The Irrawaddy dolphin (Orcaella brevirostris) is a marine mammal in the family of oceanic dolphins, part of the order of cetaceans. A particularly distinctive dolphin, the Irrawaddy has a rounded head with no beak and a flexible neck, causing visible creases behind the head. Although most closely related to the orca, the Irrawaddy dolphin is similar in body form to the Beluga whale, but darker in colour, with a pale to dark grey back and a light underside. The dorsal fin is small, triangular and rounded, and the flippers are long and broad.
|Orcaella brevirostris Source: Collection Georges Declercq|
Kingdom: Animalia (Animals)
| Common names:
The color of the irrawaddy dolphin has been described as dark bluish grey, slate grey, battleship grey, or pale grey. The underside is usually paler than the dorsal surface. Orcaella brevirostris has a high, anteriorly convex forehead which overhangs the mouth. It does not have a beak, and its U-shaped blowhole is to the left of the midline. Unlike the condition in most dolphin species, the blowhole opens toward the front. Irawaddy dolphins resemble the finless porpoise, but unlike that species, they have a small, triangular, and bluntly rounded dorsal fin (with a barely concave rear margin) set just behind the midback. The flippers are relatively large (about one-sixth as long as the body) and have a great breadth with a gently curved leading edge. The mouthline is straight, and there may be a visible neck crease. The neck is unusually flexible because only the first two cervical vertebrae are fused. The tail is also quite flexible. Orcaella brevirostrishas homodont, narrow, pointed, and peg-like teeth with slightly expanded crowns. The teeth are about 1 cm in length, and tooth counts are 17 to 20 (upper) and 15 to 18 (lower) in each quadrant. The skull is characterized by its globular shape, short rostrum, and broad facial region. The Irrawaddy dolphin does not have a cardiac sphincter; the stomach is subdivided into compartments communicating through narrow orifices.
Little is known about the reproduction/breeding behavior of irrawaddy dolphins, but according to Kampuchean fisherman, the courtship season is from March to June at 11-12 degrees north latitude. These fisherman say they observe copulation almost daily during this period, and fights between males are also often observed. Coitus is preceded by much play including chases or jumps with the partners often leaping out of the water, belly to belly. During coitus, other members of the group swim around the mating pair.
The irrawaddy dolphin has not been extensively studied, and little is known about its reproduction and breeding behavior. The mating season is believed to extend from April to June in the Semayang Lake/Mahakam River area of Kalimantan. Calves from animals caught in this area have been born in captivity in Jakarta (six degrees north of Kalimantan) in July and December. The age of sexual maturity is unknown, but there is evidence that at least some dolphins reach adult size when they are four to six years old. The gestation period is estimated to be fourteen months. A neonate born in captivity in Jakarta was 96 cm long and weighed 12.3 kg. It was born twelve days after milk was first seen discharging from its mother, and the tail was observed protruding from the genital slit more than two hours before the calf was born. The calf started suckling twelve hours after birth and eating dead fishes at the age of six months. It was fully weaned by two years of age. During its first seven months, the calf increased in length by 57 cm (59%) and in weight by 32.7 kg (266%). Little is known about the reproductive biology of the Irrawaddy dolphin, but it is thought to breed between April and June in the Mahakam River, and gestation is estimated at 14 months.
The species lifespan in the wild is estimated to be approximately 25 years.
Irrawaddy dolphins are not notably active, but they do make low leaps on occasion. They are usually seen while surfacing slowly and exposing their blowhole. The animal then usually continues forward with a smooth slow roll. They are not known to bowride.
Nothing is known of the depths to which Irrawaddy dolphins dive, but considering their coastal and riverine distribution, it is unlikely for them to dive to considerable depths. It has been reported that Orcaella brevirostris typically respires three times in rapid succession and then dives for 30-60 seconds (dive times are longer when the animal is frightened). The maximum dive time recorded is 12 minutes, and the maximum swimming rate recorded is 25 km/hr (recorded while a Orcaella brevirostris was being chased by a boat). Irrawaddy dolphins are usually seen in small groups, which usually consist of less than six individuals but which may contain as many as 10 to 15 animals. There is no information on the population dynamics of this species.
The behavior of Orcaella brevirostris suggests that they spend most of their time feeding. Irrawaddy dolphins sometimes spit water while feeding (they can expel water from their mouths for distances of up to 1.5 m), apparently to herd fish. Fishermen also allege that the dolphins sometimes catch large fish for sport by stunning them with a blow from their lower jaw. The dolphins then play with the fish like a cat with a mouse before discarding them.
The vocalizations of Irrawaddy dolphins are short time-duration bandwith signals of about 25-30 microsecond duration. The main sonar signal consists of only a few cycles of a dominant frequency of around 60 Kilohertz. Pulse trains are rather regular in nature. No audible whistles or pure tones have been recorded.
Believed to be reincarnated humans by some of the people of Laos, Irrawaddy dolphins are less active than many other dolphins, making only occasional low leaps and never bow-riding. Feeding together in groups of usually less than six, but as many as 15, the Irrawaddy dolphin can dive for up to 12 minutes to feed on bony fish, crustaceans, cephalopods and fish eggs. Irrawaddy dolphins are known to spit water to herd fish, and have even been reported to stun large fish with a blow from the lower jaw, only to play with them before casting them aside. In some areas of Asia, fishermen consider the Irrawaddy dolphin to be a competitor for fish, but in other areas the fishermen attract them to the boat and encourage them to drive fish into the nets for a share of the catch. Irrawaddy dolphins communicate with clicks, creaks and buzzes.
This dolphin species is known to carry out daily migrations from the Semayang Lake in eastern Borneo to the Mahakam River, returning to the lake in the evening. In Indonesia, Irrawaddy dolphins move into tributaries at high water and into the main river during low water. The individuals found in northern Australia, which are morphologically distinct from Asian individuals, do not appear to migrate.
Reports from various parts of Asia suggest that irrawaddy dolphins regularly assist fisherman by driving fish into their nets. In one report, fisherman in Burma were observed to attract irrawaddy dolphins by tapping the sides of their boats with oars. The dolphins swim around the boats in ever-diminishing circles, therby forcing fish into nets. The fisherman share their catch with the dolphins and consider them friends that are not to be harmed.
The Irrawaddy dolphin has a patchy distribution in the shallow, coastal waters of the Indo-Pacific from northern Australia and the Philippines to northeastern India. There are a number of subpoulations, including:
- Ayeyarwady River subpopulation of Burma
- Mahakam River subpopulation of Indonesia
- Mekong River subpopulation Laos, Cambodia and Viet Nam
- Malampaya Sound subpopulation in Palawan, Philippines
- Songkhla Lake subpopulation in Thailand
- Chika Lake subpopulation in India
Irrawaddy dolphins inhabit coastal, brackish, and fresh waters (major river systems) of the tropical and sub-tropical Indo-Pacific. They have been found as far as 1440 km upstream and can live permanently in freshwater.
Orcaella brevirostris is a generalist feeder, taking food both from within the water column and from the bottom. Bony fishes seem to be the main food of irrawaddy dolphins, but they have also been observed to eat crustraceans, cephalopods, and fish eggs. Stomachs of ten Orcaella brevirostris from coastal waters off Townsville, Australia all contained bony fishes (from 16 orders and 13 families). Nine of the ten stomachs contained crustaceans (five with shrimps, two with isopods, and four with unidentified crustaceans), and all stomachs contained cephalopod remains (ten with squid, three with cuttlefish, and two with octopods). Two species of cyprinid fish, Cirrihinus siamensis and Paralaubuca typus, are believed to be an especially important food source for irrawaddy dolphins in northeastern Cambodia and Lao PDR. Carp appear to be the most important food source for Irrawaddy dolphins in Semanyang Lake (Kalimantan).
All cetaceans are protected under Australian legislation within the Australian Exclusive Economic zone which extends 200 nm off the northern coast. The Irrawaddy dolphin is also protected by law in Laos. Some captive breeding of this species has been successful.
It has been estimated that there are fewer than 2,000 Irrawaddy dolphins left, but they are not believed to be at risk of imminent extinction. Most live captures are for the oceanarium trade in Asia, and hunting of this species is rare, occuring only in parts of India to harvest oil for the treatment of rheumatism. However, the Irrawaddy dolphin is likely to be affected by increasing pollution, construction of dams and the build-up of silt following severe erosion. Incidental catches occur and fishing with explosives also results in dolphin casualties.
The irrawaddy dolphin is likely affected by increasing pollution, the construction of barrages such as dams, and build-ups of silt. Also, shark gillnets in Australia and fish traps and other types of nets throughout the range are known to take away some irrawaddy dolphins. Furthermore, some small-scale hunting by local people probably occurs in many areas of its range (nonetheless, the irrawaddy dolphin is generally unexploited), and the irrawaddy dolphin inhabits some of the most vulnerable of aquatic habitats, those being tropical, riverine, estuarine, and coastal habitats.
The oil of the irrawaddy dolphin has reportedly been used as a remedy for rheumatism in parts of India.
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