The Insane Biology of: The Platypus

Here’s a sanitized version of the provided YouTube transcript:

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When the platypus first came to the attention of European scientists in 1798, not everyone was convinced that the creature before them was real. Some thought a prankster had combined parts of different animals to create a fake, which was not uncommon during this boom time of natural discovery. However, the platypus was a very real animal, one that confused anatomists for some time. It is a creature with fur, a bill, and webbed feet that lays eggs and can secrete venom. Was this a mammal, a duck, or some sort of furry reptile? These European scientists were asking the same questions that the Aboriginal people of Australia had been pondering for a long time before them.

There are several Aboriginal stories about the origins of the platypus, one of which tells of a union between a duck and a water rat. In scientific terms, we classify the platypus as a mammal, or more specifically, a monotreme—an egg-laying mammal of which there are only two kinds of animals in the world. This, along with its other rather reptilian traits, has made scientists scratch their heads for a long time. Where does the platypus fit exactly on the tree of life? As continents divided and the branches of the tree of life diverged, the platypus seems to have taken its own very special route.

Just like all animal adaptations, there has to be some purpose to all the platypus’s strangeness. If you look back in any textbook, the definition of a mammal is a warm-blooded vertebrate animal that has fur, secretes milk, and typically gives birth to live young. The key word here is “typically.” Platypuses are one of only two mammals that lay eggs; the other is the echidna, of which there are four species. When it’s time to lay eggs, female platypuses dig their burrows, crawl in, and seal themselves up. Here, they lay their eggs, curling them between their body and tail until they hatch about 10 days later. After hatching, the platypus acts like most other mammals, nursing their young on milk for three to four months until they are capable of swimming solo.

To understand how and why this egg-laying ability exists in a mammal, we need to wind back the clock a long time—around 340 million years ago, when the first amniotes appeared on Earth in the form of small lizard-like creatures. Amniotes are four-legged vertebrates defined by the membrane or amnion that protects the embryo during development. The amniotic egg was an evolutionary invention that first allowed reptiles to colonize dry land. Fish and amphibians must lay their eggs in water and therefore can’t live far from it, but thanks to the amniotic egg, reptiles can lay their eggs nearly anywhere on dry land. Soon, amniotes spread far and wide across Earth’s land and became the dominant land vertebrates.

Around 315 million years ago, they split into two major groups of four-legged vertebrates that still exist today. One branch contains modern reptiles and birds, while the other included mammal-like reptiles from which modern mammals later evolved. This branch of mammals eventually developed to have the amniotic egg grow inside the mother’s womb, giving rise to internal pregnancies. However, one branch of mammals did not follow suit: the monotremes, or egg-laying mammals, split off from the mammalian lineage around 200 million years ago. They never gained the ability to have an internal pregnancy and never lost their egg-laying ability.

Genome sequencing of platypus sex cells has shown there are a large number of shared genes between platypuses and birds. In particular, the platypus retains copies of the vitellogenin gene, which codes for egg protein that is a precursor to egg yolk, helping sustain growing embryos. Platypuses have fewer copies of the gene than birds and reptiles, but most mammals don’t have the gene at all. This means the platypus has the ability to lay eggs, but their young are perhaps less reliant on egg protein than birds and reptiles. This makes sense when we remember that platypuses also feed their young via lactation.

After hatching, their laying of eggs is a sort of remnant from their reptilian ancestors, and for a while, it served them well. The monotremes were the dominant mammals on what is now the continent of Australia for a long time, until they were overshadowed by the arrival of their marsupial cousins. Marsupials originated in what is now South America and migrated to Australia via the supercontinent Gondwana around 70 million years ago. Their bodies were more efficient at locomotion, and their internal pregnancies meant they could better protect their young, allowing them to out-compete the monotremes on almost every front. Slowly, all but two monotremes—the echidna and the platypus—went extinct.

So, the question is, why did they survive when the rest did not? One hypothesis is that the platypus persisted in the face of intense competition from marsupials due to its ability to take to the water, a domain where marsupials could not follow. The echidna’s earlier ancestor is also thought to have been semi-aquatic, even though it is not anymore. Marsupials could not colonize water environments because when they are born, they have to live inside their mother’s pouch for weeks to suckle milk, which would pose a drowning risk if their mothers ventured into the water. However, with their eggs secure in a nest, the platypus can happily stay in the water, avoiding predation from marsupials and exploiting its own environmental niche.

The platypus is an expert swimmer. Its ability to hunt underwater is due to several key features of its physiology. The platypus’s webbed feet help propel it through the water, using its front feet for paddling and its back feet for steering. A feature that helps the platypus stay submerged for up to two minutes at a time is its ability to become watertight when necessary. It has folds of skin that cover its ears and can close its nostrils. Despite its hunting prowess underwater, it nearly completely closes its eyes when diving, thanks to a sixth sense that almost no other mammal possesses—it doesn’t need to see to hunt.

One of the most distinct parts of the platypus is its bill. Its iconic shape is wide and flat, but unlike a duck’s bill, the platypus’s bill is described as flexible, rubbery, and a little fleshy. Its surface supposedly feels a bit like suede, and the bill is the platypus’s primary hunting tool. It can hunt with its eyes completely closed because it is super sensitive in two key ways: it is mechanoreceptive and electroreceptive. Mechanoreceptive means sensitive to external mechanical stimuli such as touch or pressure. In the case of the platypus, its bill contains mechanoreceptors called push rods—columns of densely packed cells that move independently of surrounding skin. When pressure or a vibration is applied to the push rod, it triggers the nerve at the bottom of the column. The pressure doesn’t have to be large; the slightest tremors can be felt through the water. The bill is so sensitive that it can detect freshwater shrimp from a distance of 15 to 20 centimeters away simply by sensing movements in the water.

The other sensing ability of the bill is its electroreception. All animals emit electric signals from their muscles when moving, and the platypus’s bill can sense these electric fields originating from their prey. The bill contains around 70,000 glands that assist in the electroreceptive function. The mechanism of electroreception in the bill is similar to that of elasmobranchs, like the hammerhead shark. The electrical currents from the stimulus travel through the water and then through secretions from the glands in the bill, which surround nerve endings beneath the bill’s surface. The one hundred thousand electro and mechanoreceptors on the platypus’s bill are arranged in a beautiful striped pattern, with bands of electro and mechanoreceptors alternating.

Electroreception is common in fish but has only been found in three mammals to date: the platypus, the echidna, and the Guiana dolphin. Platypus bills have up to 70,000 electroreceptors, while those of long-beaked and short-beaked echidnas have only 2 and 400, respectively. You can see evolution in progress here; since moving back onto land, the electroreceptors of the echidnas are being selected against because such sensing ability is only useful in semi-aquatic environments. Interestingly, this electroreception is not a remnant from an early fish-like ancestor; it evolved completely independently of electric fish, demonstrating how two different lineages can arrive at similar evolutionary solutions.

Electroreception isn’t the only feature of the platypus that’s rarely seen in mammals. Male platypuses have spurs on the back of their hind feet that connect to venom glands in their abdomen. While not deadly to humans, the venom can have some pretty nasty side effects, including nausea, cold sweats, lymph node swelling, and immediate excruciating pain that can’t be relieved through normal painkillers. Venomous mammals are now quite rare; there are only a handful that we know of, such as the slow loris, the only known venomous primate, which uses venom to protect itself against predators, or the American short-tailed shrew, which uses its venom to immobilize its insect prey.

It’s thought that with the development of teeth and claws, most mammals developed much faster ways of killing prey than venom, which needs time to take effect, leading to the evolutionary redundancy of venomous capabilities in mammals. The very different methods of delivery suggest that the ability evolved independently. For instance, slow lorises can look pretty cute when they raise their arms in the air as if they’re asking for a hug, but this is actually how they access their venom. It’s produced by glands in their armpits, which they lick, mixing the venom with their saliva and settling it into grooves in their teeth, ready to harm anything they bite. The short-tailed shrew also has grooves in its teeth, but its venom comes ready-made in the shrew’s saliva. Both differ from the platypus, which delivers its venom through its spurs.

Interestingly, a recent study found that many of the proteins present in platypus venom are the same as those found in reptile venom, even though reptiles split from mammals some 315 million years ago. Does this mean the venom is an evolutionary leftover from the platypus’s reptilian ancestor, or is it an example of independent convergent evolution of venom? By sequencing the platypus genomes, scientists found that the platypus’s ability to deliver venom is due to a duplication in a set of reptilian genes—the same reptilian genes that underwent the same duplication independently in snakes after reptiles and mammals split in the evolutionary tree. This suggests that platypus venom is an unlikely example of convergent evolution, where reptiles and the platypus developed similar venoms despite not having a common ancestor for hundreds of millions of years.

Scientists still don’t know exactly why platypuses have venom spurs, but it’s thought that they use them in mating practices and to defend territory against other platypuses. Despite its unusual characteristics, we now understand that the platypus is not just a funny-looking animal; it’s a highly adapted creature perfectly suited to its environment. Some of its bizarre features are remnants from ancient times, while others are more recent evolutionary inventions that happen to be similar to those in fish, birds, and reptiles. It’s a wild and unlikely mashup of traits that allows the platypus to sit on its very own branch of the evolutionary tree.

However, we also know there’s a lot more to the puzzle to put together. Scientists are constantly stumbling across new and unusual findings about this amazing animal. Just last year, researchers accidentally discovered that platypus skin glows under UV light, and we still don’t know why. There are likely many more secrets hiding within the platypus that we will continue to discover for years to come.

Australia’s unique landscape and geography have shaped the platypus into what it is today. Australia is literally teeming with biodiversity; its patchwork of different climates—from tropical forests to hostile deserts to tropical reefs—along with its isolated nature, has given rise to some of the world’s strangest and most iconic creatures. Eighty percent of the plants and animals in Australia are unique to the continent, found nowhere else on Earth. Having grown up in America, Australia’s animals seem downright alien to me, and I love learning about their evolution and their sometimes wacky behavior.

To immerse yourself in 50 minutes of the beautiful, weird, and wonderful Australian continent, you should watch “Hidden Australia” on Curiosity Stream. Believe me, I have spent my life watching nature documentaries, and this one still showed me animals I never knew existed. Curiosity Stream is a streaming platform with thousands of high-quality documentaries like this one. Now, Curiosity Stream has partnered with us to offer an incredible deal. By signing up for Curiosity Stream, you also get a subscription to Nebula, a streaming platform made by me and several other educational YouTube content creators. It’s a place where we can upload our videos and podcasts ad-free and experiment with new original content.

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