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An opossum joey. Image credit Anne Keyte

Marsupials are just plain weird when it comes to procreating.

I’m not talking about bifurcated penises (where the penis has two heads) although that’s pretty freaking weird. I’m talking about the embryos.

When a baby marsupial is born after a 4-5 week gestation, it’s a tiny pink speck of nothing much. About the same size as a jelly bean, it’s hairless, blind, and most of its brain has not developed.

In this state, the joey has to crawl across its mother’s fur and find its way into her pouch. Inside the pouch it continues to develop, growing on the nutrients it sucks from a teat.

It stays there for several months before it emerges, looking more like an animal and less like a little pink alien.

This bizarre method of procreation is the subject of a recent study into developmental biology. The writers, Anne Keyte and Kathleen Smith, found that although a joey is extremely underdeveloped when it is born, some of it features are accelerated. For example it’s forearms grow much faster than the rest of it. The joey has guns! It uses those strong arms to meander through mountains of fur and into the pouch. By comparison, its hindlegs are undeveloped and almost like jelly.

The study used opossum embryos and compared them to mouse embryos of the same age. They used two markers, Tbx4 and Tbx5, to track the development of the fore and hind limbs respectively. Both these genes were switched on earlier than in the mouse, but the forearms were especially beefy. Strangely the hindlimbs do not develop early like the forelimbs, even though the genes are switched on in both.

Not only is gene expression different in marsupials, the forearms were also allocated more red blood cells during development. This gives growing cells the energy they need to become big and strong. The spinal nerves grew into the forelimb buds differently as well.

This research rules out the assumption that limbs arise because of signaling from partially developed organs. The organs in a marsupial are simply not developed enough.

Marsupial faces also develop at an accelerated rate to allow the joey to suckle when it gets to the pouch.

But why such the strange method of procreation. Did marsupials not even THINK to evolve themselves a placenta??? “There are probably 50 explanations for why marsupials develop outside the womb, and none of them are very good,” says Anne.

Keyte, A., & Smith, K. (2010). Developmental origins of precocial forelimbs in marsupial neonates Development, 137 (24), 4283-4294 DOI: 10.1242/