Delphinids: White-sided Dolphins

Atlantic white-sided dolphin

The taxonomy of dolphins is far from settled, with exactly how we should classify some species having been an open question for years. There is a good chance that the scientific names I am using for some species in this series will not still be in use in a decade, as old genera are split and the family tree re-arranged. Such is the case, for example, with the dolphins of the genus Lagenorhynchus.

The genus was named by John Edward Gray in 1846 for a specimen of a previously unknown species sent to him for analysis at the British Museum, after having been caught somewhere off the coast of Norfolk. It translates as "bottle-nose", for the shape of the beak... which is, perhaps, unfortunate, given that the animal we refer to in English as the "bottlenose dolphin" is something else entirely. Over the centuries since, five new species have been added to the genus, giving us the six we recognise today.

The problem is that, despite the physical similarities that led to this classification, genetic analysis has shown, that the species assigned to the genus are not each other's closest relatives. This means that most of them will have to be moved elsewhere. Unfortunately, exactly where they should be moved, and which genetic analysis shows the most useful results, has been a matter of debate since at least 1999. 

Three of these species had been named even before the genus was, only later being moved into it. One of the more distinctive is what we now call the Atlantic white-sided dolphin (Lagenorhynchus acutus). Since it doesn't appear to be closely related to any other sort of dolphin, representing the last survivor of some early branch of the family tree, it's probably going to end up in a genus of its own when taxonomists finally agree on how best to sort the mess out. But even that isn't certain.

The Atlantic white-sided dolphin is similar in size to a common dolphin, but has a bulkier body and a relatively short beak. The distinctive appearance comes from its colouration. The upper body is dark grey, and the undersides white, something that helps camouflage against the light from the water surface when viewed from underneath. The flanks are paler grey, distinctly offset from both the dark back and light underside, but, more significantly, there is a white patch on the mid-flank, and a yellow one slightly above and behind it. A narrow black streak runs from a ring around the eyes to the leading edge of the flippers.

They inhabit the chilly waters of the North Atlantic, preferring temperatures between 5° and 15° C (40° and 60° F). In the south, they reach about as far as a line running from the English Channel to New Jersey, while in the north, they venture far beyond the Arctic Circle. Although they sometimes venture into fjords or other inlets, they mostly prefer waters over 100 metres (330 feet) deep, staying over the lower parts of the continental shelves and not normally into waters over 300 metres (1,000 feet). Despite this, populations on either side of the Atlantic are less isolated than one might expect, perhaps mixing in the north during the summer, since there is some evidence that they migrate with the seasons. 

Their favoured food appears to be whiting, with generous quantities of mackerel, haddock, pout, and (in the summer) herring, along with smaller amounts of other similarly sized fish and squid. How they catch their food is unclear, although dives are brief, typically only lasting around a minute, and never more than four. On at least some occasions, they act collaboratively, 'herding' fish into a tight ball that they can feed on together.

They appear to live in groups of a few dozen, averaging around 50, and consist mostly of unrelated individuals, suggesting a fluid social structure. They often travel with whales, sometimes riding in their bow wake, a habit that they have also adopted for boats. They also regularly leap out of the water, leading to the common nickname of 'jumper'. Their echolocation clicks are simple, with a relatively low tone compared with most other dolphins (although still ultrasonic from a human perspective), while whistles are long for dolpins at nearly a second, and slide up and down in tone by about an octave. Mating is apparently most common in the summer, but may occur year-round.

White-beaked dolphins

The species for which Gray named the genus in 1846, and which therefore is the only one definitively still in it, is the white-beaked dolphin (Lagenorhynchus albirostris). While it's probably not a close relative of the Atlantic white-sided species, it's easy to see why it might have been thought otherwise, given the apparent similarity between them.

The white-beaked dolphin is of similar size and build to the white-sided one, but has an even shorter beak, making it look a little like an oversized and oddly coloured porpoise. It too, has a dark grey back and a white belly and beak, but the white is less extensive, and the coloured patches on the flanks are replaced by a light grey one in front of the dorsal fin.

Having much the same temperature preferences, they live in almost exactly the same areas as Atlantic white-sided dolphins do, although their southern boundary is slightly further north at the western end, so that it reaches only as far as Massachusetts. On the other hand, stray individuals have wandered further afield for brief periods, reaching northern Portugal, North Carolina, and (in the east) the Baltic Sea. Among the few differences are that they are more willing to venture out into deeper waters; although they prefer seas less than 200 metres (650 feet) deep, they can be found in waters five times that depth.

They prefer to eat cod and whiting, although some individuals have been found with large quantities of haddock or herring in their stomachs, along with relatively small amounts of other fish, squid, and bottom-dwelling crustaceans. The few dives that have been analysed tend to be short, with one study recording a maximum of 78 seconds - although the dolphin in question only needed to go down 45 metres (150 feet) to reach the bottom, so there may have been little point in him diving for longer. Swimming speeds have been clocked at up to 30 km/h (19 mph), but are usually only half this.

By the standards of dolphins, they do not seem especially social, with group sizes often less than ten, which are often single-sex. Nonetheless, larger groups are sometimes seen, and they may associate with white-sided dolphins, or even with whales much larger than themselves. Their echolocation clicks are far higher-pitched than those of the Atlantic white-sided species, being closer to those of bottlenose dolphins. 

Their whistles, used to communicate with others of their kind, are also high-pitched, and they can shout loudly enough to be heard by other dolphins over 10 km (6 miles) away... which may suggest a more distributed community than their small social groups might imply. Mating probably takes place in the summer, with young born about 11 months later.

Pacific white-sided dolphin

Moving to the other side of the world, we come to the Pacific white-sided dolphin (Lagenorhynchus obliquidens). This has a very similar appearance to its Atlantic namesake, even though we now know that they are not especially closely related. However, it lacks the white and yellow patches on the flanks, having instead a pattern of pale grey patches and streaks behind the eyes and in front of the tail. Pure white individuals are also known from both sides of the Pacific.

They also prefer slightly warmer waters than their namesake, reaching just south of Baja California and southern Japan, and not venturing north of the Aleutian Islands. A population in the Sea of Japan is genetically distinct from the others, but not formally identified as a subspecies; it was probably cut off from its kin during the Ice Ages by low sea levels around the islands.

They prefer deep water and are thus most often found some distance from the shore, but they can approach surprisingly close where deepwater canyons reach towards the coast, as happens off British Columbia and California. They are obviously opportunistic feeders, since their diet has been recorded as varying considerably depending on where they live. Off North America, they feed on herring, sardines, hake, and salmon, and in the north, anchovy and mackerel. Near Japan, two-thirds of their diet is thought to consist of squid, with the firefly squid apparently being a favourite.

Pacific white-sided dolphins are gregarious, although the reported size of their pods is highly variable. One study concluded that, at any given time, about half of them live in groups with less than fifty members, but that larger groups are so common that the overall mean group size is around 200. This may change across the year, depending on the availability of food, with larger groups being more common when sea currents bring in warmer waters from the south. 

Their dives are reported to be even shorter than in their Atlantic namesake, lasting just 23 seconds on average, although this may depend on the local depth of the water. There is also some evidence for variation in the exact form of their calls depending on population, which may allow them to separate and feed on different types of fish without competing with one another.

Dusky dolphin

The closest relative of the Pacific white-sided dolphin is thought to be the dusky dolphin (Lagenorhynchus obscurus); the two are likely to end up being placed together in a new genus. These also prefer cool waters, but they live in the south, not the north. Their population seems oddly disjunct, with four main populations living off western South America, Patagonia, southwestern Africa, and New Zealand, respectively. They have also been spotted around remote islands in the far south of the Atlantic and Indian Oceans. At least some of the populations may be distinct subspecies, although there remains disagreement about how many that might be.

They are similar in general form to the white-sided dolphins of the north, but about 25% shorter, being typically about 210 cm (8' 2") in length and weighing less than 85 kg (190 lbs). They have the same dark back and white belly, with the paler skin reaching up to the sides of the chest, and distinctive two-pronged patches of pale grey on the flanks behind the dorsal fin.

The populations are likely disjunct because the dolphins live over continental slopes, but not over the deep oceans that separate them in the south; over the million-plus years of their existence, some must have been swept further away to colonise new shores, but it's not something they do willingly. Even so, the fact that they prefer the slopes around the continental shelves to the shelves themselves means that they are found in relatively deep waters up to 2,000 metres (6,500 feet). Mother dolphins may, however, hug closer to the shore when raising young infants, perhaps avoiding both predatory sharks and pestering males.

This is presumably a reflection of their preferred food, but this varies depending on where they live, so they can't be that picky. Anchovies, hake, and southern cod are common food off Patagonia, anchoveta off the coast of Peru, and lanternfish off New Zealand. At night, they dive to catch fish rising from the depths, while during the day they chase fish up towards the surface to feed, often providing an incidental boon to seabirds that follow them about. 

Like their close relative in the North Pacific, they live in large groups, often of over 100 individuals. Like many dolphin species, these groups have a very fluid composition, with individuals coming and going all the time, but with preferred long-term friendships that buck the wider trend. Larger groups often break up when they do not need to gather together for safety, such as when resting in shallow waters, and these subgroups often have fewer than ten members each. 

Males can be competitive, as shown by the presence of scars on their fins apparently left after being nipped by their rivals. They chase females prior to mating, with several males often pursuing a single target, with the females apparently preferring those that are fastest and most agile. As with some other dolphin species, sex has been observed even when neither sex is fertile, including some homosexual activity. Pregnancy lasts around 13 months, with mating occurring in late winter in the warm waters fof Peru, but summer further south.

Peale's dolphin

Dusky dolphins apparently delight in leaping out of the water, often in coordinated groups and as noisily as they can manage. It's generally thought that this helps maintain social bonds, perhaps as a signal to all move together to new feeding grounds. They typically swim at around 5 km/h (3 mph) but can put on bursts of up to seven times that speed while pursuing prey. 

Peale's dolphin (Lagenorhynchus australis) is known only from the coasts around Patagonia and the Falkland Islands, although it sometimes ventures into the shallower waters of the Drake Passage to the south. It is of similar size to the dusky dolphin, and has two grey patches on the flanks, separated by a diagonal black band. Although they are quite common locally, they have been little studied and are not so well-known as those found in more temperate waters.

They prefer shallow waters, less than 20 metres (65 feet) deep, and are often found in association with kelp beds, feeding on animals living amongst the seaweed. That food can be quite varied, since they seem willing to eat almost anything of the right size that lives close to the seabed. This includes hagfish, southern cod, squid, and significant quantities of both shrimp and octopus - not normally main constituents of dolphin diets. Group sizes are small, averaging as few as four individuals, although larger aggregations have occasionally been seen, often in association with other dolphin species.

Hourglass dolphins

The hourglass dolphin (Lagenorhynchus cruciger) is equally mysterious but, while closely related to Peale's species, has quite a different habitat. It is found in subantarctic waters south of 45°, where the surface temperature is between 13°C and 1°C (55°F and 34°F) and well clear of the ice fields. It prefers deeper waters, rarely visiting those less than 200 metres (660 feet), although it can sometimes be found close to the coast in southern South America. Otherwise, it is found right around the world, although southern New Zealand is the only other significant land mass in the right temperature range. 

The name of the species comes from its patterning. Aside from the black back and white belly common among species assigned to the genus, the flanks are also black, but with a distinct hourglass-shaped patch of white running along the body, with the narrow part just above the flippers. They are also unusually small dolphins, especially considering the cold waters they prefer, averaging just 165 cm (5' 5") in length.

Group sizes are small, usually with no more than a dozen members each, although they may be associated with larger whales. They are fast swimmers, regularly reaching 22 km/h (13 mph) and not only leap into the air, but seem to enjoy surfing in high waves during rough weather. 

Both Peale's and hourglass dolphins have echolocation clicks that sound more like those of other small southern dolphins than they do those of their formerly presumed relatives. Indeed, while they may belong to the same branch as the dusky and Pacific white-sided dolphins, it has also been proposed that they belong to the same, already-known genus, as those similar-sounding neighbours. It is to the species already placed in that group that I will turn next time.

[Photos by Elliott Hazen, in the public domain, Charlotte Kirchner, "Yummifruitbat", NOAA, Christopher Moning, and "Lomvi2" from Wikimedia Commons.]