Introduction to Cetaceans

Cetacean is the term used to refer to all whales, dolphins, and porpoises and comes from the Greek ketos or Latin cetus meaning sea monster.

Cetaceans are mammals like us, they are warm blooded, breathe air, and give birth to live young which they suckle on milk.


Cetaceans closest living relative is the hippopotamus

Cetaceans began evolving around 50 million years ago from four-legged land mammals which began foraging in shallow coastal waters.

Currently all cetaceans fall in to one of two groups, odontocetes and mysticetes, which separated around 30 million years ago. Odontocete means toothed (odonto) whale/cetacean (cete), and mysticete means moustached (mysti) whale/cetacean (cete), with moustache referring to their hair-like baleen plates which hang in their upper jaw.

Despite being split in to two groups, all cetaceans share some features.

Cetaceans are conscious breathers, which means that they must choose every single breath they take and can never fall asleep completely. They undergo unihemispheric sleep, shutting down only one half of their brain at a time and keeping the other half awake enough to continue swimming and breathing. They all breathe through blow-holes, the equivalent of our nostrils, which have moved through evolution to the top of their heads.

All species are entirely aquatic, meaning they spend their entire lives in water. They give birth at sea and never come on to land to rest like seals do. Cetacean calves must be able to swim from the moment they are born so they need to be well developed when they are born and grow quickly to be able to keep up with mum. In order to grow quickly they are fed on a fat rich milk which takes a lot of mum’s energy to produce. Cetaceans only ever have one calf at a time due to this demand placed on the mother.

Odontocetes – the toothed whales

includes all porpoises and dolphins

~80 species

lengths from 1.4 to 18m

all species have teeth of some kind

all use echolocation to hunt and communicate


social and gregarious

Mysticetes – the baleen whales

the ‘big’ whales

~16 species

posses filters in upper jaw called baleen plates

lengths from 6 to 30m

communicate with whale song


often solitary or small groups

Whale vs dolphin vs porpoise

Why are some species called whale, while others are called dolphin or porpoise?

While technically all cetaceans can be called whales, in English common names we tend to refer to anything larger than around 6m as a whale so this covers all mysticete species as well as around 35 of the larger species of odontocetes.

The smaller odontocetes are then referred to as dolphins or porpoises depending on their taxonomic grouping. Phocoenidae are the seven species of porpoise and tend to have a less pronounced beak and spade-shaped rather than conical teeth. The remaining 38 odontocete species are referred to as dolphins covering the Delphinidae (oceanic dolphins) and the river dolphins.

As well as a ‘common’ name (e.g. Risso’s dolphin) all species have a Latin name (e.g. Grampus griseus) which allows them to be identified around the world, regardless of the language spoken or what the local common name might be. This is particularly helpful as common names can change over time as well as location, or be used interchangeably, such as seafarers and fishermen tending to use the word porpoise for any small odontocete.

A popular trivia fact is that killer whales (orca) are actually dolphins. This is true, they are members of the Delphinidae but due to their size they are referred to with the common name whale. This is true of other dolphin species including the pilot whales and the melon-headed whale. The Risso’s dolphin is the largest dolphin species which still has the common name dolphin