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| Indo-Pacific humpback dolphin |
Our best evidence suggests that it probably isn't a "real" group, in the sense of one consisting of a common ancestor and all of its descendants. That's because three other genera of dolphin appear to be descended from that common ancestor, forming part of the same cluster of what we might describe as "typical-looking" dolphins. The common and bottlenose dolphins form two of these interspersed groups, while the third is represented by the humpback dolphins.
As it turns out, their classification has also had to undergo significant revision in recent years, albeit for different reasons.
The humpback dolphin was first identified as a distinct species by Swedish naturalist Pehr Osbeck in 1757, after he saw a couple swimming near the mouth of the Pearl River in southern China. This, however, was one year before our current system of scientific naming was invented, so it doesn't formally count as a scientific description. Osbeck published the description again in 1765, naming the animal the "Chinese dolphin", Delphinus chinensis, and it's this date that now gets listed alongside the name. (The Chinese, of course, already knew about the animal well before this, calling it by a name that translates as "white dolphin").
Over the following two and half centuries, the classification of the animal changed repeatedly, with disputes as to whether it was one species or several, and where in the tangled close-knit family tree it was supposed to fit. In 1866, John Edward Gray placed it in a newly created subgenus of the "short-nosed dolphins", and this was subsequently promoted to full genus status. In the last couple of decades, however, it has slowly become clear that what we had considered to be a single species for most of the 20th century was, in fact, three different species, which aren't necessarily each other's closest relatives.
Two of those species inevitably got their own names, and the one that's left with the original is what we now call the Indo-Pacific humpback dolphin (Sousa chinensis). This lives along the coast of southeastern Asia from just beyond the southern tip of India to just north of Taiwan, reaching the coasts of Borneo, Sumatra, and Java in the south.
The operative phrase here being "along the coast". While other closely related dolphins may prefer shallow waters, they can still be found far at sea, something that has enabled to, for example, reach both sides of the Atlantic without having to enter the chilly waters of the Bering Straits. This, however, is very much not true of the humpback dolphins, which are truly coastal animals. They do not travel more than two kilometres (just over a mile) from the shore and have rarely been reported in waters more than 25 metres (82 feet) deep, and usually less than half that. On the positive side, they remain comfortable in the brackish waters of estuaries, which most dolphins don't, and will even swim a short distance up freshwater rivers.
Physically, they are medium-sized dolphins, about 280 cm (9'4") in length, with robust bodies and distinct slender snouts. They are born dark grey, but as they age, the colour fades over most of their bodies, until it remains as just a series of spots on a pale pinkish or even white background. In some individuals, even the spots disappear, explaining the origin of their Chinese name. Most notably, of course, they have a distinct hump on their back, located in front of the dorsal fin and sloping gradually down on either side.
They feed on small shallow-water fishes, probably without much selectivity, although in practice, the bulk of their diet consists of mullets, herrings, and anchovies. They have the same sort of fission-fusion society as their relatives, living in groups of up to ten from which individuals come and go as they please. At least some, however, have more long-lasting associations, and sometimes groups will gather together to form short-lived pods with up to forty members. Like many dolphins, they are vocal, making whistles, barks, and grunts in addition to the broadband sonar clicks used to find food. Whistles, in particular, appear to be contact calls, especially between mothers and their infants, and are produced with a wide range of different pitches and vocal forms.
Breeding takes place throughout the year. Mothers give birth about once every three years, with one year taken by the 10 to 12-month pregnancy, and the remaining two weaning the resulting infant.
Living in such shallow waters, however, comes at a cost. Hundreds of years ago, it may not have been a problem, but this is where motorised boats are now most common. While slow-moving boats don't seem to worry them, sometimes even evoking curiosity, fast-moving ones are disruptive, causing the dolphins to flee in apparent stress. Despite which, their population is not as small as might be feared, with an estimated 2,600 living in the Pearl River estuary close to Hong Kong and another large population off Bangladesh. Unfortunately, that population is dropping, albeit slowly, with little sign of recovery, leading to them being listed as a Vulnerable species. The population off the west coast of Taiwan, which is too far from the mainland to be replenished by dolphins living there, is an exception, and may be close to local extinction.
Since they are slow-moving and not especially acrobatic, they are less popular as subjects for dolphin-watching than some other species, although their proximity to shore helps to counteract that.
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| Indian Ocean humpback dolphin |
This occupies, barring a few gaps here and there, all of the coasts of the Indian Ocean west of the Indo-Pacific species. The exact location of the boundary between the two is unclear; it may be the southern tip of India, or it may be somewhere in the Bay of Bengal, perhaps with the two species overlapping in the region. (Or, for that matter, there could be a fifth species somewhere in between). In the other direction, however, it is clear that the dolphin lives along the remainder of the southern coast of Asia, around Arabia, and down the east coast of Africa as far as the Cape, as well as along the west coast of Madagascar.
In retrospect, it's surprising it took so long for the species to be split apart again, given how different they look. They are about the same size, although male Indian Ocean dolphins are larger than females, which isn't true of the Indo-Pacific species. They are born pale in colour and the back of the animal becomes a darker grey as they age - almost the exact opposite of the pattern in their eastern relative. They have more teeth, and, perhaps most significantly, the hump is much larger and wider, a clear bulge rising on the back with a tiny, curved dorsal fin poking out from the top of it.
This makes it easier to examine the hump and determine what it's doing there. The precise answer isn't clear, but we can tell that it is formed of connective tissue with numerous branching blood vessels inside it. As such, it probably has a role in keeping the animal cool, acting as a heat exchanger, as the regular dorsal fin does in other dolphin species. Whether having a hump, rather than a large fin, makes it easier to retain heat or to radiate it out is what remains the mystery...
The habitat preferences of the two species are, however, similar, with Indian Ocean humpback dolphins remaining close to shore, often near rocky reefs or mangrove swamps. The best-studied population, on the South African shoreline, apparently prefer to stay in water less than 6.6 metres (21 feet) deep, although elsewhere it is not unusual for them to forage in water up to 11 metres (36 feet). While they seem to return to favoured spots regularly, they also travel significant distances along the coast, with 120 km (75 miles) being an average figure.
Off Abu Dhabi, they are reported to feed on mojarras and jack fish, both of which are common in the area, and also cuttlefish. Cephalopods are an uncommon component of the diet of their eastern relative, but have also been reported to be regular prey for African populations as well. Group sizes may be slightly larger than in the eastern species as well, in rare cases reach two dozen individuals, although they are more usually in single figures.
The overall population is, on the other hand, likely much lower than in the Indo-Pacific species. Few communities have more than a hundred individuals in total, and none reach a thousand. Since figures this low cannot withstand even relatively low levels of mortality due to the slow reproduction of the animals, the Indian Ocean humpback dolphin is, unlike its relative, formally listed as an endangered species.
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| Australian humpback dolphin |
As we might expect, given that nobody had realised it was different back in the 19th century when naming new species was all the rage, this looks much more like the Indo-Pacific humpback than does its western counterpart. Apart from the genetic differences that demonstrated its existence, it can be distinguished by a slightly different shape to the dorsal fin, and by the fact that it does not get so dramatically pale with age, retaining a solid, dark grey, back. The dorsal fin is, however, often much paler, especially in males.
Like their relatives, Australian humpback dolphins stick to shallow waters, preferring those less than 10 metres (33 feet) deep, although they have been seen to use shallow seas as deep as 36 metres (118 feet). The shallowness of the seas around Queensland in particular, however, mean that they can be found further offshore than one might expect, reaching 6 km (3½ miles) in the waters between the mainland and the Great Barrier Reef. Indeed, such coral reefs are ideal feeding grounds for the dolphins and they have been spotted around the relatively isolated Montebello Islands, a smaller set of coral reefs off Western Australia on the opposite side of the continent.
Social groups also seem to be similar to those of the other species, with some tendency to stick to preferred locations but fission-fusion between the pods and few long-term relationships. Females, however, seem to be more gregarious than males and to have a network of preferred associates with which they meet up more often than chance, if not necessarily for a long time on each occasion. While we have no reliable estimates of their total population, it is likely to be declining, and their preference for hugging close to river mouths and coral reefs where boat traffic is common probably doesn't help much. Like the Indo-Pacific species they too, are therefore considered Vulnerable, but not quite on the endangered list.
Like some other dolphin species, Australian humpback dolphins have been observed playing with objects they have discovered, most notably including sponges dislodged from the seabed. Often, this involves them placing the object on their snout while they search for food, suggesting that this may be helpful in that endeavour in some practical way, but just as often there is no obvious purpose. They throw them in the air, pass them to one another, push them around and generally seem to be having fun, perhaps as a part of social bonding with their neighbours.
It's thought that the common ancestor of these three species may have originated along the coasts of eastern Australia during the Pliocene, spreading out only in the last 1.5 million years or so, when sea levels dropped during the Ice Ages. However, the fourth species is not descended from this Ice Age stock, having been separate for far longer. Even in the 20th century, it was already considered distinct, but it is sufficiently uncommon not to have been scientifically described until as late as 1892.
Even then, it was described on the basis of a skull collected by Julius Eduard Tëusz, a botanist who found it alongside the headless remains of a manatee. Incorrectly deciding that the two parts had to belong to the same animal, he concluded that he had discovered a unique, vegetarian dolphin. Partly because the teeth looked blunt, zoologist Willy Kükenthal, who gave it its scientific name, didn't disagree. Surprisingly, it took until 1956 for scientists to prove them both wrong.
This is the Atlantic humpback dolphin (Sousa teuszii) and it lives well away from the others along the west coast of Africa, from the Western Sahara in the north to Angola in the south. There is little doubt that they are related to the others, however, and, while they remain a relatively dark grey throughout life, the hump on their back is especially prominent.
We know relatively little about this species, despite its long history of formal recognition. What we do know suggests that they are similar to their counterparts in the Indian and Pacific Oceans. Their diet has been reported to consist of mullets, sardines, and bonga fish, but this probably varies along their range. When shallow seas stretch some distance of the coast, they can be found surprisingly far out, but, when heading inland, they seem to stick to tidal waters rather than venturing into more than the very lowest parts of rivers. Shallow mudflats seem to be their preferred habitat. The only analysis of their whistles shows that they are comparable to those of the other humpback species.
As the fact that it took over sixty to prove that it ate fish demonstrates, this is not a common dolphin. There are no good estimates of its population size in any of the places it exists, but it's a heavily developed region with intensive fishing, commercial and industrial development, and general habitat degradation. Given the species' narrow habitat requirements, this cannot be good, and it's thought that whatever the population may be it has probably crashed over the last few generations. It's very unlikely that more than 3,000 survive along almost the entire west coast of Africa, leading to its classification as critically endangered, even without the rigorous evidence commonly required for such a status.
Traditionally, humpback dolphins were placed in a separate subfamily from bottlenose and common dolphins, something that molecular and genetic data has since disproved, even if we aren't sure exactly what the relationships should be. In being moved to their current taxonomic position, however, they displaced some other, apparently similar dolphins, that are actually quite distant. How they relate to anything else is even less clear, but even so, it is to those that I will turn next time.
[Photos by "Chem7", "Pavlaki 1968", and Yves Bas, from Wikimedia Commons.]
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