidnas might have also been venomous.

Monotremes can make their own L-ascorbic acid (Vitamin C) in their kidneys.

Monotreme Family Tree

Scientists usually group all monotremes into one order called Monotremata. However, some suggest dividing them into two orders: Platypoda (for platypuses and their fossil relatives) and Tachyglossa (for echidnas).

The most common idea, called the "theria hypothesis," says that monotremes separated from other mammals (marsupials and placental mammals) very early in history. This explains why monotremes still lay eggs, a trait that their ancient mammal ancestors likely had. Most evidence supports this idea.

It's not clear exactly when monotremes split from other mammals, but some genetic studies suggest it was around 220 million years ago. The oldest known monotreme fossil is a jaw fragment from Steropodon galmani, found in Australia and about 110 million years old. Other ancient fossils include Teinolophos and Obdurodon. A 61-million-year-old platypus tooth was found in Argentina, suggesting monotremes might have traveled from Australia to South America when the continents were still connected.

Scientists also use "molecular clocks" (studying genetic differences) and fossils to guess when platypuses and echidnas split from each other. Estimates range widely, from 19 to 89 million years ago. If these dates are correct, it means both types of echidnas might have evolved from an ancestor that looked more like a platypus. The oldest echidna fossil found so far is about 13 million years old.

Here's how living monotremes are classified:

Fossil Monotremes

We don't have many monotreme fossils, but the ones we have tell us a lot. The first ancient monotreme found was Steropodon galmani in Australia. Even though monotremes are very old, most of their fossils are from after the Miocene epoch (about 23 million years ago).

The oldest known monotreme fossil, Teinolophos trusleri, is 123 million years old. This shows that monotremes were already diverse a long time ago. The discovery of a platypus tooth in Argentina suggests that monotremes might have originated in Australia and then spread to South America when the continents were connected.

Known Fossil Species

Here are some of the fossil monotremes scientists have discovered:

Images for kids

  • Monotreme reproductive organs

    Monotreme female reproductive organs

  • Monotreme Egg Diagram

    Diagram of a monotreme egg. (1) Shell; (2) Yolk; (3) Yolk Sac; (4) Allantois; (5) Embryo; (6) Amniotic Fluid; (7) Amniotic Membrane; and (8) Membrane

  • Steropodon model aus

    A model of the extinct platypod Steropodon at the Australian Museum.

  • Steropodon jaw

    A 100 million-year-old Steropodon jaw on display at the American Museum of Natural History, New York City, USA

See also

Kids robot.svg In Spanish: Monotremas para niños

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