What whales and dolphins look like and where they come from

Whales, dolphins and porpoises are collectively called cetaceans as they belong to the mammalian order Cetacea.

Their closest living relative is thought to be the hippopotamus, an animal belonging to the order Artiodactyla, which comprises all even-toed ungulates such as deer, camel, cow, etc.

Based on evidence supporting the hypothesis that cetaceans and artiodactyls share a recent common ancestor, a new order called Cetartiodactyla has been proposed.

Full-time Marine Mammals

Whales, dolphins and porpoises spend their entire life in the aquatic environment.
They only share this characteristic with the Sirenians (dugong, manatees). All the other so called marine mammals, such as the pinnipeds, the polar bear and the sea otter always return to land at least at some point during their existence, tipically to breed, give birth and suckle their young.

Toothed and toothless Cetaceans

It makes a lot of sense to divide whales, dolphins and porpoises into 2 big groups, which correspond to the extant suborders of the order Cetacea:

Dolphins and porpoises, along with other species such as the sperm whale, the beaked whales, the narwhal and the beluga are Odontocetes - i.e. they have teeth.

On the contrary, baleen whales are Mysticetes and therefore they do not have teeth.
Instead, they have a large number of comb-like keratin plates, called baleen, hanging from the upper jaw. Baleen plates act like a sieve, allowing whales to filter the zooplankton (typically krill or copepods) and/or small fish they feed upon.

How do Cetaceans breathe?

Cetaceans breathe air at the surface of the water through their blowholes, the equivalent of our nostrils.

Mysticetes (baleen whales) have two blowholes while Odontocetes (toothed cetaceans) have only one blowhole.

Unlike humans and most other mammals, they are unable to breathe through their mouth, which makes it possible for them to feed underwater while keeping their respiratory system free of water.

How do Cetaceans swim?

Cetaceans use their tail to swim. Unlike fish, the tail of whales, dolphins and porpoises lies in a horizontal plane and it is moved up and down rather than from side to side.

Muscles actively work when the tail is raised, while they rest (or almost completely rest) when the tail is lowered. Their way of swimming is known as caudal oscillation, as opposed to caudal undulation which is typical of fish.