For this little guy the "Great Big World" is a Eucalyptus Tree. He only comes down from the tree to mate, fight a bit, or in this case to mark the tree as his. You can see the brown on his chest which comes from glands used for marking.
I was invited to a presentation about Koalas by the Wildlife Conservancy of the San Diego Zoo yesterday morning and learned a lot about the world of Koalas. There are many other worlds apart from ours.