"Type 6 pouches (a bag with an opening oriented to the rear) are understandably linked with burrowing in wombats, bandicoots, and the marsupial mole, but are also found in the aquatic Chironectes. However, there is a ringer: the koala possesses a backwards-opening pouch, despite the fact that it is almost strictly arboreal (lives in trees) and certainly never burrows!
We are tempted to regard this instance of an inappropriate container for the neonate as an instance of maladaptive radiation, but perhaps it is just a palimpsest (a manuscript which has been re-used by scraping off the original text and writing over the top) of the koala's shared phylogenetic history with wombats. However, M. Renfree makes the interesting suggestion that the koala's Type 6 pouch may facilitate coprophagy (eating feces) by the young, which is necessary to develop the gut flora associated with digestion of Eucalyptus leaves.
Deep pouches evolved in arboreal or saltatory taxa; backward- oriented pouches arose in burrowing forms and an aquatic species, and were retained (perhaps to encourage coprophagy and transfer of intestinal flora) in the arboreal folivorous koala."
Storytellers: Thomas J. Givnish and Kenneth J. Sytsma
Molecular Evolution and Adaptive Radiation
Story Research : Geoffrey Simmons
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