When the platypus first came to the attention of European scientists in 1798, not everyone

was convinced the creature before them was real. Some thought a prankster had mashed

together separate parts of different animals to create a fake - a not uncommon occurrence

at this boom time of naturalist discovery. But the platypus was a very real animal, and

one that confused anatomists for some time. A creature with fur, a bill and webbed feet,

that lays eggs, and can secrete venom? Was this a mammal? A duck? Some sort of furry

reptile? These European scientists were asking the

same questions that the aboriginal people of Australia had asked 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 science terms, we have ultimately classed 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. And 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. But just like with all animal adaptations,

there has to be some purpose to all of the platypus’s strangeness. But what evolutionary

question was this the right answer to?

If you look 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.

Typically being the key word there. Platypuses are one of only two mammals that lay eggs.

The other is the only other monotreme – the echidna, of which there are 4 species.

When it’s time to lay the eggs, female platypuses dig their burrows, crawl in and seal themselves

up. Here they lay their eggs, curling them between body and tail, until they hatch 10

days later. From here, 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 long time.

Around 340 million years ago, the first amniotes appeared on earth, in the form of small lizard-like

creatures. Amniotes are four legged vertebrates that are 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 cannot live far from water. But thanks to the amniotic egg, reptiles

can lay their eggs nearly anywhere on dry land.

Soon, amniotes spread far and wide around Earth's land and became the dominant land

vertebrates. And around 315 million years ago, they split into the two major groups

of four legged vertebrates which still exist today.

One branch contains the modern reptiles and birds. The other included the 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. But 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, which in turn helps sustain growing embryos.

Platypuses have fewer copies of the gene than birds and reptiles, but most mammals don’t

have the gene at all. [4] This means the platypus has the ability to lay eggs, but that 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 hangover 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. That is, until they got swept aside by the

arrival of their marsupial cousins.

Marsupials originated in what is now South America, and migrated over to what is now

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, and thus they outcompeted the monotremes on almost every front.

And slowly, all but two, the echidna and the platypus, went extinct.

So the question is, why did the platypus and certain echidnas survive, when the rest did

not?

One hypothesis as to how the platypus persisted in the face of intense competition from the

marsupials is its ability to take to the water - a domain where the marsupials could not

follow. The echidna’s earlier ancestor, too, is thought to have been semi-aquatic,

even though it is not any more. 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. The babies would drown if their mothers ever had to venture into the water.

But with their eggs secure in a nest, the platypus can happily stay in the water, avoiding

predation from the marsupials, and exploiting their very own environmental niche.

The platypus is an expert swimmer. Its ability to hunt below the water is down to a few,

key features of its physiology. The platypuses' webbed feet help power them

through the water, using their front feet for paddling, their back feet for steering.

And a feature that helps the platypus stay submerged for up to two minutes at a time

is its ability to become watertight where necessary. It has folds of skin that cover

its ears, and it can close its nostrils, too. And, despite its hunting prowess underwater,

it nearly completely closes its eyes when diving. Thanks to a 6th sense that almost

no other mammal possesses, it doesn’t need to see to hunt.

One of the most distinctive parts of the platypus is its bill. It’s iconic shape is wide and

flat, but unlike a duck’s bill, the platypus bill is described as flexible, rubbery, and

a little fleshy. Its surface feels a bit like suede.

And the bill is the platypuses 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 stimulus, such as touch or pressure.

In the case of the platypus, its bill contains mechanoreceptors called pushrods. These are

columns of densely packed cells that move independently of the 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, it can detect

freshwater shrimp from a distance of 15-20 centimetres, 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 moving, and the platypus bill can sense these electric fields

originating from their prey. The bill contains around 70,000 glands, that assist in the electroreceptive

function of the bill The mechanism of electroreception in the bill

is much like that of the elasmobranchs, like the hammerhead shark. The electrical currents

from the stimulus travel through the water, then through secretions from the glands in

the bill, which surround the nerve endings beneath the bill’s surface. [9]

The 100,000 electro- and mechanoreceptors on the platypus 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, and those of long-billed and short-billed

echidnas have 2,000 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

But this electroreception is not a remnant from an early fishier ancestor. It evolved

completely independently of electric fish, and is the answer two different lineages came

up with to a similar evolutionary pressure.

Electroreception isn’t the only feature of the platypus that is rarely seen in mammals.

Next time you pick up a platypus, you’d be wise to keep its back legs pointing out

of reach. Male platypuses have spurs on the backs of their hind feet that connect to venom

glands in their abdomen. And that, while not deadly to humans, can have some pretty nasty

side effects: nausea, cold sweats and lymph node swelling, and immediate, excruciating

pain that can’t be relieved through normal painkillers.

Venomous mammals are now pretty 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 immobilise 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, and so venomous

capabilities became evolutionarily redundant. Of those mammals that kept their venom, the

very different methods of delivery suggest the ability evolved independently. Take those

slow lorises mentioned earlier: they can look pretty cute when they raise their arms in

the air as if they’re asking for a hug, but don’t be fooled, this is actually how

they access their venom. It’s produced by glands in their armpits, which they lick.

The venom mixes with their saliva and settles into grooves in their teeth, ready to harm

anything the slow loris bites. The short tailed shrew also has grooves in its teeth, but its

venom comes ready made in the shrew’s saliva. Both of which differ to the platypus, which

delivers its venom through it’s spurs.

But interestingly, for the platypus, a recent study found that many of the proteins present

in its venom are the same as those found in reptile venom, even though the reptiles split

from mammals some 315 million years ago. Does this mean the venom is an evolutionary leftover

from the platypuses' reptilian ancestor? Or is it an example of independent, convergent

evolution of the venom? By sequencing the platypus genome, scientists

found that the platypus’s ability to deliver venom is down to a duplication in a set of

reptilian genes that underwent the same duplication independently in snakes after reptiles and

mammals split in the evolutionary tree.

This suggests then, that platypus venom is an unlikely example of convergent evolution.

In this case, 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 they use them in mating practices, and to

defend territory and mates from other platypuses.

Despite its weirdness, we now understand that the platypus is not just a funny looking animal,

it is a highly adapted creature perfectly suited to its environment. Some of its bizarre

features are remnants from an ancient time, and others are more recent evolutionary inventions

that happen to be similar to ones in the fish, birds, and the 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. But we also know there’s a lot more of the

puzzle to put together. Scientists are constantly stumbling across new, unusual findings about

this amazing animal. Just last year, researchers accidentally discovered that platypus skin

glows under UV light – called biofluorescence. Why? We still don’t know. 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

made the platypus what it is today. Australia is literally teeming with biodiversity. It’s

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. 80% of the plants and animals in Australia are unique to Australia,

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 CuriosityStream. Believe me,

I have spent my life watching nature documentaries, and this one still showed me animals I never

knew existed. Like, what even is this guy.

This is such a fun film, and will make you daydream about getting to visit such an incredible

and diverse place.

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And if you are looking for something else to watch right now, you can watch our previous

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