[Note: This post is inspired partly by Erika, partly by a kangaroo documentary Ariel and I watched today, and partly by a book about two cute little animals (“Platypus” and “Echidna”) having a birthday party, which we read in the library this morning.]
When I was a kid at school and paid as little attention in Biology as I thought I could get away with, all I picked up about kangaroos is that they are marsupials which means they have a pouch to put their babies and all I picked up about the platypus is that it is a bit weird, something like a cross between an otter and a duck.
So now I learn stuff. If anyone else is as uneducated as me in these matters, prepare to be amazed. I mean, wow!
Mammalian reproduction comes in not just one but THREE basic types.
The one we all know about, because it’s what happened to us, is the placental variety. Sperm and egg meet and set up home in the uterus, growing into a foetus that is nourished by the placenta and is then born at a fairly well-developed stage. Lactation then occurs, usually for a relatively brief period, before the new animal becomes more or less independent of its mother.
Then there is the monotreme type – the rarest and most primitive method of mammalian reproduction, where a mammal lays eggs. Yes, we’re talking otter meets duck: also the echidna (spiny anteater) but that’s about it. Lactation happens after the eggs hatch.
Finally, those marsupials. The male has a two-pronged penis, which is just as well because females have two vaginas, each leading to a separate uterus. Don’t ask me why they need two, I don’t know.
There is no placenta to nourish the marsupial foetus, it just has a yolk-like sac to keep it going until it is developed enough to exit the uterus (though a third vagina!) and crawl a short distance through its mother’s fur to reach the pouch where it latches itself onto a nipple. Since lactation starts at such an early stage, it is relatively prolonged and, at least in kangaroos, continues until well after the joey first starts to explore the world outside its mother’s marsupium.
Another remarkable thing about the female kangaroo is that she can hold a joey foetus in suspended animation within her uterus.
At any given time, assuming conditions are favourable, a mature female kangaroo might have one joey who has recently left the pouch but is still coming back for milk, another inside the pouch attached to a nipple (her body makes two different kinds of milk – one for the older joey and one for the new foetus), and a third foetus in her uterus ready to be born as soon as the joey in the pouch is ready to leave. She manages to time things so that almost as soon as the joey leaves the pouch, the next one is ready to be born and take up residence. And as soon as that joey foetus leaves her uterus, she comes into heat and will mate: the resulting zygote will develop for a few days and then stop, dormant until she is ready for it to move to the next stage. Isn’t that amazing?