The difference between whale, sperm whale and dolphin

The words “whale“, “sperm whale” and “dolphin” are often used interchangeably in everyday speech. They do not, however, refer to the same animals. All belong to a single order, the cetaceans (Cetacea), which today contains about 94 living species, but they fall into clearly distinct biological groups, with very different morphologies, behaviours and ways of life.

This guide explains in detail the difference between whale, sperm whale and dolphin: how to recognise them, what sets them apart biologically, and why the sperm whale, contrary to a common assumption, is more closely related to the dolphin than to the blue whale. We shall also see that the classification of these animals remains, on several points, a scientific work in progress.

One large family: the cetaceans

All cetaceans are marine mammals fully adapted to aquatic life. They breathe air, give birth to live young, nurse their offspring and regulate their body temperature internally. Descended from land-dwelling ancestors related to the artiodactyls (the closest living relative of the cetaceans is the hippopotamus, whose lineage diverged from that of the cetaceans about 53 to 55 million years ago), they took to the ocean over a long evolutionary history, a subject we set out in our page on the evolution of cetaceans and the land ancestor of cetaceans.

This large family divides into two clearly distinct suborders:

• The mysticetes (“baleen whales”): these are the “true” whales, without functional teeth, which filter their food through horny plates called baleen. The suborder contains about 15 living species (blue whale, fin whale, humpback whale, grey whale, right whales, and so on). See our file on the mysticetes.
• The odontocetes (“toothed whales”): these are all the cetaceans that possess teeth. This suborder, by far the richer with about 79 species, brings together sperm whales, dolphins, porpoises, killer whales, belugas, narwhals and beaked whales. See our file on the odontocetes.

In other words, the sperm whale and the dolphin belong to the same broad group (the odontocetes), while the blue whale and the humpback whale fall into another group (the mysticetes). This is the first real surprise for many readers. These species counts follow the reference list of the Society for Marine Mammalogy (SMM, Committee on Taxonomy), which is updated regularly; they shift with taxonomic revisions.

Summary table of the differences

Criterion	Whale (mysticete)	Sperm whale	Dolphin
Suborder	Mysticetes	Odontocetes	Odontocetes
Teeth or baleen	Baleen (no teeth)	Teeth (mainly the lower jaw)	Teeth (both jaws)
Blowhole	2 blowholes (vertical blow, distinctly V-shaped, especially in right whales)	1 blowhole, offset to the left (angled blow)	1 blowhole
Head	Massive, no melon	Enormous, boxy, ~1/3 of the body	Elongated, with rostrum and melon
Size	~6 to 30 m depending on the species (the large ones: 15-30 m)	10 to 20 m (male); ~11 m (female)	1.5 to 9 m (depending on the species)
Feeding	Krill, plankton, small fish (filter feeding)	Cephalopods (squid, up to the giant species)	Fish, cephalopods
Diving	Often < 500 m	Regularly 1000-2000 m	Variable, often < 300 m
Echolocation	No	Yes, very powerful (clicks up to ~230 dB)	Yes
Social behaviour	Solitary or small groups	Matriarchal groups, acoustic clans	Often large groups

How to read this table: the size ranges given for “whales” refer here to the large baleen whales. In the strict sense, the suborder also includes small mysticetes such as the common minke whale (Balaenoptera acutorostrata, ~8-9 m) and above all the pygmy right whale (Caperea marginata, ~6-6.5 m), the smallest mysticete known. For a figure-by-figure comparison, see our cetacean scale comparator and our page on morphometrics.

Whales: the filter-feeding giants

In the strict sense, a “whale” is a baleen whale. Its main characteristic is the absence of functional teeth: in their place it has hundreds of horny plates, the baleen, attached to the upper jaw, which act as an enormous sieve. The whale takes in huge volumes of water laden with krill or small fish, then pushes the water out through the baleen while retaining the food alone. This filter-feeding method is set out in detail in our page on cetacean diets.

• V-shaped blow: whales have two blowholes side by side, producing a characteristic V-shaped blow at the surface (see identifying a cetacean by its blow).
• No echolocation: unlike the odontocetes, mysticetes do not use echolocation to hunt. They do, on the other hand, produce long low-frequency vocalisations in order to communicate.
• Animals that are rather solitary or in small groups, except when feeding or migrating.
• Long migrations: some travel more than 10,000 km a year between cold waters (feeding) and warm waters (breeding). See our annual migrations.

Among the emblematic whales: the blue whale (Balaenoptera musculus), the largest animal ever to have lived on Earth (up to ~30 m and 150-180 tonnes), listed as Endangered on the IUCN Red List; the humpback whale, famous for its complex songs; the fin whale, present in the Mediterranean; and the grey whale (Eschrichtius robustus), champion migrant and globally listed as Least Concern. Its western Pacific subpopulation (Sakhalin-Kamchatka) was reclassified by the IUCN in 2018 from “Critically Endangered” to Endangered, while the North Atlantic subpopulation has been extinct for centuries. Conservation statuses species by species are gathered in our page on the IUCN conservation status of cetaceans.

A taxonomy still in flux: Rice’s whale

The classification of the large whales is not fixed. In 2021, a new rorqual species was formally described: Rice’s whale (Balaenoptera ricei), endemic to the Gulf of Mexico and long confused with the Bryde’s whale complex (Balaenoptera edeni). It is today one of the most threatened large cetaceans in the world, listed as Critically Endangered by the IUCN, with probably fewer than 50 mature individuals. That complex remains, moreover, a debated point of taxonomy: Balaenoptera edeni is today treated as having two subspecies (edeni and brydei), and B. ricei was recognised as a distinct valid species in 2021.

The sperm whale: a cetacean apart

The sperm whale (Physeter macrocephalus) is a special case that often causes confusion. It is the size of a large whale, males can exceed 18 metres and 50 tonnes, yet it is an odontocete, a toothed whale. Its lower jaw carries functional teeth that can weigh close to a kilogram each; the upper jaw bears only vestigial teeth, often unerupted. It is listed as Vulnerable on the IUCN Red List. See our detailed profile of the sperm whale (Physeter macrocephalus).

Several features make the sperm whale unique:

• A colossal head, box-shaped, taking up close to a third of the body. It houses the spermaceti organ, filled with a waxy substance. Contrary to what is often read, spermaceti is not a simple “oil”: it is a mixture dominated by wax esters (65% to 95%, mainly cetyl palmitate), liquid at body temperature but which solidifies as it cools. This property lies at the heart of the hypotheses about its acoustic role (focusing the clicks) and, according to some authors, about buoyancy control. See our page on the melon and acoustic anatomy.
• A single blowhole set to the left, which produces a characteristic angled blow (rather than a vertical one as in the dolphin).
• A champion of deep diving: it regularly descends to between 1000 and 2000 metres in search of squid. The mammalian diving record, however, is not held by the sperm whale but by the Cuvier’s beaked whale (Ziphius cavirostris), which holds two separate records: depth, with a documented dive to ~2992 m (Schorr et al., 2014), and duration, with a record dive of ~3 h 42 min (222 minutes) measured off Cape Hatteras (Quick et al., 2020). See our cetacean diving records.
• Extremely powerful echolocation: its clicks are among the most intense sounds produced by any animal (up to ~230 dB re 1 µPa at 1 m).
• A highly specialised diet: it feeds almost exclusively on cephalopods, mainly large squid, including the giant squid (Architeuthis) and the colossal squid (Mesonychoteuthis).
• A matriarchal social organisation structured into clans that share their own click dialects, a genuine cultural acoustic profile.

Phylogenetically, the sperm whale is indeed closer to the dolphins than to the baleen whales, since it belongs to the odontocetes. But it would be misleading to make it out to be a “giant dolphin”: the sperm whales (superfamily Physeteroidea) form, on the contrary, one of the oldest and most isolated odontocete lineages, having diverged very early (around the Eocene-Oligocene transition) before the radiation that led to the dolphins and the porpoises. They are therefore not particularly closely related to the Delphinidae: they form a branch of their own on the toothed whale tree. Our interactive cetacean phylogeny makes this early divergence possible to visualise.

Dolphins: agile and social odontocetes

The word “dolphin” covers above all the odontocetes of the Delphinidae family, the richest in the whole order with about 38 species: common bottlenose dolphin, common dolphin, killer whale, pilot whales, Risso’s dolphin, and others. These are generally small or medium-sized cetaceans, with an elongated rostrum, conical teeth on both jaws and great agility.

• Highly social, they live in groups that sometimes number several hundred individuals.
• Excellent communicators: whistles, clicks, individual vocal signatures, dialects that differ between groups.
• Developed cognition: mirror self-recognition, tool use (sponges, shells), social learning.
• Varied habitat: from the coast to the open sea, from fresh water to polar waters.

One caution, though: “dolphin” is a vernacular term, not a single biological group. The “river dolphins”, the Ganges river dolphin (Platanistidae), the Amazonian boto (Iniidae), the franciscana (Pontoporiidae) and the baiji (Lipotes vexillifer, Lipotidae), listed by the IUCN as “Critically Endangered (Possibly Extinct)”, do not form a natural group: their resemblance is the result of convergent evolution (independent adaptation to fresh water and to turbid waters) in at least three separate lineages. The Ganges river dolphin (Platanistidae) in particular is markedly more divergent from the true dolphins than the boto or the franciscana are. Using the word “close” without qualification would therefore be inaccurate.

The killer whale (Orcinus orca), despite its common name and its size, really is a dolphin, the largest in the Delphinidae family (up to ~9 m). The same goes for the pilot whale and for Risso’s dolphin (Grampus griseus). It is worth noting that the classification of the killer whale itself is under discussion: several ecotypes, with very different diets and behaviours, could in time be split into distinct species or subspecies (the names Orcinus rectipinnus and Orcinus ater have been proposed). This is one example among others of points of classification still open.

And the porpoise, the killer whale, the beluga, the narwhal?

Many odontocetes do not carry the name “dolphin” and do not belong to the Delphinidae family:

• The porpoises (family Phocoenidae, ~7 species) differ from the dolphins in their short snout and a dorsal fin that is rather triangular. Contrary to a widespread idea, they are not the closest relatives of the Delphinidae: within the Delphinoidea, the Phocoenidae are in fact closer to the Monodontidae (beluga, narwhal) than to the true dolphins. Among them, the vaquita (Phocoena sinus), a small porpoise of the Gulf of California, is the most threatened cetacean on the planet: Critically Endangered, with only a handful of individuals thought to remain.
• The beluga and the narwhal form the Monodontidae family (2 species). Lacking a true dorsal fin, they live in Arctic waters and show unique traits (the white skin of the adult beluga, the spiral tusk of the male narwhal).
• The beaked whales (Ziphiidae, the second richest cetacean family after the Delphinidae), such as the Cuvier’s beaked whale, are odontocetes too: despite their name, they are not “whales” in the strict sense. Yet it is they that hold the mammalian diving records.

Telling them apart at sea

A few visual clues make it possible to tell these groups apart quickly during a sighting. For a full method, see how to identify a cetacean and our best watching sites.

• The blow
  • Whale: vertical or V-shaped blow, several metres high.
  • Sperm whale: blow angled forward and to the left, short but clearly visible.
  • Dolphin: barely visible blow, often mistaken for spray.
• Size
  • Large whale: between 15 and 30 m; small mysticetes ~6 to 10 m.
  • Sperm whale: 10 to 20 m (male); ~11 m (female).
  • Dolphin: generally between 1.5 and 4 m (up to 9 m for the killer whale).
• Body shape
  • Whale: massive outline, smooth back or with a small dorsal fin.
  • Sperm whale: enormous square head, low dorsal fin, wrinkled skin.
  • Dolphin: streamlined body, pronounced rostrum, large falcate dorsal fin.
• Swimming
  • Whale: slow, with a stately lifting of the tail fluke.
  • Sperm whale: vertical dive, tail raised clear of the surface.
  • Dolphin: fast swimming, frequent leaps, bow-riding.

How many species, how many families?

According to the reference list of the Society for Marine Mammalogy (SMM, Committee on Taxonomy), which is updated regularly, about 94 living cetacean species are counted, divided between:

• ~15 mysticetes (baleen whales), gathered into 4 families: Balaenopteridae (rorquals and humpback whale, 9 species), Balaenidae (right whales and bowhead whale, 4 species), Eschrichtiidae (grey whale, 1 species) and Neobalaenidae (pygmy right whale, 1 species). Where this last one belongs is debated (see below).
• ~79 odontocetes (toothed whales), spread across 10 families: Delphinidae (38 species), Ziphiidae (24), Phocoenidae (7), Monodontidae (2), Kogiidae (2), Platanistidae (2), Physeteridae (1), Iniidae (1), Pontoporiidae (1) and Lipotidae (1). The Delphinidae family alone accounts for about 38 species. The baiji (Lipotes vexillifer, Lipotidae) is still counted in this total, although it is listed as “Critically Endangered (Possibly Extinct)” by the IUCN.

These totals are not fixed: cetacean taxonomy is revised regularly (description of Balaenoptera ricei in 2021, debates about killer whale ecotypes, and so on). To go further into numbers and distribution, see our pages on cetacean demography and life cycles.

An ongoing debate: the pygmy right whale

Where the pygmy right whale (Caperea marginata), the smallest living mysticete, belongs is a good illustration of these unsettled points. The SMM reference list places it in its own family, the Neobalaenidae. But several studies (notably Fordyce & Marx, 2012) attach it to the Cetotheriidae, a family that was thought to be entirely extinct, which would make Caperea the “last of the cetotheres”. The molecular and morphological data do not yet agree perfectly, so the question remains open. We choose here to set out the debate rather than settle it arbitrarily. Another example of disagreement concerns the Ganges river dolphin, whose Ganges and Indus populations are sometimes treated as a single species and sometimes split into two.

Frequently asked questions

Is the sperm whale a whale or a dolphin?

Neither one nor the other in the strict sense. The sperm whale is an odontocete (toothed whale), and so more closely related to the dolphins than to the baleen whales. But it is not a “giant dolphin”: it belongs to a very old and isolated odontocete lineage (Physeteroidea), which diverged well before the dolphins appeared.

What is the difference between a whale and a dolphin?

The whale (mysticete) has no teeth but baleen for filtering krill, has two blowholes and does not use echolocation. The dolphin (odontocete) has teeth, a single blowhole, a melon, and practises echolocation. Whales are among the largest animals, while dolphins are generally much smaller.

Is the killer whale a dolphin?

Yes. Despite the name “whale” and its great size, the killer whale (Orcinus orca) is the largest member of the Delphinidae family, which is to say a dolphin.

Which cetacean dives the deepest?

The mammalian diving record is held by the Cuvier’s beaked whale (Ziphius cavirostris), which holds two separate records: depth, with a documented dive to ~2992 m (Schorr et al., 2014), and duration, with a dive of ~3 h 42 min (222 minutes) measured off Cape Hatteras (Quick et al., 2020). The sperm whale, for its part, dives regularly to between 1000 and 2000 m.

How many cetacean species are there?

About 94 living species according to the Society for Marine Mammalogy: ~15 baleen whales (mysticetes) and ~79 toothed whales (odontocetes). The figure shifts with taxonomic revisions.

In summary

• All whales, all sperm whales and all dolphins are cetaceans (about 94 living species).
• Only the baleen whales (mysticetes) are “whales” in the strict sense.
• The sperm whale is an odontocete, closer to the dolphins than to the whales, but it is an old and isolated lineage, not a “giant dolphin”.
• “Dolphin” is a vernacular term: the true dolphins (Delphinidae, ~38 species) and the “river dolphins” do not form a single natural group.
• Classification remains a living matter: Rice’s whale described in 2021, debates about the pygmy right whale, about the Bryde’s/Rice’s complex and about killer whale ecotypes.
• Watching the blow, the outline, the dorsal fin and the behaviour makes it possible to tell these groups apart at sea.

To go further, see our visual identification guide, our cetology glossary, our cetacean FAQ and our species profiles, including the one on the sperm whale.