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Cetacea

Cetacea is an order of mammals, including about 90 existing fish-like, naked-skinned aquatic mammals and their extinct early relatives, collectively known as whales or cetaceans. The larger ones are usually called whales, and the smaller ones are called dolphins. Most of them live in shallow seas, and a few live in fresh water.

I. Introduction to Cetacea

1. Defining Characteristics:

  • Cetaceans are fully aquatic mammals with streamlined, fusiform bodies.

  • Forelimbs are modified into flippers; hind limbs are absent externally.

  • They possess a thick layer of blubber for insulation, and most have a dorsal fin or ridge.

  • Breathing is through a blowhole (or pair of blowholes in baleen whales) located on top of the head.

  • Reproduction, birth, and nursing occur entirely in the water.

2. Diversity and Distribution:

  • About 90 extant species of cetaceans inhabit all the world’s oceans and some rivers.

  • They range from the enormous blue whale (the largest animal ever known) to small porpoises and dolphins.


II. Evolutionary History

1. Origins:

  • Cetaceans evolved from terrestrial ancestors during the Eocene epoch (around 50 million years ago). Early fossils like Pakicetus indicate that their ancestors were land-dwelling, wolf-sized mammals. Over millions of years, these ancestors adapted to increasingly aquatic lifestyles.

  • The discovery of transitional fossils (e.g., Ambulocetus, Rhodocetus) shows the gradual modification from land locomotion to aquatic propulsion using tail flukes and forelimbs as flippers.

2. Transition to Full Aquatic Life:

  • By the late Eocene, early cetaceans were fully aquatic. Two major lineages diverged: the baleen whales (Mysticeti) and the toothed whales (Odontoceti).

  • Molecular and morphological data confirm that cetaceans share a close common ancestor with even-toed ungulates (Artiodactyla), especially hippopotamuses. This discovery led to the combined grouping known as Cetartiodactyla.

3. Adaptive Radiations:

  • Mysticetes adapted filter-feeding apparatuses (baleen plates) to exploit large amounts of small prey (krill, zooplankton).

  • Odontocetes developed echolocation abilities to navigate and hunt in often murky underwater environments, feeding on fish, squid, and sometimes marine mammals.


III. Classification Overview of Cetacea

Cetacea is traditionally divided into two suborders:

  • Suborder Mysticeti (Baleen Whales): Characterized by baleen plates instead of teeth, used to filter small prey from the water. They generally have two blowholes.

  • Suborder Odontoceti (Toothed Whales): Possess teeth (in varying numbers and shapes) and echolocation abilities. They have a single blowhole.

Below are the major families and some representative genera:

Suborder Mysticeti (Baleen Whales)

FamilyRepresentative Genera/SpeciesNotes
Balaenidae (Right and Bowhead Whales)Eubalaena (Right whales), Balaena (Bowhead whale)Large heads, long baleen plates, slow-moving filter feeders.
Balaenopteridae (Rorquals)Balaenoptera (Blue whale, Fin whale), Megaptera (Humpback whale)Streamlined, often pleated throats, gulp-feeding strategy.
Eschrichtiidae (Gray Whale)Eschrichtius (Gray whale)Bottom-feeder, short baleen, migratory along coastal routes.
Cetotheriidae (Pygmy Right Whale)Caperea (Pygmy right whale)Rare and enigmatic family; smallest baleen whale.

Suborder Odontoceti (Toothed Whales)

FamilyRepresentative Genera/SpeciesNotes
Physeteridae (Sperm Whales)Physeter (Sperm whale)Large head with spermaceti organ, deep divers feeding on squid.
Kogiidae (Pygmy Sperm Whales)Kogia (Pygmy and Dwarf sperm whales)Small, elusive whales with squid-based diet.
Ziphiidae (Beaked Whales)Ziphius (Cuvier's beaked whale), Mesoplodon (various species)Deep-diving, elusive, many species poorly known.
Monodontidae (Narwhal, Beluga)Monodon (Narwhal), Delphinapterus (Beluga)Arctic whales with unique features (tusk in narwhal, white beluga).
Phocoenidae (Porpoises)Phocoena (Harbor porpoise), Neophocaena (Finless porpoise)Small, robust, lacking a prominent beak, spade-shaped teeth.
Delphinidae (Dolphins)Tursiops (Bottlenose dolphin), Orcinus (Killer whale), Delphinus (Common dolphin)Highly diverse, social, and intelligent; largest Odontoceti family.
Iniidae (New World River Dolphins)Inia (Amazon river dolphin)Freshwater habitats in South America, long beaks, poor vision.
Pontoporiidae (La Plata Dolphin)Pontoporia (La Plata dolphin)Coastal and estuarine waters of eastern South America.
Platanistidae (South Asian River Dolphins)Platanista (Ganges and Indus river dolphins)Nearly blind, rely on echolocation in muddy river waters.

(Additional notes: The family Lipotidae, representing the Yangtze River dolphin (Lipotes vexillifer), is functionally extinct. Classification may vary slightly as some taxonomists treat river dolphins differently.)


IV. Summary

Cetaceans represent a remarkable evolutionary journey from land to water. Their extensive adaptive radiation has produced giants like the blue whale as well as highly social and intelligent dolphins and elusive deep-diving beaked whales. Modern molecular studies have situated cetaceans within the order Cetartiodactyla, reaffirming their close kinship with terrestrial ungulates such as hippos. Yet, historically and for the sake of clarity, Cetacea remains a well-recognized grouping for whales, dolphins, and porpoises, illustrating one of the most extraordinary transitions in mammalian evolution.

Carnivora Primates Rodents Chiroptera Eulipotyphla Ungulata Cetacea Lagoiformes Pholidota Proboscidea Monotremata
porpoise

porpoise

Cetacea LC

Features:It's a relatively small toothed whale

The porpoise (Phocoena phocoena in Latin), also known as porpoise, is the most common toothed whale in the North and Baltic Seas.Porpoises prefer calmer waters at a medium depth of about 20 meters near the coast, but occasionally swim deeper into the ocean.Porpoises feed almost exclusively on fish,...

Sotalia fluviatilis

Sotalia fluviatilis

South American long-beaked dolphin, Amazon River white-sided dolphin

Cetacea LC

Features:The body is stout, the beak is prominent, and the forehead is slightly rounded.

Tucuxi dolphin (scientific name: Sotalia fluviatilis) is also known as tucuxi in English. There is no subspecies.Although they may let ships approach, most Tucuxi dolphins are wary of ships. They may ride on the waves caused by passing ships, but will not perform bow riding. Often seen peeping, tail...

Cephalorhynchus heavisidii

Cephalorhynchus heavisidii

South African dolphin, Hiwig's dolphin, Benguela dolphin

Cetacea LC

Features:The dorsal fin is a distinct triangle, and the body color is a striking black, white and gray pattern.

Haviside's dolphin (scientific name: Cephalorhynchus heavisidii) is called Haviside's dolphin in foreign language, and has no subspecies.Haviside's dolphin is cautious by nature and sometimes quite shy. It is rare to see them jumping, but they have been known to leap more than 2 meters o...

Cephalorhynchus eutropia

Cephalorhynchus eutropia

black dolphin, black dolphin, Chilean dolphin

Cetacea LC

Features:After death, the body color will soon darken, so it is also called "black dolphin".

Chilean dwarf dolphin (scientific name: Cephalorhynchus eutropia) is also known as Chilean Dolphin and Black dolphin in foreign languages. There is no subspecies.Little is known about the Chilean dwarf dolphin, but it is generally believed that this species of dolphin is cautious by nature; it rarel...

Cephalorhynchus commersonii

Cephalorhynchus commersonii

Black and white dolphin, panda dolphin, Connor's dwarf dolphin

Cetacea LC

Features:The beak is quite sharp, with unclear boundaries between it and the forehead, and the pattern on its body is very similar to that of a panda.

The spotted beaked dolphin is the common name of Commerson's dolphin (scientific name: Cephalorhynchus commersonii), also known as Commerson's dolphin in English, and has two subspecies.Spotted beaked dolphins usually live in groups of 1 to 3, and there are also large groups of more than 100...

Cephalorhynchus hectori

Cephalorhynchus hectori

Hector's dolphin, Atlantic black and white dolphin, white-headed dolphin, New Zealand dolphin, white-headed beaked dolphin, Hector's dwarf dolphin

Cetacea LC

Features:One of the rarest species and the smallest oceanic dolphin

New Zealand black and white dolphins (scientific name: Cephalorhynchus hectori) are called Hector's Dolphin and Dolphin in foreign languages. There are two subspecies.New Zealand black and white dolphins usually gather in small groups of 2-10. These small groups sometimes temporarily unite to fo...

Orcaella brevirostris

Orcaella brevirostris

Irrawaddy dolphin, fin dolphin

Cetacea LC

Features:The body is stout, round and streamlined, with a large forehead and a blunt and round head.

Irrawaddy dolphin (scientific name: Orcaella brevirostris), foreign name Irrawaddy dolphin, no subspecies.Irrawaddy dolphins are social animals, usually 3 to 6 in groups, the social behavior of group members occurs within and outside the group, and they also swim with other groups. When Irrawaddy do...

Globicephala melas

Globicephala melas

Atlantic pilot whale, black whale, pilot whale

Cetacea LC

Features:Its forehead is round and its dorsal fin is sickle-shaped. Its flippers are very long, accounting for 15-20% of its body length.

Long-finned pilot whale (scientific name: Globicephala melas) is also known as Atlantic pilot whale and black round-headed whale. There are two subspecies.Long-finned pilot whales are a group of organisms that like to live in cold salt water areas with a temperature of 13-30℃. The diving depth is 3...

Lissodelphis peronii

Lissodelphis peronii

Southern whale dolphin, beaked dolphin without dorsal fin

Cetacea LC

Features:The only dolphin in the Southern Hemisphere without a dorsal fin

The Southern Right Dolphin (scientific name: Lissodelphis peronii) is also known as the Southern Right Whale Dolphin. It has no subspecies. It is the only dolphin in the Southern Hemisphere without a dorsal fin, and has a distinct black and white pattern on its body. If the Southern Right Dolphin is...

Lissodelphis borealis

Lissodelphis borealis

Northern whale dolphin, warm water finless beaked dolphin

Cetacea LC

Features:The body is slender, tapering towards the tail like an eel, and the whole body is generally black with a white belly.

Northern right whale dolphin (scientific name: Lissodelphis borealis) is called Northern right whale dolphin in foreign language, and has no subspecies.When the northern right dolphin swims and jumps quickly, the overall impression given is that of a leap; each leap can reach up to 7 meters. Very ea...

Stenella longirostris

Stenella longirostris

Spinner dolphin, spinner dolphin, long-beaked spinner dolphin

Cetacea LC

Features:The snout is slender and accounts for 8.1% to 9.9% of the body length, the longest among the genus Prodolphins.

Long-beaked dolphin (scientific name: Stenella longirostris) is also known as Spinner Dolphin in English. There are 4 subspecies.There are about 118 long-beaked dolphins in the northern Mozambique Channel surrounding Mayotte Island. They often travel in groups of more than 200, and large groups of 1...

Lagenorhynchus albirostris

Lagenorhynchus albirostris

White-nosed dolphin, white-beaked dolphin, squid-hunting dolphin

Cetacea LC

Features:Not all beaks are white, but from a close distance, the pure white beaks are particularly obvious.

White-beaked dolphin (scientific name: Lagenorhynchus albirostris) is called white-beaked dolphin in foreign language, and has no subspecies.White-beaked dolphins especially like to perform bow riding in front of large, fast-moving ships, but usually lose interest quickly. Some groups are very elusi...

Lagenorhynchus acutus

Lagenorhynchus acutus

Atlantic white-sided dolphin, jumping dolphin, bouncing dolphin

Cetacea LC

Features:Atlantic white-sided dolphin, jumping dolphin, bouncing dolphin

Atlantic White-sided Dolphin (scientific name: Lagenorhynchus acutus) is also known as Atlantic White-sided Dolphin. It has no subspecies. Atlantic White-sided Dolphin is currently classified in a polymorphic genus (Cipriano 1997). However, molecular biological studies have shown that this species i...

Pontoporia blainvillei

Pontoporia blainvillei

La Plata River Dolphin

Cetacea LC

Features:It is the only freshwater dolphin that lives in the sea.

The scientific name of the Lapu-Lapu river dolphin is Pontoporia blainvillei. It is the only freshwater dolphin that lives in the sea, but it prefers shallow coastal waters.The pufferfish is a carnivore that often swims to the shallow water near the shore to hunt at dawn and dusk. It usually swallow...

Inia geoffrensis

Inia geoffrensis

Amazon porpoise

Cetacea LC

Features:The largest dolphin in the world

The scientific name of the river dolphin is Inia geoffrensis. It is the largest dolphin in the world and one of the few dolphins that is still common. During the dry season, river dolphins gather in groups of about a dozen; in other seasons, they live in groups. The sound of the puffs can be loud or...

Tasmacetus shepherdi

Tasmacetus shepherdi

Shea's beaked whale, Tasman's whale, Tasman's beaked whale

Cetacea LC

Features:

Shepherd's beaked whale, scientific name Tasmacetus shepherdi, foreign name Shepherd's Beaked Whale, is a little-known cetacean, with only about 20 stranding incidents and a few possible sightings. Increasing research in the Southern Hemisphere may find more specimens, but the evidence shows...

Indopacetus pacificus

Indopacetus pacificus

Longman's beaked whale, Pacific beaked whale, Indo-Pacific beaked whale

Cetacea LC

Features:There is a pair of teeth at the front of the lower jaw

Longman's Beaked Whale (Scientific Name: Indopacetus pacificus) is also known as Longman's Beaked Whale (English), Baleine à bec de Longman (French), Zifio de Longman (Spanish), formerly known as Longman's Beaked Whale, also known as Pacific Beaked Whale and Indo-Pacific Beaked Whale. I...

Mesoplodon hotaula

Mesoplodon hotaula

Cetacea LC

Features:

The Sri Lankan Mesoplodon hotaula was first described in 1963 and was considered a synonym of the Ginkgotoothed Mesoplodon ginkgodens in 1965, but until recently, mitochondrial and nuclear studies have shown that the genes of the Mesoplodon hotaula are different from those of the Ginkgotoothed Mesop...

Mesoplodon traversii

Mesoplodon traversii

Cetacea LC

Features:It is a rare species living in the ocean.

Spade-toothed whale (Mesoplodon traversii), foreign media reported that scientists discovered the world's rarest whale for the first time in New Zealand. In December 2010, two spade-toothed beaked whales, a mother and her calf, were found stranded on Opape Beach on New Zealand's North Island...

Mesoplodon stejnegeri

Mesoplodon stejnegeri

Stein's beaked whale, North Pacific beaked whale, Saber-toothed beaked whale, Bering Sea beaked whale

Cetacea LC

Features:The teeth are in the center of the lip line, and the dark forehead is gently sloping.

Stejneger's Beaked Whale, scientific name Mesoplodon stejnegeri, foreign name Stejneger's Beaked Whale, is not easy to see at sea, and it is rare to see it alive. Maybe it is rare, but it may just live in an area where few people study it and avoid people's attention. Small groups someti...

Mesoplodon peruvianus

Mesoplodon peruvianus

Cetacea LC

Features:It is the smallest beaked whale in the genus Mesopharma.

The Peruvian Beaked Whale is the smallest of the Mesoplodon peruvianus. The relevant information is only obtained from 13 specimens and a few possible sightings at sea. Scientists noticed their existence after part of a strange head was found in a fish market in San Andres, Peru in 1976; the head wa...