What Is a Monotreme? 9 Facts About Earth’s Weirdest Mammals

A monotreme is any mammal that lays eggs. You probably know about the platypus, the most famous monotreme. But have you heard about echidnas? In addition to being the only mammals that lay eggs, these animals have a number of strange characteristics that set them apart from most other mammals alive today. Here are several fascinating facts about them. 

1. Monotremes Don’t Have Stomachs

Neither the platypus nor the four species of echidnas — spiny anteaters — has a stomach. This trait is highly unusual in vertebrate animals, although they share this bizarre characteristic with lungfish, chimeras, and some carp, all of which are fishes. 

When a monotreme eats, food travels straight from its esophagus to its intestines. No one is sure why monotremes lost their stomachs over the course of history, but it might have to do with the simplicity of their diets. The jury is still out on that one.

2. Adult Monotremes Have No Teeth

Platypuses have teeth when they first emerge from the egg, but they lose them by the time they reach adulthood. Baby echidnas only have an egg tooth — a single, temporary tooth that helps them hatch — but they lose it shortly after leaving the egg. 

Thankfully, neither type of monotreme needs teeth. Spiny anteaters only eat ants and termites. Platypuses eat worms, shellfish, and insects, using a little help from gravel and grinding pads in the back of their jaws to chew their food. 

A few other groups of mammals lack teeth, including the baleen whales, pangolins, and giant anteaters, which are not closely related to spiny anteaters. 

3. They Also Lack Nipples

Another strange trait that sets monotremes apart from other mammals? Their mammaries. 

Neither the platypus nor echidna has nipples. Instead, female monotremes have mammary glands that slowly secrete milk in a process similar to sweating. Their babies — called puggles, which might be the most adorable name for any animal ever — lick the milk off their mothers’ skin. 

4. They Can Sense Electric Fields

As if it wasn’t enough to be the only mammals that lay eggs and ooze milk, monotremes also had to develop superpowers. Platypuses and echidnas are the only land mammals with electroreception, the ability to sense weak electrostatic fields in the environment. Guiana dolphins are the only other mammals with this ability.

Echidnas use electroreception to sense ants and termites underground. Platypuses use their 40,000 electric sensors — located in their bill — underwater to find invertebrates hidden in the riverbed. For the prey animals doing their best to hide, this ability must come as quite a shock!

5. Monotremes Have Cloacas

“Monotreme” means “single opening.” Much like reptiles, amphibians, birds, and some fishes, monotremes have a cloaca. A cloaca is a single hole used for mating, urinating, defecating, and laying eggs. How convenient!

Neither placental nor marsupial mammals, the two other groups of mammals besides monotremes, have cloacas. Most female mammals have three separate openings — one for urinating, one for mating and giving birth, and one for defecating. One exception is the spotted hyena, in which females only have two orifices and must give birth through their urethra. Ouch.

6. Platypuses Have Venomous Spurs

Platypuses are one of the few venomous mammals on Earth, a title that slow lorises, some bats, and a few shrews and shrew-like animals called solenodons can also claim. Males have spurs on their hind legs that inject an excruciating venom. The spurs are so strong that the platypus may hang from them after stinging a victim.

Although not lethal to humans, the pain from a platypus sting can incapacitate a person. The venom causes the sting site to swell rapidly and the pain can last for months. 

Why do platypuses pack such a painful punch? The current hypothesis is that during mating season, male platypuses may attack each other with their spurs to compete for females. They normally avoid each other except during the mating season.

7. Monotremes Have Low Body Temperatures

Like all other mammals, monotremes are endothermic. That means they maintain their own body temperature without relying on the sun or surrounding air to warm themselves. 

Despite being endotherms, however, platypuses and spiny anteaters have very low body temperatures, averaging just 89.6°F compared to 98.6°F in placental and marsupial mammals.

8. Monotremes Glow Under Blacklights

Still not strange enough? Platypuses and echidnas — their spines, anyway — also glow an eerie blue-green color under UV light. This phenomenon is dubbed biofluorescence, and only a handful of mammals exhibit it. Wombats, some flying squirrels, African springhares, and ghost bats also glow under a blacklight. 

No one knows for sure why some mammals are biofluorescent. Does it serve any purpose, or is it merely a colorful evolutionary accident?

9. Monotremes Are Ancient

No one knows for sure when monotremes diverged from other mammals, but the split may have occurred as early as 220 million years ago. Monotremes aren’t the ancestors to any other living mammals. Instead, they took a separate evolutionary path. They still retain several primitive traits that other early mammals and therapsids — mammal-like reptiles that gave rise to all modern mammals — had. 

For example, they are missing part of the middle ear, their shoulder girdles have extra features, and they have ribs on some of their neck vertebrae. They also hold their femur and humerus parallel to the ground when they walk, much like a reptile.

It’s fascinating to think that monotremes once foraged for bugs alongside dinosaurs. 

Nature’s Strangest Mammals

Monotremes are unique in almost every way. They’re the only mammals that lay eggs, have a cloaca, lack a stomach, have no nipples, and can sense electric fields. Platypuses, in particular, have venomous spurs and glow neon blue under blacklights. 

Of the five species of monotremes, three are doing poorly in the wild. The Western long-beaked echidna and Sir David’s long-beaked echidna are critically endangered, while the Eastern long-beaked echidna is vulnerable. Overhunting and habitat loss are the biggest threats to these animals. 

Time will tell if monotremes — which survived the extinction of the dinosaurs 65 million years ago — will survive into the future. For now, the existence of these relics from the past hangs in the balance.