Photo courtesy of Fir0002/Flagstaffotos.
It’s hard to be a biologist without getting into the history of science, at least a little bit. And with a background in Anthropology as well I might be getting a bit boring when I keep saying that some of the things we do tell us just as much about us than anything else. How were you taught about marsupials and monotremes at school?
This might be an Australian thing, seeing we’re surrounded by them, but it’s one of the early things kids are taught about animals over here. We have special mammals called marsupials that have a pouch! And then it trails off into an embarrassed silence because they’re not as good as the real mammals they have in the northern hemisphere. And monotremes may be really cute and have special talents like electrolocation, but that doesn’t count because, cough, they lay eggs. Shhh.
How unfair. We’re small, we’re new, we invaded and dispossessed people and our nation was started by convicts, and we don’t even have proper animals. How backwards.
And this isn’t just my warped childhood memories, you should see the latin names. Placental mammals are ‘eutheria’, true or good animals. Marsupials and their relatives are ‘metatheria,’ the sort of animals or behind animals. Poor monotremes are ‘single holes,’ because they have a single cloaca like reptiles and birds, rather than separate genital, urinary and rectal openings. You could say that study of Australian mammals was based firmly on the deficit model.
But slowly, at the same time as we have developed our own national identity and come out of the shadow of England, we have also started looking at marsupials differently. Not as failed mammals or ones that got ‘left behind,’ but as animals who are cleverly adapted to a harsh and unpredictable environment.
Now these are major groups of animals we’re talking about and there are all sorts of anatomical and physiological details that I’m not going into in a blog post, plus I’m going to ignore monotremes for the moment. Sorry echidnas. I’m just going to point out some obvious bits about reproduction. Specifically, something about the costs.
As most of the people reading this are parents, you know that placental pregnancy is very, very costly to mothers. They need more food to support the developing young, and if they don’t get it the resources are scavenged from their own bodies. They are physically vulnerable as they carry around a weight, their organs are squashed and their joints loosen, even their immune system is suppressed so they don’t destroy their own babies. Pregnancy is very dangerous.
A change during a long pregnancy, such as social changes in the pack or bad weather leading to low food, could mean that the baby won’t survive and the mother has put in a large investment from her limited reproductive life for no return.
And ending pregnancy can be just as dangerous, whether it’s on time or not. A miscarriage is physically hard on a mother and bleeding makes her vulnerable, and while there are hints that some animals can control their pregnancies it seems to be very difficult, especially later. The day you are born is the most dangerous day of your life, and giving birth is one of the most dangerous human activities, although for most mammals it’s a bit easier. The pay off is that your baby is the best protected while it is developing because it is inside the mother, using her organs as a life support system. And when it is born it needs relatively less support.
Marsupial pregnancies are obviously different. The egg is released and fertilised in the uterus, where the embryo develops for a few weeks, relying on its yolk and a simple placenta. When it is still the equivalent of a human embryo it is born simply and easily, and drags itself to its mother’s pouch with its large forelimbs. Once in the pouch it attaches to a nipple and the mother begins lactating.
The pouch is not a simple flap of skin or bag to hold the baby – it is functionally equivalent to a uterus. It controls moisture and even has secretions that act as an immune system to protect the joey. The milk is also very complex, including antibodies and hormones and changing significantly over the months the joey is in the pouch.
It depends on the species, but joeys stay in the pouch for several months before beginning to emerge, then spend a long time going back and forth before staying out completely. It is functionally similar to a placental pregnancy, but the pouch and milk take the place of the uterus and placenta, and the joey hopping in and out is the equivalent of a placental infant born but nursing.
Some marsupials such as kangaroos have another special trick called embryonic diapause. As soon as the joey is born the mother is sexually receptive and produces another embryo, which is then developmentally frozen in the uterus. This allows the mother to match reproduction with resources – if there is not enough food around, the joey will die and the embryo remains frozen, so the mother can use scarce resources to stay alive herself. If there is plenty of food the second embryo can be born as soon as the first joey leaves the pouch and the mother can produce different milks for them both.
Live birth has advantages over eggs which allow or even force social living. This then has advantages in terms of learning complex behaviour, protection and things like hunting, where a group working together is more likely to be successful. Rather than being some type of earlier evolutionary stage that never moved on, marsupials and placental mammals are two alternative ways of caring for a foetus over months of development, one using a placenta and the other using milk.
In a harsh and unpredictable environment, such as, oh, Australia, marsupials are better adapted than placental mammals. A mother is physically safer and in the case of droughts or flooding rains* they can take resources from their babies or have another quickly, maximising their chances of successfully raising as many as they can. Far from backwards, that sounds pretty cool to me.
*For non-Australians – My Country by Dorothea Mackellar is a pretty good way to get to know Australians.
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