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The absence of koalas outside Australia is more a matter of geology than anything else. Australia and New Zealand were once part of a supercontinent called Gondwanaland, which broke up into what we know now as South America, Africa, India, Antarctica and Australia.
The two islands that make up New Zealand were pulled away from Australia by continental drift (a.k.a. plate tectonics) at least 90 million years ago, while koala fossils in Australia go back just 24 million years. Since New Zealand and Australia separated long before koalas existed, New Zealand had to leave without any. By the time koalas evolved, New Zealand was 1,250 miles away across a cold, blue ocean.
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Staying Where the Food Is
One of the things you notice about koalas is that they hate to hurry. They never made it across Australia's arid interior to the western states, or even around the edges.
Heck, koalas are so slow-moving that they didn't even make it from Queensland and New South Wales (eastern Australian states famous for these cuddly-looking marsupials) to Australia's only island state of Tasmania before it separated from the Australian continent a mere 10 thousand years ago. Tasmania may be only 150 miles away across the Bass Strait, but koalas -- unlike other notable Australians -- are not great swimmers.
In addition to the limiting factor of lack of speed, koalas feed exclusively on the leaves of the many species of eucalyptus trees. These trees actually all contain deadly toxins; the leaves of each species contain different amounts of toxins at different times, so the koalas need access to multiple species of eucalyptus in a specific growth sequence in order to survive without being poisoned. This makes it impossible for koalas to live anywhere but where the right varieties of eucalyptus trees can grow in the right order.
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Koalas at the Zoo
The largest breeding colony of koalas outside Australia is at the San Diego Zoo. The zoo spent 28 years planting its grounds with eucalyptus trees by the thousands to make fresh koala food available. They evaluated their climate to be sure it was suitable for koalas. They did everything they could think of to make the zoo a home for koalas, until 1952, when four koalas came to California to appear in a Hollywood movie and then to take up residence at the zoo.
The colony thrived, and in 1983, selected koalas began to travel to other zoos in the US and all over the world as educators and ambassadors, complete with an entourage of keepers and support staff, as well as daily deliveries of fresh eucalyptus leaves. You can now see koalas on regular exhibit at several major U.S.zoos, so contact the zoo nearest you to find out if koalas are there or are coming!
- Koala bear in tree, PhotoSpin
- Australasian Society of Zoo Keeping, Inc.: Koala (Phascolarctos cinereus) Captive Husbandry Guidelines, http://www.aszk.org.au/docs/koala.pdf
- San Diego Zoo: ZooNooz, June 2013, http://animals.sandiegozoo.org/zoonooz/beguiled-koalas
- NASA Science News: Continents in Collision: Pangea Ultima, http://science1.nasa.gov/science-news/science-at-nasa/2000/ast06oct_1/
- University of Sydney: How Australia Lost New Zealand: Computer Model Explains Abyss between the Two Countries, http://sydney.edu.au/news/84.html?newsstoryid=4681
- Queensland Government: Department of Environment and Heritage Protection: Koala Ecology, http://www.ehp.qld.gov.au/wildlife/koalas/koala-ecology.html