Irrawaddy Dolphin

## Description

This dolphin is easily distinguished due to its unusually short beak, rounded head and flexible neck, which makes creases behind its head. The species is most closely related to the orca. However, in its appearance, this dolphin has more similarities with the beluga whale, though the overall coloration of the Irrawaddy dolphin is darker, the underside is lighter and the back is dark grey. The animal has 12 - 19 peg-like teeth on either side of each jaw. Females of this species have smaller dorsal fin, shorter body and lighter weight.

## Distribution

The species is found in scattered areas throughout the Indo-Pacific region, occurring from the Philippines to northeastern India. The Irrawaddy dolphin can be found in the following freshwater river systems: Mahakam (Indonesia); the Ayeyarwady, previously called Irrawaddy (Burma); and finally, the Mekong (Laos, Cambodia and Vietnam). The animal is also known to live in Chika Lake in India as well as Songkhla Lake in Thailand. The Irrawaddy dolphin generally inhabits coastal areas and is not found in offshore waters. Preferred habitat of this dolphin is muddy, brackish waters at river mouths and deltas.

## Behavior

Irrawaddy dolphins are highly social animals. However, compared pods of other dolphins, groups of this species are smaller, usually containing 3 - 6 individuals. Coming to the surface, these curious animals are known to look around and explore the surrounding area. As opposed to other dolphins, they are not very active, swimming slowly with sluggish movements. As they come to the surface to breath, only the top of the head is seen; once appearing on the surface, they immediately dive back. As a matter of fact, only 14% of all surfacings include playful behavior such as rolling, splashing, limb waving and slapping. They usually surface twice before diving. When disturbed, Irrawaddy dolphins, living in Chilka Lake swim onto a sand bar and roll around. Every day, these diurnal dolphins travel from the Semayang Lake in eastern Borneo to the Mahakam River, coming back to the lake in the evening.

## Diet

Irrawaddy dolphins are carnivores (piscivores and molluscivores).The usual diet of this animal includes fish, cephalopods and crustaceans.

## Reproduction

These dolphins have polygynous mating system, where each male mates with a number of females. Male dolphins compete with each other for their mating rights. There is very scarce information on the reproductive biology of this species. However, they are likely to breed in December - June. Gestation period lasts for about 9 months, though data from two captive births has shown 14 months of gestation. Usually, females yield a single baby with an interval of 2 - 3 years. During the first seven months of its life, the offspring feeds exclusively on the nutrition from its mother. The calf learns to prey on fish, observing the behavior of its mother as well as other members of the pod. By the age of 6 months, young dolphins begin eating fish, being completely weaned at 2 years old. They are thought to become sexually mature with 7 - 9 years.

## Population

### Threats

The main threat to the population of this species is drowning in gill nets, which most often happens in December-May, at the dry season, when these animals remain in deep water bodies. On the other hand, electrocution and prey depletion from electric fishing pose a real threat to these animals, particularly to those found in the Ayeyarwady River. The dolphin also suffers from pollution, construction of dams as well as the build-up of silt and sedimentation, which leads to a high degree of erosion. In addition, the animal currently faces dangers such as overfishing, collisions with boats and resulting injuries, which can sharply reduce the population of this dolphin.

### Number

According to the IUCN Red List, the total population size of the Irrawaddy dolphin is unknown, but there are estimates available for only a few portions of the range: in Malampaya Sound (Philippines) - 77 dolphins; in the Mekong River - 125 dolphins; in the Mahakam River (Indonesia) - 70 dolphins; in the Ayeyarwady River (Myanmar) - 58-72 dolphins; in coastal waters of Bangladesh - 5,383 dolphins; in the Sundarbans mangrove forest of Bangladesh - 451 dolphins. Overall, numbers of Irrawaddy dolphins’ population are decreasing today, and the species is currently classified as Vulnerable (VU) on the IUCN Red List.

### Ecological niche

Irrawaddy dolphins act as top ecosystem predators, feeding on and regulating popelations of fish, cephalopods, and crustaceans.

## Fun Facts

These dolphins tend to spit water in order to herd fish. They are also known to stun large fish with a blow from their lower jaw and play with them.

These dolphins are extremely playful animals, known to wave and slap water with their tail and flippers, breach and leap from the water, roll sideways, blow bubbles and pause at the water surface.

During the night and in areas with low visibility, the Irrawaddy dolphins use echolocation to orientate in the river and to detect prey.

This dolphin is named after the river it’s endemic to, the Irrawaddy River.

The Irrawaddy dolphins are able to remain submerged for more than 6 minutes. However, when foraging, these dolphins typically dive for 1 - 2 minutes.

The Irrawaddy dolphins communicate with each other through a wide variety of clicks and buzzing calls, helping them identify individuals.