We’re all able to recognise a dolphin, an orca or one of the whale species. These animals are part of the order of cetaceans (Cetacea). But….

Did you know that there are 80 species of cetaceans, and that there are huge differences between them?

What is a cetacean?

The first thing to know is that cetaceans are euterine mammals, which means that they are placental. In other words, their offspring develop in the womb and are fed by a placenta, like almost all mammals. Aristotle coined the term “cetacean“, meaning whale or sea monster, to refer to aquatic animals with pulmonary respiration. These mammals are completely adapted to their aquatic environment, but they still need to come to the surface to breathe.

They have hydrodynamic bodies, similar to that of fish and are perfectly adapted to life in seas, oceans, even rivers. Evolution has transformed their front legs into fins, whereas their back legs have practically disappeared, giving way to a tail fin. This is formed by two lobes arranged horizontally to help them swim to the surface. At the top of the head they have one or two blowholes (nostrils) which allow them to breathe on the surface of the water.

Sperm whales underwater. By megablaster | Shutterstock.com

How are cetaceans classified?

There are three suborders of cetaceans, one of them containing species that are already extinct (archeocetes). The other two are divided into baleen cetaceans (mysticetes), commonly called whales; and toothed cetaceans (odontocetes).

Mysticetes, commonly known as baleen whales, are the largest animals that have ever existed on our planet – even larger than dinosaurs! The group consists of rorquals and 4 other families of whale. They are toothless carnivores with keratin beards in their upper jaws (similar to that found in nails) that filter the water coming out of their mouths, trapping the living organisms they feed on (mainly krill and small fish or crustaceans).

They have a thick layer of fat that isolates them from low temperatures of the cold waters where they live. Their lungs are adapted to obtain 80% of the oxygen in the air (in humans this figure is 20%). Their lungs collapse instead of resisting the pressure, which gives them the ability to dive to great depths where the pressure is enormous.

Odontocetes, or toothed cetaceans, have a snout with teeth. They have a single blowhole for breathing and a protruding forehead which, in most cases, conceals an organ used for echolocation called a ‘melon‘. This organ is made up of lipids (fats) that seem to function as a kind of cushion, assisting with the perception of sounds traveling through water.

This order consists of 8 families: dolphins (including killer whales and pilot whales), river dolphins, porpoises, vaquitas, sperm whales, beaked whales, belugas and narwhals. Unlike baleen whales, odontocetes can range in size and most have a dorsal fin.

A pod of long-beaked common dolphins leap out of the water in Monterey Bay, California. By Chase Dekker | Shutterstock.com

How many species of cetaceans are there in the world?

Right Whale and bowhead whale. By shanesabin | Shutterstock.com

Whales

There are three families of whales. The first of them, the balenidae, includes three types of right whales and the Greenland whale. The second, eschrichtiidae, is a single species: the gray whale. And the third family, the neobalenids, includes the pygmy right whale. These animals can measure from 5 to 17 meters, weigh up to 80 tons and live for 30 years.

They have a narrow and arched jaw, which gives them a convex profile. They mainly feed on small crustaceans. Unlike rorquals, whales do not have gular folds (folds in the skin of the lower body) and have a more robust body that lacks a dorsal fin (except the pygmy whale).

Rorquals and humpbacks

Rorquals are the most abundant and diverse family of baleen cetaceans. They include 8 species of rorquals and the humpback whale. The main characteristic of this family is the presence of gular folds in the throat and ventral region. These folds expand enormously when they swallow large amounts of water to filter out the krill they feed on. Also, they have a dorsal fin.

Of all the species of rorqual, the blue whale stands out as the largest animal that has ever existed, up to 34.5 meters long, weighing 173 tons and capable of storing 90 tons of water and food in its mouth at one time. Their offspring grow an average of 4 kg per hour during the first months of life. Then there’s the humpback, which is characterised by longer fins and a knobbly head. Humpbacks communicate with long, characteristic calls that can last for hours and can be heard in the oceans.

Blue whale and humpback whale. By shanesabin | Shutterstock.com
Common bottlenose dolphin, orca (killer whale) and pilot whale. By shanesabin | Shutterstock.com

Oceanic dolphins

There are currently 34 species of oceanic dolphins, including killer whales and pilot whales. These animals measure 2 to 9 metres, have a fusiform body (elongated, with very short limbs and adapted for swimming at high speed), an elongated snout (less sp in the case of pilot whales) and a large brain with a more developed cortex than most mammals. All species also have the echolocation organ (melon).

Interestingly, when these animals sleep, half of their brain remains awake. This allows them to breathe and stay afloat, as their breathing is voluntary. In general, they are very sociable and intelligent animals that live in groups. In the case of pilot whales, their small snout is almost unnoticeable as they have a very prominent “forehead”. Meanwhile, killer whales are the largest dolphins, with a length of up to 9 meters.

River dolphins

There are two families of river dolphins, divided into 4 genera and 7 species. All of them live in estuaries and river. They have long, slender snouts. Depending on the species, they have limited or zero vision, which has led them to develop a sense of echolocation. They have underdeveloped dorsal fins and, unlike oceanic dolphins, which have fused cervical vertebrae, river dolphins can turn their heads.

The Amazon pink dolphin, as its name suggests, is pink and is the largest river dolphin up to 2.5 meters long. Its flexibility allows it to move between the flooded forests to hunt its prey, but this also makes it slower than oceanic defines.

Ganges river dolphin and Amazon river dolphin. Pic of DKfindout.com and American Cetacean Society
Finless porpoise and vaquita. By Alona K | Shutterstock.com

Porpoises and vaquitas

Porpoises are the smallest of all the cetaceans. The term porpoise is usually – and incorrectly – applied to any small dolphin. They are more robust than dolphins and produce more offspring than dolphins, as they reach sexual maturity much earlier. They have flatter teeth, a less prominent, beaked snout and have either a small, triangular dorsal fin or none whatsoever.

The vaquita is the smallest of the porpoises. It measures 150 cm and weighs 50 kg. It lives in the Gulf of California and is the most endangered species of cetacean. It can be distinguished by the dark lines around its eyes and lips. There are currently some 40 specimens and, although recovery programmes are in place, it is at imminent risk of extinction.

Sperm whales

Sperm whales, are in fact two families that include three species: the sperm whale, the pygmy sperm whale and the dwarf sperm whale. As their names suggest, the difference between them lies mainly in their sizes. The sperm whale is the largest toothed animal in existence, up to 20 meters long and 50 tons. They have the ability to dive down 3 kilometres and for as long as 90 minutes. It is the largest predator that has ever existed. They have a huge head (one third of the length of their body). They have no dorsal fin and their skin is rough. In addition, they produce the most intense sound in the animal kingdom, like a snap.

A characteristic of sperm whales is spermaceti, a whitish wax present in the cavities of the skull. It is believed that the spermaceti organ serves as a biological ballast for sperm whales to aid their buoyancy. Before immersion, with cold water, this wax solidifies and its density generates a downward force that helps them to submerge. When they hunt, oxygen produces heat by melting the spermaceti, increasing buoyancy. This substance, highly valued by humans for the creation of different products, led to intensive hunting, which has endangered their populations.

Sperm whale and pigmy sperm whale. By 3drenderings | Shutterstock.com
Beaked whales. (c) Namu-the-orca

Beaked whales

These cetaceans are made up of 21 described species. They can measure between 4 and 13 meters and weigh from 1 to 15 tons. Their main characteristic is that they have an elongated and slender snout similar to that of some dolphins, and two furrows that converge at the throat. In addition, in almost all species of beaked whales, the jaw has only one set of teeth. They have small fins in relation to their long and very robust body. They are very elusive animals, spending most of their time submerged at great depths, so little is known about them.

These animals are great deep sea divers. The longest recorded was 137.5 minutes to a depth of 2,992 metres, made by a Cuvier’s beaked whale. Eight specimens were tracked via satellite from the Cascadla Research Collective in the United States.

Belugas and narwhals

These two species of cetaceans belong to the same family: the monodontides. They inhabit the glacial waters of the Arctic and northern Pacific and Atlantic oceans. The beluga whale is the only white whale and, in fact, is also referred to as ‘the white whale’. It has no dorsal fin, and its forehead is very prominent, which accommodates a large, malleable melon organ. They measure up to 5.5 metres and weigh up to 1,600 kg. Their bodies have more fat than other cetaceans to protect them from the cold waters of their habitat. Echolocation and large ears allow belugas to find air pockets under the Arctic ice sheets. One fact about belugas is that, thanks to their malleable melon, they have more facial mobility than any other cetacean.

However, perhaps the most remarkable cetacean of all is the narwhal. Males of this species have a tusk, up to two meters and 10 kilograms; helically twisted, it resembles a large horn. These animals can dive to great depths to obtain food (fish and crustaceans). They measure between 4 and 4.5 metres and weigh between 100 and 1600 kg. They are grey with speckled markings and white undersides. They are known as unicorns of the sea.

Beluga and narwhal. By shanesabin | Shutterstock.com
Long-finned pilot whale stare. By Andrew Sutton | Shutterstock.com

The extinction of cetaceans

Many international measures are in place to protect cetaceans. Measures to ban whaling and whaling conservation efforts have moved some species, such as humpback whales, from a “vulnerable” to “minor concern” status on the IUCN list. However, small coastal and river cetaceans have not been so lucky. Some, such as the vaquita, are in imminent danger of extinction.

In fact, a quarter of all cetacean species are endangered and, considering that more than half of these animals are listed by the IUCN in the “Data Deficient” category, there are likely to be many more. There are several reasons for their decline: collisions with boats, fishing nets, deterioration of their habitats, reduction of their food sources and noise pollution in their habitats generated by human activity, especially boats.

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Cetaceans. The small and giant mammals of the oceans
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Cetaceans. The small and giant mammals of the oceans
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Cetaceans are the best adapted mammals for life under water. In this article we'll learn all about them and how to tell them apart.
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Zoo Portraits
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