Monday, 30 April 2007

Why does a platypus lay eggs anyway?

Children are taught many ‘rules of thumb’ to help them identify animals, for example: mammals, are furry (or hairy), give birth to live young, make milk and take care of their children. So why does the furry platypus lay eggs? The discovery of this bizarre, egg-laying, duck-billed, web-footed mammal initially baffled naturalists, but now we understand them much better because we know much more about their special place in the tree of evolution.


All animals on Earth are related, even though these relationships aren’t always obvious. The reason these relationships are hard to figure out is that despite the diversity of animals we see, they represent a mere fraction of the life that has ever existed on this planet. In fact, one famous palaeontologist estimated that we know of less then 1% of the Earth’s diversity through time.

To understand the relationships between animals better, we have to look back in time, at the extinct ancestors of the animals alive today (animals living today are called ‘extant’).

This diagram (called a cladogram) shows how the five major groups of vertebrate animals are related:
-Fish gave rise to the first amphibians, which crawled on to land about 400 million years ago.
-Amphibians and reptiles share a common ancestor about 350 million years ago.
-Mammals and birds evolved from reptiles much later at different times during the Mesozoic Era.

Each of these major transitions was slow. It must be understood that a fish doesn’t become an amphibian overnight. There were many different species of animals that formed that transitory stages between these groups and possessed a unique set of characteristics, a sort of ‘mosaic’ between the two groups.

Now let’s look more closely at the evolution of mammals. Mammals evolved from mammal-like-reptiles, a very diverse groups of animals which are all (unfortunately) extinct. Through time three major groups of mammals evolved:
Prototherians - which lay eggs (eg. platypuses and echidnas)
Metatherians - which let young develop in pouches (marsupials such as kangaroos and koalas)
Eutherians – ‘modern mammals’ which give birth to well-developed offspring (this includes many familiar species such as rabbits, elephants, horses, and humans)

The platypus is a prototherian. This ancient group branched off of the mammal tree of life early on before the other two groups. There are many different prototherians in the fossil record but only platypuses and echidnas are still around. So the past there was a great diversity of egg-laying mammals, but sadly, all of those animals except for platypuses and echidnas) are now extinct.

If you are interested in this topic I recommend visiting Date A Clade, which has an excellent (and more comprehensive) cladogram with detailed information on when major groups of animals split away from each other.

16 comments:

Max said...

Hi, I just started reading your blog and have been enjoying it very
much! I do have a question about the platypus post you made though:
you said that "Mammals and birds evolved from reptiles much later...",
which implies to me that mammals and birds share a common ancestor
that reptiles do not, but doesn't your cladogram show that reptiles
and birds are the ones that share a common ancestor that mammals
don't? And in this case, where does the bill come from? I understand
that the egg-laying was lost after meta- and eu- therians branched
away from protherians, but the fact that reptiles and birds branched
off of the whole bunch of mammals before before that would seem to
mean that either birds and protherians independently evolved
beaks/bills, or that metatherians/eutharians, and reptiles
independently LOST beaks/bills. Anyway, I look forward to hearing
back about my confusion :)

Sarda Sahney said...

Hi Max, I will clarify.
Mammals evolved from mammal-like-reptiles sometime around 320 million years ago and birds evolved from reptiles 250 million years ago.

Mammals, reptiles and birds share a common ancestor but that ancestor is older then 320 million years. As I said, the Date A Clade website has more information on the topic.

Also, you are correct in that the platypus' bill is not related to the bill of a bird and that it is an analagous structure of different origin.

the domestic minx said...

Oh there are many weird creatures in Australia - many of them human!!

I love the platypus!

Laelaps said...

It's interesting that when the platypus was first brought to the attention of Richard Owen the eggs of the animal had yet to be found (although anecdotal evidence existed). Owen apparently found no evidence of egg-laying (i.e. internal membranes that would secrete shell material), but of course we now know the real story.

Zach Miller said...

I keep hoping that the term "mammal-like reptile" will go away, because it's not true. Reptilia is a clade within Amniota. The amniotes, of course, being mammals and reptiles. "Mammal-like reptiles" are actually synapsids, whereas the other major branch of egg-layers are reptiles.

Turtles, lizards, birds, and Petrolacosaurus are reptiles. Dogs, cats, bears, and Dimetrodon are synapsids.

Mammals did not evolve from reptiles--they evolved from lower-tier synapsids like pelycosaurs or something much like them. Actually, I think I'll blog about this later today...

Gufo said...

so, the playtipus lays eggs. that much I knew. What I am not clear about is, those reptiles (and some sharks, but that's probably a different story) who give birth to their babies, do they just keep their eggs within until they're hatched? are they hard-shelled? or not? Do they nourish them somehow? do they have specialised structures/organs to provide the embryos with oxygen, if not food altogether?

Gufo said...

Is it protherians, or prototherian as I found on wikipedia? Ehy today's featured picture in wikipedia is an Echidna! I blogged about this Synchronicity.

Sarda Sahney said...

Gufo, in regards to sharks, I am not sure because as you said some do bear live young. I will look into it. And I stand corrected on my spelling, you are right it is prototherian not protherian. Thanks!

Lucy said...

While I found this informative it didn't quite answer the question I had. While this post was made a while ago, perhaps you could answer my question and point to some further sources I can look at.

Lucy said...

While I found this informative it didn't quite answer the question I had. I realise this post was made a while ago, but, perhaps you could answer my question and point to some further sources I can look at.

So, we know that the Platypus lays eggs because sometime historically its ancestors did, but then why has the Platypus since not evolved to be viviparious like other mammals? How does laying eggs advantage the Platypus in a way that bearing live young doesn't?

Thanks in advance.

James said...

Platypuses like other monotremes have a cloaca which is one opening that the kidneys, the reproductive tract, and the alimentary tract all empty into. This means that if a baby is being born it's going to pass through an area where waste is being expelled. A shelled embryo may be more protected in this environment since the waste could wash back and damage an unprotected embryo. In therians (marsupials and eutherians), the ureters developed separate of the embryo canal to prevent damage to the embryos.

In regards to vivparious sharks, they do keep the eggs within them. They are soft leathery eggs but the way the embryos are nourished depends on the type of shark. Some are completely nourished by the yolk of the egg and other have a yolk-placenta so they get nutrients from the yolk but once they eat that and the egg shell up they still receive nutrients from the uterus. They can also get nutrients once they are out of the shell but still in the uterus by eating other eggs or siblings in the womb.

stephanie said...

if your theory of elevolution is correct where are the evolving skeletons that would show the evolving transformation. birds have always been birds different sizes and breeds but a bird will never have an egg that is half bird half reptile nor will any other creature.

Anonymous said...

I think this is very interesting. After watching "Phineus and Ferb", a show on Disney Channel that my daughter watches, I was confused on how platypus's can be a mammal if it lay's eggs. Now it all sort-of makes sense!

Anonymous said...

The author of fishfeet2007.blogspot.com has written an excellent article. You have made your point and there is not much to argue about. It is like the following universal truth that you can not argue with: A Hermit never has peer pressure. Thanks for the info.

Ella said...

hi, im doing a science experiment for school about why platypuses lay eggs, although this explains alot, and i dont understand most of it, i was wondering if there is a simpler way of explaining, and i didnt see anything about how they lay eggs. if you dont know the answer, or have no idea what i'm talking about, then its ok, but ifyou do, do you mind posting it? thanks
Ella

Alex said...

Wow, this is extremely interesting. I have always been, well, obsessed with the platypus because of its uniquity, but now I know WHY it is so odd. I am also doing a minor school assignment about platypi, and I thank Sarda and james as well for the clarification as to why a platypus lays eggs. Ella, you might want to look at james' post above as well.