Marvelous Monotremes

Eggs, venom and four-headed penises: Inside the wild world of Australia’s Monotremes

Some of the earliest ancestors of mammals were reptiles that lived some 260 million years ago. These tiny reptilian rodent-like creatures, known as cynodonts, would eventually give rise to actual mammals, the first of which occurred around 160 million years ago. These minuscule shrew-like mammals scurried around a world dominated by the prehistoric rockstars known as the dinosaurs. These early mammals evolved into the advanced mammals we see today: the marsupials and the placentals (which includes us). Those primitive reptiles, known as synapsids today, also led to the third and strangest modern group of mammals, the monotremes.

Though they are still around Australia today, monotremes are primitive and reptile-like, lacking many features we equate with being a mammal. They have no teats, but instead secrete milk through pores along the belly of the female. They also lay eggs and have a cloaca, or single opening for their intestinal, urinary, and genital tracts for excretion purposes (the term monotreme, in fact, means “one hole”). These are traits they retained from their reptilian ancestors.

But monotremes do possess several distinct mammalian features, including a segmented jaw. They are warm-blooded and lactate, albeit in the way you would think a lizard would lactate. Although their primitive features make monotremes seem ancestral to the more advanced mammals of today, they just radiated into a different group fairly early on in mammalian history. After millions of years doing their own thing, they simply did not adapt the way other mammals have, thus retaining their archaic features.

Today there are five species of monotremes that all share the prerequisite strangeness of laying eggs, secreting milk through their stomach and having a cloaca. But they each add their own wacky features on top of those that distinguish them as the strange black sheep of the mammal family.

Probably the most famous monotreme is the platypus (Ornithorhynchus anatinus), an iconic Australian species. It was so strange that early specimens brought back to Europe from the mysterious land Down Under were thought to be hoaxes. This is both due to its composite body plan, combining aspects of a beaver and a mole with the bill of a duck, and the unbelievable assertion that these creatures laid eggs. Their otherworldliness led someone to shoot a vulnerable platypus while it laid its eggs just to prove it.

But there are benefits to the strange amalgamation of parts that make up a platypus. Their duck-like bill is used to sift through the sand on the river bed in search of crayfish. It is studded with touch and electro-receptors that can detect the movement of prey (check out our Instagram page for a video of a platypus using its bill). Its webbed feet and beaver-like tail help propel it through the water. Its otter-like fur helps it stay warm and quickly dry while out of the water. They are so adapted to an aquatic lifestyle that flaps of skin even cover their eyes and ears to keep water out as they let their bills do the prey-detecting.

Perhaps the most outlandish things about platypuses is that males sport a venomous spur on their hind foot capable of delivering a painful sting to any predator or other aggressive platypus. Echidnas, the other group of monotremes, also have a similar venom gland in their hind feet, but it has become vestigial, which means it no longer produces venom. The existence in both animals does point out an evolutionary common ancestor, linking both groups of monotremes.

The four remaining species of monotremes are all echidnas. There is one species in Australia, the short-beaked echidna (Tachyglossus aculeatus), and three, longer-beaked species in New Guinea. They look like a large hedgehog (weighing as much as 20 some pounds) with a long snout (not actually a beak), that encases a long, sticky tongue to lap up termites and ants. They need to eat small prey because they no longer have teeth. They sport large claws to dig through the ground and break apart logs in search of insects and are covered in spines to deter predation. Unlike porcupines, their spines are hollow and barbless.

Like the platypus, echidnas have electro-receptors in their nose that help them feel vibrations. Male echidnas find a use for the spurs on their hind feet even though they no longer secrete venom. Instead, a milky substance oozes out during the breeding season as a means of scent communication to help the lady echidnas track him down.

When breeding does ensue, it is wildly strange. To start things off, the echidna has one of the oddest and most terrifying penises on the planet. It has four heads! Only two of them are working at a time, but it makes for an ungainly sight. The belief as to why their penises have three extra heads is that it helps create more sperm, which comes in handy when competing with other males.

During the winter months, they hibernate. Horrifyingly, sometimes males will come into a female’s burrow and mate with her while she hibernates and she will awaken pregnant. However the female becomes impregnated, she usually lays one egg at a time which she incubates in her stomach pouch, similar to Australia’s marsupials. After about a week, the egg hatches and the baby echidna (called a puggle) stays in the mother’s pouch for eight more weeks.

Although there is only one species that lives in Australia, it is the most widespread native mammal in the continent. It lives, preferably alone, in a variety of habitats from alpine meadows to dense forests to even desert. They could possibly live for up to 45 years in the wild, although there is no definitive proof about how long the lifespans actually is. Two of the species in New Guinea, including the Sir David’s long-beaked echidna (named after famed British naturalist David Attenborough), are classified as critically endangered. Habitat loss is likely the main culprit as the Western long-beaked echidna has possibly seen its population decline by close to 80%.

The platypus is listed as near threatened by the IUCN, but its population is trending downward. Their biggest threat is the alteration of their river habitat by development and damming.

Australia’s marvelous monotremes, which are two of the strangest creatures on the planet, are an incredible look into an alternate route of mammal evolution. While placentals and marsupials diverged one way, monotremes went another way and remained very reptile-like mammal. Although they are not our ancestors, it truly is like looking at a living fossil when a platypus darts through the water or an echidna lumbers around the forest floor.

All artwork and photography by Jack Tamisiea.

Sources:

https://www.nationalgeographic.com/animals/mammals/p/platypus/

https://www.livescience.com/57267-echidna-facts.html

https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/monotreme

https://www.scientificamerican.com/article/meet-the-ancient-reptile-that-gave-rise-to-mammals/?redirect=1

https://australianmuseum.net.au/learn/species-identification/ask-an-expert/what-is-a-monotreme/