Up a Gum Tree with a Down Pouch

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Up a gum tree with a down pouch: Richard Dawkins notes that the koala pouch “opens downwards, instead of upwards as in a kangaroo.” According to Dawkins this is “not a good idea in an animal that spends its time clinging to tree trunks.” Dawkins’ explanation for this apparent anomaly is: “Koalas are descended from a wombat like ancestor. Wombats are champion diggers.” He then goes on to explain that it has not been possible for evolution to change the back facing wombat pouch into a forward facing one. Dawkins suggests “the embryological upheaval attendant on such a major change would render the intermediates even worse off than the koala coping with the existing state of affairs.”
Richard Dawkins, The Greatest Show on Earth, Bantam Press, 2009, pp369-370

Editorial Comment: Since us Aussies who live with koalas in our backyards don’t ever worry about “drop bears” hitting us on the heads when we walk under gum trees, perhaps some local knowledge will help. It is obvious that Dawkins’ faith in evolution as the explanation for all things is rather limited. He is able to believe evolution could turn the wombat pouch around when it was evolving from a possum-like creature with front facing pouch, so that the wombat pouch does not fill with dirt while it digs burrows, but evolution has not been able to do the reverse when the wombat evolved into a tree climbing Koala.

In fact it is an upward facing pouch that is not a good idea for a gum tree climber. Koalas have short legs, so when they climb trees their abdomens are close to the surface of the tree, if not actually scraping it. If the koala pouch opened upwards the pouch would collect bark and other detritus as the koala moved about the tree. When they are not moving (which is most of the time) they sit, well supported, in forks of trees, rather than spending their time “clinging to tree trunks”. Furthermore, the koala pouch is well equipped with a strong muscle around the opening to keep it closed, and whilst the baby is feeding on milk alone the mother's nipple swells in its mouth keeping it firmly attached. Koala babies are not in danger of falling out.

The following information comes from the "Save the Koala" website:
"Female koalas have been described as having a ‘backward-opening’ pouch, in common with wombats and in contrast to an upward-opening pouch like kangaroos. However, that's not strictly true. When a female koala first gives birth to young her pouch opening faces neither up nor down, although it is located towards the bottom of the pouch rather than at the top. It faces straight outwards rather than 'backwards'. It sometimes appears to be ‘backward-facing’ because when the joey is older and leans out of the pouch, this pulls the pouch downwards or 'backwards'."

The reason the joey pulls the pouch in a backwards direction is that during weaning process the mother produces a mix of pre-digested leaves in her caecum (part of large intestine) and passes this out as droppings for the baby to eat. The koala baby knows where its food source is: down, not up.

Dawkins should consider the whole animal, including its behaviour and reproduction, not just one feature, when assessing what is good and bad design. (Ref. marsupials, mammals, reproduction)

Evidence News, 26 May 2010