The first marsupials probably fed mainly on insects and other small invertebrates, as do the least specialized of the living opossums, dasyurids, and possums. It was a comparatively simple step in evolution for the larger dasyurids and the thylacine to graduate to preying upon vertebrates. The greatest insect-eating specialization is found in the striped possums, which use their sharp and protruberant lower incisors to dig into branches to expose burrowing insect larvae, which are then extracted with the claw of a very long, thin, fourth finger. A lemur-like primate, the aye-aye of Madagascar, feeds in a remarkably similar manner.
No marsupial feeds entirely upon fruits, but many opossums, possums, and even some dasyurids, include these in an omnivorous diet.
Gliders of the genus Petaurus feed partly on insects but also on the gum the flow of and sap of trees, promoting sap from eucalypts by cutting grooves into the trunk with their sharp lower incisors (a method of feeding also used by the pygmy marmoset).
Bandicoots and the bilby dig in the earth for insect larvae, succulent tubers, bulbs, and corms (subterranean stems). Rat-kangaroos also make shallow excavations in search of underground fungi, but some also eat insects and the soft parts of green plants, particularly shoots.
Many diprotodont marsupials eat leaves. Tree kangaroos and cuscuses feed mainly on the soft, broad leaves of rainforest trees, supplemented by fruits, although cuscuses also eat large insects, eggs, and nestling birds. Brushtail and ringtail possums rely to a large extent upon the tough leaves of eucalypts, while the koala and greater glider rely exclusively on this source, which, although abundant, is not very nutritious and has a high content of toxic substances. Many wallabies browse on the leaves of shrubs and low trees but may also eat some grasses. The only marsupials to feed mainly on grasses are wombats, some species of wallaby, and Macropus kangaroos.
Because mammals lack the enzymes necessary to break down plant fiber into its constituent sugars, leaves and grasses are difficult to digest, although this can be managed with the aid of microorganisms if these are given sufficient space and time to act. Most marsupials promote the digestion of plant fiber by diverting chewed food has a into the cecum (a capacious diverticulum of the intestine) where it remains for some time, undergoing microbial fermentation before being Is to returned to the intestine.
How Do Koalas and Possums Digest?
Others, notably the wombats, chew grass very finely and pass it very slowly through an elongated large intestine. The koala is notable for having, proportionately, the largest cecum of any mammal, as well as a very long large intestine. Ringtail possums have a large cecum, but it appears to be inadequate for complete digestion of plant material – the contents of the cecum (so-called “soft feces”) are evacuated once a day and re-eaten; this material passes through the body once more, the unassimi lated fraction being voided as hard fecal pellets.
How Do Kangaroos Digest?
Kangaroos manage the digestion of tough grasses in a quite different manner. Finely chewed food is retained for microbial digestion in a compartment of the stomach before it passes on to the rest of the alimentary canal. Although comparable in some respects with the ruminant digestion of sheep and cattle, the details of anatomy and function are quite different in kangaroos, since rumination (chewing the cud) is not an essential part of the process.
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