Whales: Facts about the largest animals on Earth

Whales are mammals — the same overall group that includes humans, dogs, cats, monkeys, pigs and others. Unlike fish, whales have lungs, not gills. That means they must return to the ocean's surface to breathe.

Like other mammals, whales are warm-blooded, which means their body temperature stays the same, even in cold water. They have a thick layer of fat under their skin. This fat, called blubber, keeps the heat in and the cold out. Whales' large size also helps them stay warm, since bigger animals lose heat more slowly than smaller ones do.

Whales, like most mammals, give birth to live babies . Whales give birth to one calf at a time. A whale calf stays in its mother's womb for between 10 and 16 months, depending on the species. It must swim to the surface immediately for its first breath. The whale calf feeds on its mother's milk for between six months and two years. Whale milk is very fatty, rich and thick — about as thick as toothpaste.

Whales eat a variety of prey. The size of their meals depends on if they have teeth. Some whales, like sperm whales and beaked whales, are classified as "toothed whales" because they have teeth.

Baleen whales, on the other hand, have large, comb-like "baleen plates" that are made from the same material as our fingernails. These whales use their baleen plates to filter tiny animals — such as plankton, krill and small fish — out of the water. Examples of baleen whales include blue whales, fin whales and humpback whales.

Toothed whales usually eat larger prey, which can include fish, squid, octopus, seabirds, seals, penguins, sharks, and even other whales and dolphins. Sperm whales dive up to 3,000 feet (900 m) below the ocean's surface to hunt for giant squid and other deep-sea prey. Some toothed whales use only their teeth to capture and grab prey and swallow it whole, while others chew their food.

Some whales may hunt by opening their mouths wide and pushing out a lot of water through their baleen to trap krill. Humpback whales sometimes work together to hunt. They use a technique called bubble netting, in which several whales blow bubbles to trap fish in a spiral and then swallow them.

The blue whale is the largest whale species. It is the biggest animal alive today and the largest animal that has ever lived. Blue whales can grow to be 110 feet (33.5 m) long — about as long as two semitrailers — and weigh as much as 165 tons (150 metric tons) when fully grown, according to the National Oceanic and Atmospheric Administration. A blue whale's heart is as big as a Volkswagen Beetle and weighs up to 400 pounds (180 kg).

The smallest whale species is the dwarf sperm whale, which is about 7 to 9 feet (2 to 2.7 m) long and weighs between 400 and 600 pounds (181 to 270 kg).

Blue whales can eat as much as 6 tons (5.4 metric tons) — about the same weight as an African elephant — of krill every day.

Blue whales are massive even at birth. When they are born, they measure about 25 feet (7.6 m) long. These gigantic babies drink over 100 gallons (379 liters) of their mother's milk every 24 hours and grow by 200 pounds (91 kg) each day.

Blue whales are listed as an endangered species, with only between 10,000 and 25,000 left in the world. This is mostly because they were heavily hunted during the late 1800s and early 1900s. Before then, there were an estimated 300,000 blue whales worldwide.

There are many more toothed whale species than baleen whale species — 77 species of toothed whale (including dolphins and porpoises), compared with only 15 baleen whale species. Well-known species of toothed whale include sperm whales, dwarf and pygmy sperm whales, belugas and narwhals.

Some whales have hundreds of teeth. Others, like the narwhal, have only one or two. In fact, the narwhal's famous, unicorn-like tusk is actually a tooth growing through its upper lip.

Toothed whales hunt using a method called echolocation. They send out high-pitched clicks from their melon, a fatty organ in their forehead. By listening for the echo of these clicks, the whales build a picture of their surroundings in . Echolocation can also allow a whale to determine how hard or soft an object is and how fast it's moving.

Sperm whales can spend over an hour diving thousands of feet beneath the surface to hunt, using echolocation to track down giant squid, octopuses and other prey in the pitch-black waters. Occasionally, these whales have been found covered in strange, circular scars that result from deep-sea battles with enormous squid.