Composition of the Milk of the Koala, Phascolarctos cinereus, an Arboreal Folivore

The composition of milk from free-ranging koalas was examined as part of a study of the energetics of reproduction in the koala, an arboreal folivorous marsupial that displays a number of energetically conservative characteristics often associated with arboreality and folivory. Koala milk composition showed a number of deviations from the general marsupial pattern that suggest koalas have adopted a lactational strategy different from that of most other marsupials previously studied. Milk solids decreased at the time of exit of the young from the pouch, in contrast with the solids content of the milk of most marsupials, which rises at that time. Lipids provided the major source of milk energy even in early to midlactation and did not rise at pouch exit, unlike lipid levels in other marsupials, which are low in early to midlactation and rise sharply at pouch exit to high levels in late lactation. The carbohydrates of late-lactation koala milk contained mainly lactose and some oligosaccharides, but little or none of the monosaccharide found in the milk of many other marsupials. Koalas share most of these features with the common brushtail possum (Trichosurus vulpecula) and common ringtail possum (Pseudocheirus peregrinus), the only two other marsupial arboreal folivores in which milk composition has been studied. This suggests that the koala'spattern of changes in milk composition may be common to marsupial arboreal folivores and may be associated with energetic limitations imposed directly by their folivorous diet or indirectly via selection for their long period of lactation and slow weaning. However, koalas' milk was more concentrated than that of the other two marsupial folivores, the importance of which, to the lactational strategy of the koala, can be resolved only by further study of milk production.