Irrawaddy dolphin


The Irrawaddy dolphin is a euryhaline species of oceanic dolphin found in scattered subpopulations near sea coasts and in estuaries and rivers in parts of the Bay of Bengal and Southeast Asia. It closely resembles the Australian snubfin dolphin, which was not described as a separate species until 2005. It has a slate blue to a slate gray color. Although found in much of the riverine and marine zones of South and Southeast Asia, the only concentrated lagoon populations are found in Chilika Lake in Odisha, India, and Songkhla Lake in southern Thailand.

Taxonomy

One of the earliest recorded descriptions of the Irrawaddy dolphin was by Sir Richard Owen in 1866, based on a specimen found in 1852 in the harbour of Visakhapatnam on the east coast of India. It is one of two species in its genus. It has sometimes been listed variously in a family containing just itself and in the Monodontidae and Delphinidae. Widespread agreement now exists to list it in the family Delphinidae.

Etymology

The species' name, brevirostris, is from the Latin meaning "short beaked".

Description

The Irrawaddy dolphin's colour is grey to dark slate blue, paler underneath, without a distinctive pattern. The dorsal fin is small and rounded behind the middle of the back. The forehead is high and rounded; the beak is lacking. The front of its snout is blunt. The flippers are broad and rounded. The finless porpoise is similar and has no back fin; the humpback dolphin is larger and has a longer beak and a larger dorsal fin.
It ranges in weight from with a length of at full maturity. The maximum recorded length is of a male in Thailand.
The Irrawaddy dolphin is similar to the beluga in appearance, though most closely related to the killer whale. It has a large melon and a blunt, rounded head, and the beak is indistinct. Its dorsal fin, located about two-thirds posterior along the back, is short, blunt, and triangular. Dorsal fin shapes differ from one Irrawaddy dolphin to another. The flippers are long and broad. These dolphins are usually two-toned, with the back and sides being gray to bluish-gray and the belly lighter. Unlike any other dolphin, the Irrawaddy's U-shaped blowhole is on the left of the midline and opens towards the front of the dolphin. Its short beak appears very different from those of other dolphins, and its mouth is known for having 12-19 peg-like teeth on each side of the jaws.

Behaviour

Communication is carried out with clicks, creaks, and buzzes at a dominant frequency of about 60 kilohertz, which is thought to be used for echolocation. Bony fish and fish eggs, cephalopods, and crustaceans are taken as food. Observations of captive animals indicate food may be taken into the mouth by suction. Irrawaddy dolphins are capable of squirting streams of water that can reach up to ; this distinct behaviour has been known for herding fish into a general area for hunting. They do this sometimes while spyhopping and during feeding, apparently to expel water ingested during fish capture or possibly to herd fish. Some Irrawaddy dolphins kept in captivity have been trained to do spyhopping on command. The Irrawaddy dolphin is a slow swimmer, but swimming speeds of were reported when dolphins were being chased in a boat. In a year, females can sometimes swim as fast as an average of, and males can swim up to an average of.
Most Irrawaddy dolphins are shy of boats, not known to bow-ride, and generally dive when alarmed. They are relatively slow-moving but can sometimes be seen spyhopping and rolling to one side while waving a flipper and occasionally breaching. They are generally found in groups of 2-3 animals, though sometimes as many as 25 individuals have been known to congregate in deep pools. Groups of fewer than six individuals are most common, but sometimes up to 15 dolphins are seen together. Traveling and staying in groups not only enables Irrawaddy dolphins to hunt, but it also creates and maintains social bonds and allows copulation to occur. There are a few groups of dolphins in Brunei Bay seen near the mouths of the Temburong and Aloh Besar rivers, which are tame and gather around fishermen's boats that catch fish during the fishing season from around December to February.
It surfaces in a rolling fashion and lifts its tail fluke clear of the water only for a deep dive. Deep dive times range from 30 to 150 seconds to 12 minutes. When 277 group dives were timed in Laos, the mean duration was 115.3 seconds with a range of 19 seconds to 7.18 minutes.
Interspecific competition has been observed when Irrawaddy dolphins were forced inshore and excluded by more specialized dolphins. When captive humpback dolphins and Irrawaddy dolphins were held together, reportedly the Irrawaddy dolphins were frequently chased and confined to a small portion of the tank by the dominant humpbacks. In Chilika Lake, local fishers say when Irrawaddy dolphins and bottlenose dolphins meet in the outer channel, the former get frightened and are forced to return toward the lake.

Mating

A female or male dolphin will attempt to pursue a mate for about a few minutes. They intertwine, facing their bellies together, and begin to copulate for 40 seconds. Once copulation has occurred, the dolphins will break away from each other and set off in different directions.

Reproduction

These dolphins are thought to reach sexual maturity at seven to nine years. In the Northern Hemisphere, mating is reported from December to June. Its gestation period is 14 months; cows give birth to a single calf every two to three years. Length is about at birth. Birth weight is about. Weaning is after two years. Lifespan is about 30 years.

Feeding

There are plenty of food items that this dolphin feeds upon. They include fish, crustaceans, and cephalopods. During foraging periods, herds of about 7 dolphins will circle around prey and trap their victim. These prey entrapments occur slightly below the water surface level.

Distribution and habitat

The Irrawaddy dolphin is an oceanic dolphin that lives in brackish water near coasts, river mouths, and estuaries. It has established subpopulations in freshwater rivers, including the Ganges and the Mekong, as well as the Ayeyarwady River, from which it takes its name. Its range extends from the Bay of Bengal to New Guinea and the Philippines, although it does not appear to venture offshore. It is often seen in estuaries and bays of Borneo, with sightings from Sandakan in Sabah to most parts of Brunei and Sarawak, and another specimen was collected at Mahakam River in East Kalimantan. Its presence in Chinese, Taiwanese, and Hong Kong waters has been questioned, as the reported sightings have been considered unreliable.
It has been extinct in Laos since 2022.

Interaction with humans

Irrawaddy dolphins have a mutualistic relationship of co-operative fishing with traditional fishers. Fishers in India recall when they would call out to the dolphins by tapping a wooden key, also known as a lahai kway, against the sides of their boats, asking the Irrawaddys to drive fish into their nets. In Burma, in the upper reaches of the Ayeyarwady River, Irrawaddy dolphins drive fish towards fishers using cast nets in response to acoustic signals from them. The fishermen attempt to gain the attention of the dolphins through various efforts such as using a cone-shaped wooden stick to drum the side of their canoes, striking their paddles to the surface of the water, jingling their nets, or making calls that sound turkey-like. A pod of dolphins that agrees to work alongside the fisherman will entrap a school of fish in a semicircle, guiding them towards the boat. In return, the dolphins are rewarded with some of the fishers' bycatch. Historically, Irrawaddy River fishers claimed particular dolphins were associated with individual fishing villages and chased fish into their nets. An 1879 report indicated legal claims were frequently brought into native courts by fishers to recover a share of the fish from the nets of a rival fisher that the plaintiff's dolphin was claimed to have helped fill.

Folk stories

Laotians and Cambodians have a common belief that the Irrawaddy dolphins are reincarnations of their ancestors. Some even claim that the dolphins have saved drowning villagers and protected people from attacks by crocodiles. Their beliefs and experiences have led the people of Laos and Cambodia to live peacefully alongside one another for ages. The West Kalimantan people have a similar story where the dolphins were naughty children who ate a pot of rice reserved for the shaman, but their mouths scalded, and they jumped into the water to cool themselves, and later transformed into these dolphins.
Buddhist Khmer and Vietnamese fishermen have regarded the Orcaella as a sacred animal. If caught in fishing nets, they release the dolphin from the rest of the catch. In contrast, Muslim Khmer fishermen kill them for food. This has led to the dolphin becoming reputed to recognize the local languages of the area and approaching areas of the Khmer Muslim community with caution.