Kangaroos are very well suited to their environment. They have adapted over millions of years to fit into their ecological niche. They have large ears, and relatively small heads. Their front limbs are very small, but their paws are very nimble. Their hind legs are very large and powerful. When they are moving slowly, kangaroos walk on all four limbs, with the front limbs barely touching the ground. But when they feel the need to move quickly, they stand nearly upright, and start hopping on their hind legs. When they begin to hop, kangaroos use their muscular, agile tails to balance themselves, and to help them change direction.