Believed to be reincarnated humans by some of the people of Laos (2), Irrawaddy dolphins are less active than many other dolphins with only the uppermost dorsal surface of the animal becomes visible during a slow rolling dive; they make only occasional low leaps and never bow-ride. Feeding together in groups of usually less than six, but as many as 15 (5), 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 (2). 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 where the dolphins also benefit by preying on fish whose movements are confused by the nets and those that are momentarily trapped around the edges or in the mud (6). Irrawaddy dolphins communicate with clicks, creaks and buzzes (7) at a dominant frequency of about 60 kilohertz which is thought to be used for echolocation (8).
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 and weaning after two years (2).
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 (4).