It is the biggest of all cetacean families, containing 35 species.

Since the animals belonging to it are sometimes more loosely related to each other than animals in other families, family Delphinidae has been called a taxonomic trash basket.

The only characteristic shared by all family members is the presence of conical teeth.

The family contains a single genus and a single species, the Boto (Inia geoffrensis).

Also called Amazon river dolphin, the Boto inhabits riverine habitats in South America, such as the Amazon and Orinoco rivers.

It contains 2 species, the Pygmy sperm whale (Kogia breviceps) and the Dwarf sperm whale (Kogia sima).

The biggest individuals reach no much more than 3 metres in length. These are elusive animals that only superficially look like miniatures of their cousin the Sperm whale.

Family Lipotidae contained (the past tense is sadly not a mistake) a single species - the Baiji (Lipotes vexillifer).

It is the first cetacean species that got extinct due to the impact of Homo sapiens.

The Baiji was a river dolphin that used to inhabit the Yangtze River in China. It could not survive the huge industrialisation that led to a dramatic change of its riverine habitat.

The family contains two genera, each containing a single species.

Family members are the Beluga (Delphinapterus leucas) and the Narwhal (Monodon monoceros).

Both animals inhabit arctic or subarctic waters in the Northern Hemisphere, often following the advance and retreat of ice.

Family members lack a dorsal fin and a rostrum.

The Beluga is completely white, while the Narwhal presents a more complex pigmentation pattern, with black-brown irregular spots over a white background.

The Narwhal is well known for its tusk, which is a modified canine tooth that grows horizontally and can be over 2.5m long.

The family comprises 7 species of animals that look somewhat similar to dolphins.

Their bodies are though more stocky, they lack a rostrum, they have spade-like teeth (as opposed to the conical teeth of dolphins) and a much more triangular dorsal fin.

Family Physeteridae contains a single genus and species, the Sperm whale (Physeter macrocephalus).

The very large head of the Sperm whale contains two barrels of oil sitting on top of each other. The upper barrel contains the spermaceti oil, the substance whalers were after. The lower barrel contains oil that was less valuable to whalers, hence its name junk.

Absolute master of free diving and deep diving, the Sperm whale feeds almost exclusively on deep water squids.

Teeth are only present on the mandible (lower jaw).

The family comprises a single genus and species, the South Asian river dolphin (Platanista gangetica).

As it is often the case for river dolphins, the South Asian river dolphin has very tiny eyes and is almost blind, which does not appear to be a disadvantage in the murky waters it inhabits, where echolocation is pretty much the only mean by which prey and obstacles can be detected.

The rostrum is very long and narrow.

A true dorsal fin is absent.

This family comprises a single genus and species, the Franciscana (Pontoporia blainvillei).

Only found in the estuaries and costal areas of the South Atlantic, the Franciscana is one of the river dolphins that do not exclusively inhabits riverine habitats.

The beak is in proportion the longest of any other cetacean.

The family comprises 21 species divided in 5 genera, commonly named beaked whales.

They have elongated beaks, bearing teeth only on the lower jaw, a feature typical of cetaceans feeding on cephalopods.

Interestingly, beaked whales only have a pair of teeth and these teeth only erupt in males.

All family members are exceptional deep divers and they can stay submerged for a very long time.

To the surprise of many Sperm whale fans, a female Cuvier's beaked whale (Ziphius cavirostris) is the current record holder among mammals, with a dive to 2992 metres depth, lasting 137.5 minutes¹.

Most beaked whales are poorly known because of their deep diving behaviour and tendency to be elusive. They also spend less time at the surface compared to the other champion of deep diving: the Sperm whale.