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The Koala is a thickset arboreal marsupial herbivore endemic to Australia. Koalas live almost entirely on eucalyptus leaves. This is likely an evolutionary adaptation that takes advantage of an otherwise unfilled ecological niche, since eucalyptus leaves are low in protein, high in indigestible substances, and contain phenolic and terpene compounds that are toxic to most species. Like wombats and sloths, koalas have a very low metabolic rate for a mammal (which conserves energy) and rest motionless for about 20 hours a day, sleeping most of that time. Photo credit: Guillaume Blanchard |