(Gray, 1866) - Irrawaddy dolphin
The Irrawaddy dolphin resembles the finless porpoise, but unlike that species, it has dorsal fin. The fin is small and triangular, with a bluntly rounded tip, and is set just behind midback. 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. The U-shaped blowhole is open toward the front, the reverse of the situation in most dolphin species.
The back and sides of Irrawaddy dolphins are grey to bluish grey; the belly is somewhat lighter.
Tooth counts are 17 to 20 (upper) and 15 to 18 (lower) in each row. The teeth have slightly expanded crowns.
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 distinguishable because neither of the other species has a dorsal fin.
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.
Irrawaddy dolphins inhabit coastal, brackish, and fresh waters of the tropical and subtropical Indo-Pacific. They range from northern Australia and New Guinea to the Bay of Bengal, including at least the Irrawaddy, Mahakam, Mekong, Ganges, and Brahmaputra rivers. The range is poorly documented and is thought to be more extensive than shown.
Biology and Behaviour
Groups of fewer than 6 individuals are most common, but sometimes up to 15 dolphins are seen together. Irrawaddy dolphins have been seen in the same area as bottlenose and Indo-Pacific hump-backed dolphins. Irrawaddy dolphins are not particularly 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 from June to August, but 1 captive female gave birth in December.
Fishes, cephalopods, and crustaceans are taken as food. They sometimes spit water while feeding, apparently to herd fish.
Shark gillnets in Australia and fish traps and other types of nets throughout the range are known to take some Irrawaddy dolphins. Some small-scale hunting by local people probably occurs in many areas of its range.