Delphinids: Dolphins in the Irrawaddy

Irrawaddy dolphin

While we normally think of dolphins as being sea-dwelling animals, there are no fewer than eight species referred to as such that are commonly found in rivers. Six of these, however, are not true members of the "dolphin family", or Delphinidae, their ancestors having split off from that group even before those of some of our modern whales did. Of the two exceptions, one is entirely freshwater, and lives in South America. The other is more varied in its habitat and lives in Asia.

The Irrawaddy dolphin (Orcaella brevirostris) was first described in 1866, from a specimen caught, not in a river, but off the northeast coast of India. We now know that this is at the far western edge of its range, and that it is also found all along the coast from northeast India, around the Malaysian Peninsula, to as far east as southern Vietnam. It is also found further south, around Borneo and along the north coasts of Sumatra and Java. In 1999, a very small population was discovered in the Philippines, living in a couple of isolated bays very far from the remainder of the animal's range, presumably the result of some having been swept away in a storm decades or even centuries before.

More significantly, the dolphins also live in at least three major rivers: the Irrawaddy (Ayeyarwady), Mekong, and Mahakam. Perhaps surprisingly, they aren't found close to the mouths of these rivers, but in isolated stretches a considerable distance inland. As their common name might suggest, they reach furthest in the Irrawaddy, where they have been identified as far as 1,400 km (870 miles) upriver. Both Chilika in India and Songkhla in Thailand, which are brackish coastal lakes/lagoons, but not fully freshwater, also host significant populations.

The scientific name translates as "small killer whale with a short snout" and one can see what Richard Owen was thinking when he named it. The overall body shape of the dolphin is similar to that of a killer whale, especially in that it has a short, rounded, snout, rather than the beak seen in many other dolphins. Beyond this, however, they are quite obviously different animals. For one thing, the Irrawaddy dolphin is medium-sized as dolphins go, at about 230 cm (7'6") in length and 120 kg (260 lbs) in weight, males being larger than females.

Furthermore, the Irrawaddy dolphin has a mobile neck, allowing it to move its head far more than a killer whale can, and a tiny dorsal fin placed unusually far back along the body. A distinctive crease runs down the middle of its back from the neck to the fin, a feature not seen in any other species. The colour is uniform, and generally described as a bluish grey, becoming paler on the underside. 

As one might expect from their distribution, Irrawaddy dolphins prefer shallow water, with about 50 metres (160 feet) being their maximum, and around 10 metres (33 feet) being preferred. Similarly, they are not seen more than a few kilometres offshore and often cluster around river mouths. They are happy in brackish waters in estuaries and along coastal margins, although both true saltwater and fresh river water are equally good. They do, however, avoid the shallowest parts of rivers, which restricts their range during the dry season when river levels drop. At these times, at least in the Mekong, they have been reported to stick to short stretches where the water is at least 8 metres (26 feet) deep, something that would also prevent them from venturing into narrower tributaries - which they are known to do during the wet season.

Groups of Irrawaddy dolphins are typically small, with no more than six individuals each. They feed during the day, using cooperative dives that usually last only two to three minutes, although they can hold their breath for at least five times this long if they have to. Their diet consists largely of shallow water fish, supplemented by crustaceans, and a few small squid. Although the details are likely to vary with their specific environment, a report from the Mekong indicated that catfish and carp were the most common prey in that river. Perhaps reflecting such differences in diet, or perhaps to stand out from background sounds, the echolocation clicks used by the dolphins vary across their range, as do the whistles they use to communicate.

While they can breed throughout the year, this is apparently most common just before the monsoons start, which, with a 14-month gestation period, would indicate a peak in births during the wet season, when rivers are at their height. Although solitary mating between pairs does occur, groups of males have been seen congregating around a female, presumably to entice her into mating with one or more of them; such temporary "herds" can be much larger than the usual social groups.

While, at least during the 20th century, the Irrawaddy dolphin was not considered varied enough to contain subspecies, George Heinsohn and others reported in 2002 that there was a surprising amount of variation in skull shape between two distinct populations. Four years later, genetic analysis showed that the two populations were not merely subspecies, but distinct species in their own right, perhaps having split not long before the last common ancestor of all modern Irrawaddy dolphins lived 0.76 million years ago. Thus, the southern population was split off as a new species: the Australian snubfin dolphin (Orcaella heinsohni). 

Even without having to open it up to have a good look at the skull shape, the two species can be distinguished by virtue of the Australian snubfin having a browner colouration, usually a more visible crease around the neck and, perhaps most significantly, lacking the long crease that runs along the back from the neck to the fin in the Irrawaddy dolphin. 

As now defined, the Australian snubfin dolphin lives along most of the northern coast of Australia, albeit in scattered populations rather than a single continuous one across the entire region. It has also more recently been reported from the south coast of New Guinea, although alleged sightings elsewhere have (so far) not stood up to scrutiny. It is reported to have very similar habitat requirements to its more northerly relative, rarely venturing out into water more than 15 metres (50 feet) deep and being common near river mouths and in seagrass beds. Having said which, it does not appear to have established any freshwater populations, perhaps because there are few rivers in the region large enough for it to do so.

Because it was identified so recently - at the time, it was the first new cetacean species to be recognised for 56 years - we know little about the species as distinct from its northern cousin. It doesn't help that, despite living close to the shore, it's relatively secretive, being difficult to spot even when it surfaces, since it tends to lie low in the water and has a very small fin. Its diet is probably versatile, although the most commonly recorded components are cardinal fish and cuttlefish; it may also be notable that their largest known population is in an area especially rich in shrimp. Their social behaviour, echolocation clicks, and mating habits are all, so far as we can tell, also similar to those of their close relative.

Living as they do along coastlines with limited space, and often feeding on the same fish that human fishmermen are trying to catch, both species are in a precarious position. The Australian snubfin is thought to be doing the better of the two, with an estimated population of around 10,000, but that is slowly declining, and the lack of detailed information means that it may be an overestimate. At the time of writing, it isn't listed as an endangered species, but that could well change as things progress, or simply as we learn more.

The Irrawaddy dolphin, however, is at greater risk, and has been formally recognised as an endangered species since 2017. The overall population is not much lower, at around 7,000. but most of that is scattered across small patches of water, in almost all of which the risk of entrapment in fishing nets is high. The largest number are thought to survive in the vast delta and of the Ganges/Brahmaputra and its neaby mangrove swamps in Bangladesh, but most other populations are thought to number less than 200 with, for example, that along the Irrawaddy itself estimated at no more than 72 in 2007. It seems hard to imagine that this is sustainable for long.

Forunately, dolphins further out at sea tend not to face such risks since, unlike their larger cousins, they are not actively hunted. Next time, I will be looking at two of those species, the last two on my list that look at all the way we expect typical "dolphins" to look...

[Photo by Stefan Brending, from Wikimedia Commons.]