The Irrawaddy dolphin resembles the finless porpoise, but unlike that species, it has dorsal fin. The fin is small and rounded, and is set just behind mid-back. The large flippers have curved leading edges and rounded tips. The head is blunt, with no beak; the mouthline is straight, and there may be a visible neck crease, especially in Australian animals. In Asian specimens, there is generally a distinct groove running along the back from the neck region to the dorsal fin. The U-shaped blowhole is open toward the front, the reverse of the situation in most dolphin species.
Coloration is highly variable. Australian animals have a tripartite pattern, with dark brown cape, lighter sides, and whitish belly. Asian animals are two-tone - the back and sides are gray to bluish-gray; the belly is somewhat lighter. The light ventral coloration extends up as an ‘elbow’ onto the underside of the flippers.
Tooth counts are 8-22 (upper) and 11-19 (lower) in each row. The teeth have slightly expanded crowns. Tooth counts in Australian dolphins are significantly higher than those in animals from Asia. This is a relatively small dolphin; adults range from 2 to 2.75 m. Scant evidence indicates that the length at birth is about 1 m.
Can be Confused With
Irrawaddy dolphins can be confused with finless porpoises or dugongs in the parts of their range where these species overlap. When a clear view is obtained, Irrawaddy dolphins are easily distinguishable because neither of the other species has a dorsal fin.
Irrawaddy dolphins inhabit coastal, brackish, and fresh waters of the tropical and subtropical Indo-Pacific. They range from northern Australia and New Guinea, north to Palawan in the Philippines, and west to the Bay of Bengal, including freshwaters in the Ayerawaddy, Mahakam, and Mekong rivers, and Songkhla and Chilka lakes. The range is poorly documented.
Ecology and Behavior
Groups of fewer than six individuals are most common, but sometimes up to 15 dolphins are seen together. Aggregations of up to 25 dolphins can be found in deepwater pools of the Mekong River during the dry season. Irrawaddy dolphins have been seen in the same area as bottlenose and Indo-Pacific humpback dolphins, as well as finless porpoises. Irrawaddy dolphins are not especially active, but do make low leaps on occasion. They are not known to bowride.
The calving season is not well known. Some calves appear to have been born in June-August, but one captive female from the Mahakam River give birth in December. There is probably great geographic variation in life history parameters.
Feeding and Prey
Irrawaddy dolphins appear to be generalist feeders, taking a wide variety of fishes (including freshwater species such as catfish), cephalopods (squid, cuttlefish, and octopus), and crustaceans (prawns and isopods). Dolphins from freshwater populations sometimes spit water while feeding, apparently to herd fish.
Threats and Status
Irrawaddy dolphins may not be threatened with extinction on a global scale, but certain populations appear to be reduced and are in serious danger of local extinction. This is especially true for freshwater stocks, such as those in the Mahakam, Ayerawaddy, and Mekong rivers, as well as that in Songkhla Lake. The nearshore and freshwater occurrence of this species makes it particularly vulnerable to threats from human modification and degradation of the coastal and freshwater environments. These result in fishing net entanglement, habitat loss, dam and waterway construction, prey depletion, environmental contamination, and vessel strikes. Irrawaddy dolphins have been hunted directly in the past, at least in Cambodia, but are also revered by local people in many other areas of Asia. Currently, irrawaddy dolphins are listed as “Data Deficient” (IUCN) and “Not Listed” (ESA).
Arnold, P.W. 2002. Irrawaddy dolphin Orcaella brevirostris. pp. 652-654 in W.F. Perrin, B. Würsig and J.G.M. Thewissen, eds. Encyclopedia of marine mammals. Academic Press.
Beasley, I., P. Arnold, and G. Heinsohn. 2002. Geographical variation in skull morphology of the Irrawaddy dolphin, Orcaella brevirostris (Owen in Gray, 1866). Raffles Bulletin of Zoology Supplement 10:15-34.
Marsh, H., R. Lloze, G.E. Heinsohn and T. Kasuya. 1989. Irrawaddy dolphin Orcaella brevirostris (Gray, 1866). pp. 101-118 in S.H. Ridgway and R. Harrison, eds. Handbook of marine mammals, Vol. 4: River dolphins and the larger toothed whales. Academic Press.
Smith, B.D. and T.A. Jefferson. 2002. Status and conservation of facultative freshwater cetaceans in Asia. Raffles Bulletin of Zoology Supplement 10:173-187.
Stacey, P.J. and P.W. Arnold. 1999. Orcaella brevirostris. Mammalian Species 616:1-8.
Stacey, P.J. and S. Leatherwood. 1997. The Irrawaddy dolphin, Orcaella brevirostris a summary of current knowledge and recommendations for conservation action. Asian Marine Biology 14:195-214.