After the age of man, the beaver's semiaquatic lifestyle proved beneficial enough to allow the animal to survive. It expanded upon this lifestyle by adapting further into the water: its hind legs became fused to its tail, and its facial sensory organs moved to the top of its head, so as to keep above water when the rest of the animal is submerged. It uses this tail to climb up tree trunks as well as propell itself through the water, giving access to even more building materials and food.