Delphinids: Small Dolphins of Shallow Southern Seas

Commerson's dolphin

While related, dolphins and porpoises are regarded as distinct types of animals. Each is placed in its own family, with the two separating at least 15 million years ago. However, telling the two apart is not always easy, at least on a superficial look at their external anatomy. Porpoises are, generally speaking, smaller than dolphins and they have a blunt nose rather than a 'beak'. The problem is that we can say exactly the same about some species that really are dolphins.

In 1766, naturalist Philibert Commerson accompanied explorer Louis-Antoine de Bougainville on what would become the first successful French circumnavigation of the globe. While passing through the Straits of Magellan the following year, he spotted an unusual-looking dolphin close to the ship and sent a description of it back to France. (As a side note, later on in the voyage, it was discovered, much to the crew's shock, that Commerson's assistant was secretly a woman; she is now remembered as the first woman to circumnavigate the globe).

The letter found its way to another naturalist, Bernard de Lacépède, who wrote up the first formal scientific description of the species in 1804. It is now known as Commerson's dolphin (Cephalorhynchus commersonni) and belongs to a genus that has at least three other, similar-looking species in it. I say "at least" because there is some evidence that the hourglass and Peale's dolphins may also belong in the genus, being descended from its last common ancestor. However, while they probably don't belong where they are currently placed, the exact details remain debated, and besides, I already looked at them last time.

Commerson's dolphin ties with three other species for the honour of "smallest species of dolphin". They are typically no more than 150 cm (5 feet) in length and 66 kg (145 lbs) - approximately the same size as a typical porpoise. Like porpoises, they have a rounded snout, without the distinct beak seen in many other dolphin species, and their dorsal fin and flippers are also a similar shape. Perhaps their most distinctive feature, however, is their colour. They have a black head, fin, and flippers, and the part of the back behind the fin is also black. The underside and flanks, however, are white, with the patch wrapping round to the front part of the back, between the head and fin. 

While black-and-white porpoises do exist, none have this particular pattern, and the internal anatomy makes it clear that they are distinct kinds of animal.

The species was long assumed to be restricted to a relatively small area of southern South America. First sighted in the Straits of Magellan, it lives around all the coasts of Tierra del Fuego, and along the Patagonian coast of Argentina to around 42°S latitude. They are also found off the Falkland Islands, although, not being a deep water species, they rarely enter the Drake Passage to reach more southerly islands closer to Antarctica.

Which makes it all the more surprising that, in the 1950s, a second population of Commerson's dolphins was discovered off the coast of Kerguelen. Which is about the same latitude as the Falkland Islands and has a similar climate... but is about 8,500 km (5,300 miles) away in the southern Indian Ocean. 

So far as anybody has been able to tell, there is no other population of these dolphins anywhere in between - although one individual, apparently lost, was allegedly sighted south of South Africa in 2004. Somewhat larger and with less distinct colouring than their South American counterparts, those off Kerguelen have been recognised as a separate subspecies since 2007, and have presumably been there a long time.

Commerson's dolphin is a shallow-water species, inhabiting coastal seas less than 100 metres (330 feet) deep, and sticking to fjords and river mouths where possible. They prefer cold waters, apparently preferring surface temperatures below 8°C (46°F) although they are prepared to brave temperatures up to 15°C (60°F) at the northern edge of their range. They feed primarily on small squid, although they are perfectly happy to eat fish such as anchovies and hake, and younger dolphins seem to dine mostly on sprats. 

The dolphins typically travel in small groups of no more than ten individuals although it is likely that, as in other dolphin species, the membership of these groups is not fixed. In particularly cold weather, groups may become much larger, possibly because their preferred food becomes more accessible and they gather together to hunt it. Like other dolphins, they continue to swim while they sleep; one study reported muscle jerks during slow swimming that may be analogous to the way humans and other animals twitch while dreaming.

An unusual feature of Commerson's dolphins is that the front edge of the left fin has serrated markings that are especially prominent in males but which are much less prominent on the right fin. The markings are thought to be the result of the dolphins using the fin to rub the bodies of their fellows. That they are most notable on the left suggests that the species is "left-handed" while the fact they are heavier in males suggests that such social rubbing is more beneficial for them - perhaps the females find it sexy.

They use a variety of different echolocation clicks, but some of these are only employed when larger groups gather together, suggesting that they may be a form of communication that helps with social cohesion, rather than being used solely to find food. Although it was long thought that Commerson's dolphin, along with its close relatives, did not produce the whistle sounds made by other dolphin species, this is apparently not true, although such sounds have so far only been noted when a mother is looking after her calf.

It's thought that, during the Ice Ages, even the cold-loving ancestors of Commerson's dolphin were forced to swim north from their original home around the Cape Horn, ending up split into two by the  South American continent. By the time the ice retreated and they headed back, the populations on either side had diverged into separate species, with Commerson's being the one on the Atlantic side.

Chilean dolphin

On the Pacific, western, coast, the common ancestor had instead evolved into the Chilean dolphin (Cephalorhynchus eutropia). The two species overlap in the Magellan Strait and along the west coast of Tierra del Fuego, and may hybridise there, but the Chilean species is found only along the shores of its namesake country, reaching around 32°S latitude, about the level of Santiago. It's about the same size and shape, but more of a dark grey than black, with the white patch restricted to the underside and, even then, split by a grey stripe running underneath the body between the flippers. There are also smaller white patches at what we could loosely call the dolphin's "armpits". 

Chilean dolphins have similar habitat requirements to their Atlantic cousins, living in cold shallow waters, especially in fjords and river mouths. They appear to prefer water with good currents, whether that be from the outflow of rivers or tidal bores and rip tides pushing into narrow inlets; these probably bring nutrients that their food relies on. In fact, they are rarely seen in water more than 20 metres (65 feet) deep or more than 500 metres (1,600 feet) from the shore. 

Pods are small, with no more than 15 members, and spend much of their time feeding in very small localised areas no more than 200 metres (650 feet) across. They are known to feed on lobsters, small squid, anchovies, and herring, but we don't know much more about them than that. They ride in the bow waves of boats less frequently than their relatives, but this may just be because they have learned that such boats in their area often carry fishing nets that they get entangled in. Their echolocation clicks are similar to those of their relatives, and they have never been recorded to make whistles (although given how little we know of their parenting, and how long it took to discover them in Commerson's species, this may not mean much).

Hector's dolphin

Hector's dolphin (Cephalorhynchus hectori) looks remarkably similar to the Chilean dolphin, with a grey body and a white underside, complete with patches near the "armpits". Both species also have a faint dark streak running down either side of the face from the blowhole to the eyes. A lobed extension to the white patch reaching up onto the rear flanks is more dramatic than in the Chilean species, and the grey stripe across the chest between the fins is fainter, or missing altogether, but the main distinguishing feature is the presence of a black patch across the snout and around the eyes.

When Pierre-Joseph Van Beneden first described the species in 1881, he did so based on a specimen sent to him by museum curator Sir James Hector from "the north coast of New Zealand". It's unclear what he meant by this, or if the original collector (whose identity is also a mystery) had been any more specific, but the consensus is that the animal was actually caught off the northeast coast of South Island. Found in no other country of the world, today the vast majority of Hector's dolphins live along the coasts of this island, in three separate populations that apparently get no opportunity to interbreed. A fourth, much smaller, population lives off the northeast coast of North Island and is now regarded as a separate subspecies.

It prefers shallow, murky coastal waters less than 8 km (5 miles) from the shore and less than 39 metres (125 feet) deep, doubtless explaining its restriction to such an unusually small area for a dolphin. It isn't quite so keen on cold water as its South American kin; while it can forage successfully at lower temperatures, it prefers seas between 14 and 22°C (57 and 72°F). It feeds almost entirely on cod, sprats, and squid, and has been observed following behind small inshore trawlers, presumably because they may disturb fish and make them easier to catch.

They live in small single-sex groups, typically with fewer than eight members, and, like the Chilean species, never seem to travel more than about 60 km (37 miles) from their home, which doubtless explains the reproductive isolation of the different populations. These figures may be different for the North Island ("Maui") subspecies, which is, if anything, even more restricted in how far it travels, but which often does so in mixed-sex groups. 

The fact that Hector's dolphin is unique to New Zealand means that it inhabits the smallest region of any dolphin species, and the second-smallest of any seagoing cetacean (after a species of porpoise). The total population was estimated at a little over 7,000 in 2005 and is thought to be rapidly declining, largely due to entanglement in fishing nets. As a result, it is formally listed as an endangered species. The news is worse for the Maui subspecies, now thought to number only around 100 surviving animals and on the verge of extinction.

Given how far apart South America and New Zealand are, it's legitimate to ask where the common ancestor of these three species came from. This is thought to be southern Africa, with the original populations descended from individuals dragged westward with the subantarctic current, first to New Zealand, and then to South America (the population on Kerguelen must have got there much later, presumably lapping its ancestors around the world). However, as one might expect, the entire population did not take part in this (presumably accidental) migration, leaving many behind in their original home.

Heaviside's dolphin

Today, these have evolved into the species known as Heaviside's dolphin (Cephalorhynchus heavisidii). This looks much like Hector's species, but is blander in colour, lacking most of the darker patterns on the grey parts of the body and with the white patches on the belly and armpits being smaller. There is often a darker "cape" on the back behind the dorsal fin, but that's about it for distinctive features.

First identified off the Cape of Good Hope, this is still the area where it is most commonly found today. The remainder of the population is sparsely distributed along the west coast of Africa, placing it much further north than the other three species, reaching about 15°S in southern Angola. This is possible because of the cold Benguela Current, since it, too, seems to prefer temperatures below 15°C (60°F). 

Named for surgeon John Heaviside under the mistaken impression that he had collected the first specimen (it was actually an entirely different man, whose surname happened to be "Haviside"), we know less about this species than any of its close relatives. They live in small groups, and are most commonly seen in pairs rather than pods, although larger temporary groups can form where food is abundant. 

Their habitat is similar to the other species, with them not venturing more than 8 km (5 miles) from the shore or into waters more than 100 metres (330 feet) deep. While they hug even closer to the shore during the day, they move outwards at night, feeding on hake rising up from the bottom as the light dims and, while they can go deeper if they have to, they only rarely dive below 30 metres (100 feet). While hake forms the largest part of their diet, they also eat gobies, kingklip (a type of South African cusk eel), squid, and octopuses.

Their echolocation clicks are very similar to those of their relatives - and to those of porpoises - including a rapid series of high-pitched sounds that come out sounding like a "cry" to human ears. As with the other species, the pitch of these sounds is at a level that's generally inaudible to predators, although they use louder and more detectable sounds when they need to communicate with their own kind.

The small ranges of the Cephalorhynchus species, especially of Hector's dolphin, are unusual, if not unique. Most dolphins range far more widely, especially if they are not concerned about travelling far from the shore. Next time, I will be looking at three species found in tropical and subtropical waters across the globe... and two of which aren't commonly called "dolphins", despite the fact that they manifestly are.

[Photos by Sebastián Saiter (x2), Gregory Smith, and Cindy Heppelmann, from Wikimedia Commons. Cladogram adapted from Pichler et al. 2002.]