Whales are warm-blooded mammals, and whale mothers feed their babies milk; a blowhole is a whale's nostril, the opening it uses to breathe. The first whales lived on land. Pakicetus was about one meter long and looked more like a wolf than a whale. Over about 10 million years, early whales lost their hind limbs and grew powerful tails, as their legs stopped holding up the body and took on new jobs; by about 49 million years ago, early whales such as Maiacetus and Dorudon were fully adapted to life in the sea. DNA points to the hippopotamus as the closest living relative of whales, while fossils of a small deer-like animal called Indohyus suggest it was even closer kin.

There are two kinds of whales: toothed whales and baleen whales; toothed whales have one blowhole, and baleen whales have two. Baleen whales have no teeth in their mouths. Plates called baleen hang from the upper jaw. Baleen plates work like a sieve: water goes out and food stays in; a sperm whale has large teeth in its lower jaw, about 20 to 26 on each side, while the upper teeth rarely come through the gums. Baleen is made of keratin, the same protein as human nails and hair. Because keratin rarely fossilizes, it is hard to tell when baleen first evolved, though the earliest known baleen whales lived about 30 million years ago. A bowhead whale has the longest baleen plates of any whale, about 3 to 4.5 meters (10 to 15 feet) long; rorqual whales such as humpbacks, fins and blues feed by lunging: they gulp a huge mouthful of water and prey, then push the water out through the baleen.

A sperm whale's head is huge, about one-third of its body length, and holds 3 to 4 tons of an oily substance called spermaceti. Toothed whales hunt using echolocation: they send out clicks and listen for the echoes. Beluga whales use their ears and sound to find their way and to hunt because they can echolocate; orcas click to echolocate, and they also rely on sound to feed, talk and find their way, and belugas make many sounds, including whistles, squeals, moos, chirps and clicks, and they are very talkative whales. Humpback whales sing songs, so people call them the singing whale.

A narwhal's tusk is a left canine tooth that usually grows through the upper lip of males at 2 or 3 years old, and most females keep both teeth hidden in the jaw, though some narwhals of either sex grow one or two tusks. A narwhal's tusk spirals to the left and can grow almost 10 feet (3 meters) long. Adult narwhals are about 13 to 16 feet (4 to 5 meters) long; a baby narwhal is about 5 feet (1.5 meters) long when it is born. The belugas and some other Arctic whales have no dorsal fin, which helps them swim under ice and cuts heat loss. A beluga has a round, bulging forehead called a melon.

Whale falls are powered by microbes: bacteria break down bones and give off hydrogen sulphide, and other microbes live off the energy from those chemical reactions; whale bodies that sink to the sea floor are called whale falls. Scavengers strip the soft tissue in a matter of months. A dead whale on the sea floor feeds a whole community, from large scavengers to tiny bacteria. A dead whale's skeleton can feed deep-sea life for years or decades; a single whale fall can give the deep sea as much food as several thousand years of marine snow, the slow drizzle of dead material from above. Zombie worms of the genus Osedax, whose name means bone devourer in Latin, were first found on a whale fall in 2002.

Words to know

• keratin: a body protein
• fossilizes: becomes a fossil
• microbes: tiny life
• canine: pointed tooth