Whales are large ocean-dwelling mammals ranging in size from the 8.5 foot dwarf sperm whale to the 98 foot blue whale, which is the largest creature to have ever lived. Their closest living relatives are hippopotamuses, but they split from them about 40 million years ago and since have become so adapted to sea life that they are unable to survive on land. Whales are warm-blooded and have a layer of blubber to protect themselves from the cold temperatures of the oceans, which they used to be hunted for. Though required to breathe air, some whales can spend much time underwater, with some species capable of being underwater for 90 minutes before needing air. Whales communicate through various vocalizations, with the most well-known being the long drawn out songs of the humpback whale.