The price of the pouch

Jack Ashby writes about the last UCL Lunch Hour Lecture of 2011, held on 8 December.

Every zoologist has their own favourite group of animals, and mine is marsupials. However, this group sometimes suffer a lot of stick from the more common type of zoologist who studies placental mammals. They say marsupials are boring, stupid, primitive, too few in number and are altogether inferior to ‘normal’ mammals. I was hoping that the Lunch Hour Lecture by Anjali Goswami (UCL Genetics, Evolution and the Environment and UCL Earth Sciences) would set some of these accusations straight.

Whenever I go to Australia to undertake ecological fieldwork I am struck by the diversity of the mammals there. You can travel 200km and find a different species of marsupial mouse doing a similar thing to the one you saw the day before, only in a slightly different environment. Go another 200km and you could find a third.

However, the three species do look pretty similar. One of the major downsides of marsupials, from a biodiversity point of view, is that they haven’t evolved the range of forms that placental mammals have. While there is a semi-aquatic species of marsupial – the yapok – it could hardly be compared with a whale or a seal; there are gliding marsupials too, but they can’t do what bats can do. Marsupials and placentals have both been evolving for the same length of time – 125 million years; why did flying, swimming or event galloping never arise in marsupials? Anjali put it down to methods of reproduction.

Mammals produce babies in one of three ways. The five species of monotreme (platypuses and echidnas) lay eggs. The 335 species of marsupial (e.g. kangaroos, quolls, bandicoots, opossums and wombats) give birth to highly under-developed (altricial) young that do most of their growing by suckling milk on a teat. The 4,500 species of placental (e.g. dogs, primates, rodents and nearly everything else) give birth to well developed (precocial) young who finish off development with a short period of suckling. What is it about these strategies that have limited marsupials so much?

When marsupials are born, which is after just a week or two of gestation, they look like a bean with massive lips and strong arms. That makes sense if you have to climb up to a pouch and attach onto your mother’s teat and suckle milk for weeks or months. Using some cutting-edge technology – laser scanners, micro-CT scanners, 3D digitisations and 3D microscopes – Anjali and her colleagues have studied the morphology of developing young. They found that new-born marsupials have well developed facial and forelimb skeletons, as would be expected, and a very basic nervous system – the bare minimum for that first expedition up the mother’s belly. In contrast placentals develop a complex nervous system earlier, and limbs come later as they aren’t needed early on.

This would explain why marsupials are so constrained in their evolution. Given that climbing arms are so important in their first moments, they can’t be changed later in life to a wing like a bat, a fin like a whale or a hoof like a horse (actually this last development did somehow happen once – the pig-footed bandicoot had hooves, but sadly Europeans drove it to extinction in the 1950s). Anjali also explained how the skeleton of the mouth is similarly restricted when compared to placentals, because of the importance of suckling.

Having explained this limitation, which I would argue doesn’t make them any less interesting than placentals, the lecture went on to explore the concept that marsupials are stupid. Sure, koalas sit around in a kind of inebriated stupor, but that’s down to the fact that their food is so toxic. It has been a widely held belief that placentals have bigger brains because for big brains to develop, a foetus needs a long time in the womb.

However, the data don’t hold up this popular view. Anjali has shown that marsupials don’t have smaller brains than placentals, at least excluding primates. In fact small bodied marsupials have bigger brains than similarly sized placentals. She proved that a long lactation period (like marsupials) or a long gestation (like placentals) can lead to big brains. So while marsupials may be less morphologically diverse in what they do with their hands and heads due to the pouch strategy, this has no effect on brain size.

To continue the rebuttal of marsupial jibes, Anjali argued that they aren’t primitive. Human-centric views have always led people to suggest that anything we do must be the best, most advanced method, but actually the marsupial method is a more advanced strategy. Human reproduction has more in common with the primitive tactics of the platypus than does the marsupial.

So why be a marsupial? Anjali ended by explaining that giving birth to altricial young is a much less risky strategy than putting all that energy into growing a big baby in the womb. If times are hard and the environment changes, if the animal would struggle to support both itself and its young, all they have to do is throw the little bean out of their pouch and try again when conditions improve. That’s certainly not the case for placentals.

Perhaps this explains how they managed to get all the way from China, where they first appeared, to colonise Europe and North America, down South America, across Antarctica and up into Australia – their current strong-hold. We placentals didn’t get there that way.

Jack Ashby, Acting Manager, Grant Museum of Zoology

Image: Photo by DAVID ILIFF. License: CC-BY-SA 3.0 Source: Wikimedia Commons