Sousa chinensis
Indo-Pacific humpbacked dolphin

Indo-Pacific humpbacked dolphins, Sousa chinensis, are found the Indian and Pacific Ocean, from the coast of Africa to the coast of China and Australia. There are two distinct forms of this species: Indian humpbacked dolphins Sousa chinensis plumbea and Pacific humpbacked dolphins Sousa chinensis chinensis. Indian humpbacked dolphins are mainly found along the coasts of the Indian Ocean, while Pacific humpbacked dolphins are mainly found along the coasts of Southeast Asia, New Guinea, and Australia. Members of this species have been observed off the coast of over 30 different countries. They do not, however, occur around the Philippines due to the presence of deep oceanic waters. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Indo-Pacific humpback dolphins have a highly tropical and subtropical distribution. They live in warm waters, generally warmer than 15 degrees Celsius, and at an average depth of 20 m, rarely traveling to waters deeper than 25 m. They are often found in or near bays, estuaries, mangrove forests, sandbanks, rocky and coral reefs and large river mouths. They generally remain close to the shore, but occasionally venture further if water depth remains shallow. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Indo-Pacific humpback dolphins are medium-sized dolphins, ranging from 1.8 to 3 m in length and weighing 250 to 285 kg when fully grown. Indo-Pacific humpback dolphins in waters near southern Africa express sexual dimorphism, with males larger than females, but sexual dimorphism is not observed in other areas. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

The dorsal fin and hump of Indo-Pacific humpbacked dolphins varies with geographical region. In eastern waters, the dorsal fin is short and sits on a wide base that gradually slopes into the body. The tip of the fin is lightly recurved, and the hump is only 5 to 10% of the total body length. In western waters, the dorsal fin is shorter and more recurved, however it sits atop a much wider and longer base that reaches to about 30% of the body length. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Coloration of Indo-Pacific humpbacked dolphins varies greatly with developmental stage and with geographic region. In general, subadults are a mottled grayish-pink color and calves are dark gray. Individuals found in southern African waters are typically dark gray with a lighter ventral surface. They develop a pinkish-white spot on the dorsal fin as they age. Calves in this region are much lighter than those of other regions. Individuals found in the northern Indian Ocean are more brownish-gray in color. In waters around China and other areas of southeast Asia, individuals are pure white, often with a pinkish tint. White Indo-Pacific humpback dolphins also often have a speckling of dark flecks on their body. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Little information about the mating systems of Indo-Pacific humpbacked dolphins is documented. However, the most likely reroductive strategy of males is mate searching. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Indo-Pacific humpback dolphins breed once yearly, though births typically occur in the spring and summer. After a gestation period of 10 to 12 months, females usually give birth to 1 offspring that measures approximately 100 cm in length. Young are weaned around 2 years of age, although they are capable of eating solid foods after 6 months. Females reach sexual maturity around 9 to 10 years of age, while males reach sexual maturity around 12 to 13 years of age. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Female Indo-Pacific humpback dolphins provide considerable care to their young. Calves are weaned around 2 years of age and remain in association with their mother for 3 to 4 years. Allomothering, or non-maternal infant care, has been observed off the coasts of South Africa and Hong Kong. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Lifespan of Indo-Pacific humpback dolphins is fairly high in the wild; they generally live 40 or more years. Indo-Pacific humpback dolphins are not commonly raised in captivity. Many die after 3 months in captivity, and one individual in India died after 28 days due to starvation. One individual, however, lived 31 years in captivity. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

Indo-Pacific humpback individuals have been observed alone and in groups. Groups tend to be small, consisting of less than 10 individuals but can consist of up to 25 individuals. Larger groups are typically associated with trawlers. Groups consist of individuals of all ages, although one half to two thirds of the group generally are adults. Strong social ties appear to be uncommon, except between mother and calf. One group in Maputo Bay, Mozambique, however, displayed strong social affiliations. (Folkens and Reeves, 2002; Jefferson, 2000)

Indo-Pacific humpback dolphins are more cryptic than other dolphins, which make them appear less playful. They are typically slower than bottlenose dolphins. Indo-Pacific humpback dolphins, however, can lift their head above the water's surface and have been observed flapping the water with their flippers and breaching the surface. They typically avoid boats and do not bow ride. (Folkens and Reeves, 2002; Jefferson, 2000)

Indo-Pacific humpback dolphins have been observed associating with other species. When following behind trawlers for food, they are often observed with groups of bottlenose dolphins and other animals, such as southern right whales, Cape fur seals, and even gulls and terns. (Jefferson, 2000)

The home range of Indo-Pacific humpback dolphins varies by group. In South Africa some groups have been observed occupying a home range spanning hundreds of kilometers of coast range. More commonly, small groups like those in the Pearl River Estuary occupy a range of 10 to 400 sq km. (Jefferson, 2000)

Indo-Pacific humpback dolphins communicate with each other through clicks, whistles, and screams. Clicks are frequently heard, while screams are the least common and have only been observed in groups exceeding 4 or 5 individuals. (Jefferson, 2000)

Indo-Pacific humpback dolphins primarily feed on abundant estuarine fish and fish associated with reef environments. They generally feed close to the ocean floor. Some groups feed with the rising tide. Indo-Pacific humpback dolphins are also known to follow trawlers, feeding on discarded organisms. (Folkens and Reeves, 2002; Jefferson, 2000)

Sharks are the only known predator of Indo-Pacific humpback dolphins. While unconfirmed, it is likely that killer whales, Orcinus orca also prey on this species. Indo-Pacific humpback dolphins have been known to flee from sharks and to chase sharks to avoid predation.

Indo-Pacific humpback dolphins eat a variety of fish and are prey to some sharks. They also host some parasites, such as nematodes (Anisakis alexandri), which affect the stomach. In Hong Kong, lungworms (Halocercus pingi) have been observed in their orbits. Barnacles (Halocercus pingi) have also been observed living on the skin of Indo-Pacific humpback dolphins.

Indo-Pacific humpback dolphins are occasionally hunted by humans, but not on a commercial scale. They are not generally held in captivity by aquariums because of high mortality rates for captive individuals. (Folkens and Reeves, 2002; Jefferson, 2000; Shirihai, et al., 2006)

There are no known adverse effects of Indo-Pacific humpback dolphins on humans.

Indo-Pacific humpbacked dolphins are listed as near threatened by the IUCN and on Appendix I by CITES. Because they live in lose proximity to the shore, they often get tangled in fishing nets and, in areas in Africa, in anti-shark nets. Destruction of habitats is most likely the greatest threat to this species. This destruction is caused by environmental contaminants and reclamation of coastal waters. (Folkens and Reeves, 2002; Jefferson, 2000; Van Parijs, et al., 2002)