Sunday 14 July 2013

Is a Platypus a Mammal?


Oh, you wanted more than that? Well, let's see what I can do.

I've been asked this question a couple of times recently, and I was somewhat surprised on both occasions. Yet why should I be? When I conducted a very brief straw poll at work this week, I discovered that most people I spoke to weren't sure, with all but one of those saying "probably not". (I had one response of "definitely not, because they're marsupials", an answer that's wrong on so many levels I'll ignore it from here on in). Only one was confident of the correct answer.

It's all very well for me to say "what else could it be?" because, if I'm honest, I spend a lot of time on this blog pointing out that porpoises aren't dolphins, rabbits aren't rodents, musk deer aren't deer, and so on. If a porpoise can be its own kind of thing, without being a dolphin, then why can't a platypus also be its own kind of thing - albeit at a higher level of strangeness?

And platypuses are, undeniably, pretty strange. (Incidentally, before anyone asks - yes "platypuses". There's no such word as "platypi" - unlike "hippopotami", it's flat out wrong). This, I dare say, is why most people I asked were so uncertain. There is no other mammal that looks at all like a platypus.

For a start, there's the beak. The platypus (Ornithorhynchus anatinus) is the only mammal to have anything that looks like a beak. The scientific name even means "duck-like bird-beak" in Latin. True, we call the snouts of dolphins and certain kinds of whale a "beak", but that's clearly not what we're talking about.

In fact, the platypus's beak is quite different from that of a bird. It's rubbery, rather than horny, and has an internal bone structure supporting it. It's actually there to house special sense organs that detect tiny electric currents in the water that give away the presence of the shrimps, worms, and so on that it eats.

But, odd though the bill is, I doubt it's the main reason people think platypuses might not be mammals. There are other, equally odd, adaptations among mammals - an armadillo is kind of scaly, but most people probably don't think they're reptiles. And, when you think about it, the bill is really no stranger than, say, an elephant's trunk.

No, the kicker is surely the fact that platypuses lay eggs.

Indeed, I've seen a rant online where somebody complained that you can't trust Wikipedia, because it claims that platypuses are mammals when they "obviously can't be" because they wouldn't lay eggs if they did.

Yet the platypus is, indeed, an egg-laying mammal. It isn't even the only one, because the four species of echidna also lay eggs. Mind you, that is it - just five living species out of a total of over 5,700 kinds of mammal. This is not a lot.

So we are, perhaps, entitled to ask "why is it a mammal?" It's not as if the egg-laying is the only thing that's strange about it. For instance, with its legs splayed out to the side, it even walks like a lizard, and not like any other mammal - even echidnas. Although it is warm-blooded, it has one of the lowest body temperatures of any mammal. And so on.

So couldn't we just say that it's very-nearly-a-mammal, and put it and the echidnas together in some group of their own? Well, yes, we could, if we really wanted. Platypuses and echidnas together constitute a group called the monotremes, which is distinct from all other mammals. The group that contains absolutely everything else are called the "therians", so platypuses and echidnas are the only mammals that are not also therians.

Technically, if we wanted, we could rename the therians "mammals", and leave the monotremes to be something else. But, despite their odd egg-laying habits, there are many things that they do have in common with other mammals.

When we're looking at fossils, the definition of what counts as a mammal is that the creature must have three bones in the inner ear. The platypus does, so, by that definition, it must be a mammal.

This, incidentally, also relates to the similar question "is a platypus a synapsid"? To which the strict answer is an unqualified "yes", since all mammals are synapsids, by definition. But what people are often thinking of when they use the word "synapsid" (those that do so at all, that is) is "pre-mammalian synapsid", a sort of almost-mammal. And, because of the ear structure, that's not a description that fits platypuses. They are proper, true, mammals.

But, let's face it, if you want to know whether a platypus is a mammal or not, most people aren't going to open one up to check the bones in its ears. If it were a fossil, that would be your only option, but, fortunately for platypuses, we don't have to do that with living animals.

We can look at them, and see that they have fur. In addition to demonstrating that they're warm-blooded, and must have some body heat for the fur to be keeping in, fur is something we only ever see in mammals. (Among living vertebrates, anyway. Because, yes, bumblebees). It's plausible that some of the pre-mammalian synapsids also had fur, but they're all long gone, and we'll probably never know for certain anyway, so we can ignore those.

The real clincher, though, is that platypuses and echidnas suckle their young, providing them with milk. This, among living species, is the absolute defining feature of mammals. There are mammals that don't have fur, such as whales. There are reptiles, such as sea snakes, that don't lay eggs, and give birth to live young. But milk, that is an absolutely mammalian feature: platypuses have mammary glands and, as the very word indicates, that makes them mammals.

The oldest known marsupial fossil is the 125-million year old Sinodelphys, but, because the oldest known placental fossil is the 160 million year old Juramaia, the split between placentals and marsupials must have happened at least that far back. And this, in terms of mammalian evolution, is a staggering long time - even the likes of Tyrannosaurus rex only lived 65 million years ago. The split between monotremes and therians must be further back still.

But, unfortunately, we have almost no fossils to go on. That may be partly because monotremes aren't exactly common now, and possible never were. It doesn't help that one of the main tools we use to assign fossil mammals to particular groups is their tooth structure. But, while baby platypuses have teeth, the adults don't, and echidnas don't have them at all. Without a reasonably complete skeleton, there's often very little to go on. Fortunately, baby platypus teeth are highly distinctive, and we do have the odd fossil tooth here and there that looks similar.

But even on this scanty basis, the oldest fossil monotreme, Teinolophus, is 123 million years old - impressively early, but from long after even the marsupials had become distinct. Interestingly, the second oldest, Steropodon, from about 100 million years ago, apparently had a bill, which may place it closer to platypuses than echidnas. [Although not necessarily - see comments].

Assuming that monotremes have evolved genetically at about the same speed as other mammals (which, admittedly, is hard to know), we can get some idea of how long ago they first appeared by comparing their genes and biochemistry with that of therians. This gives an estimate of around 220 million years ago, which is actually rather further back than the oldest definitive mammal fossils we have. Unlike marsupials, they may well have originated in Australia, although the continents were such a different shape back then that it's debatable what that would actually mean.

Either way, it may be deeply weird, but the platypus is, without any shadow of a doubt, a mammal.

[Picture by Stefan Kraft, from Wikimedia Commons]


  1. I'd like to point out that echidnas and platypus apparently only diverged during the Oligo-Miocene according to some recent studies. Hence, *Steropodon* is morphologically closest to platypus but not phylogenetically and echidnas derive from platypus-like ancestors. Fossil platypuses *Obdurodon* and *Monotrematum* might thus be basal to this split. Amazingness unending!

  2. More a matter of semantics, but one might say that the unequivocally furry Castorocauda is not a mammal as it's not a member of the crown group.

    It may also be worth pointing out that monotremes only lay eggs because they retained the ancestral condition. It so happens that most living mammals are livebearers, but at the dawn of mammalhood egg laying may well have been the normal mode of reproduction.

    1. As you say, a matter of semantics. I admit to using a rather broader definition here, that goes beyond merely the crown group (I'd probably count Morganucodon, for instance). But, yeah, I'll also confess to not having even thought of Castorocauda!

      And, absolutely, one would be rather surprised if the very first mammals/mammaliformes/whatever-you-want-to-call-them didn't lay eggs. It would seem an odd thing for monotremes to re-evolve.