Taxonomy

Five species of the genus Sousa have been described: S. chinensis (Osbeck, 1765), S. plumbea (G. Cuvier, 1829), S. lentigenosa (Owen, 1866), S. teuszii (Kukenthal, 1892) and S. borneensis (Lydekker, 1901). Some authors recognize all 5 species, whereas other recognize only S. teuszii, the Atlantic form and S. chinensis, the Indo-Pacific form, which includes the other 3 species (S. plumbea, S. lentigenosa and S. borneensis). The genus Sousa is closely related to the genera Steno (rough-toothed dolphin) and Sotalia (tuxuci) (Ross et al, 1994).

Appearance

There is a wide variety in coloration of this species and the colors may change with age. The following variations have been described:

• slate-grey dorsally and pale grey ventrally (Atlantic dolphins)
• dark plumbeous grey dorsally, gry flanks and off-white ventrally (South African dolphins). The fin tip and the fin base may turn white with age.
• uniformly plumbeous or brownish grey (Indian Ocean dolphins)
• plumbeous or brownish grey with dark bluish-black longitudinal flecks (Indian Ocean, S. lentigenosa form
• pinkish white with dark eye patches (Chinese dolphins). Calves are dark grey, subadults become mottled greyish pink
• pale-grey back and flanks, fading to off-white ventrally (Queensland, Australia, dolphins). Older animals may develop a white fin.

These animals are rather robust. Marked dorsal and ventral ridges are present on the peduncle. The melon has a distinct apex, blending indistinctly with the snout. There are some regional variations in size, but on average adult hump-backed dolphins are 210-220 cm long (range: 180-279 cm) (Ross et al, 1994). They weigh 80-100 kg. They have 29-38 pairs of peg-like teeth in each jaw (26-31 for S. teuszii) (Evans, 1987).

Hump-backed dolphins are slow swimmers (4.8 km/hr) and surface briefly at long intervals (40-60 seconds). Their characteristic high roll when surfacing probably gave them their common name.

Distribution

The Indo-Pacific hump-backed dolphin can be found in the coastal waters of the Indian Ocean and the Western Pacific Ocean, from the northern East China Sea to New South Wales, Australia. This is a distinctly tropical species, associated with warm waters (15-36°C). They usually live in shallow waters (less than 20 m deep). In Moreton Bay, Australia, all sightings of this species occur in waters of 10 m deep or less and within 6 km of the coast.

Population dynamics and life history

Neonate calves are about 1 m in length. There appears to be no seasonality in reproduction. Very little is known about maturation and mortality in this species.

Population status

There is very little data on the abundance of the species anywhere in its range.

Exploitation

In the Arabian Sea, Red Sea and Persian Gulf, small numbers have been taken for their meat. They do occassionally get entangled in shark nets off South Africa (about 8/year) (Klinowska, 1991). Some animals have been captured for live display. One animal, a female captured in 1966, is still on display at Sea World, Gold Coast, Australia. The dolphins of Imraguens de N'memghar, Mauretania, co-operate with local fishermen in their fisheries for mullet. This co-operation seems to benifit both humans and dolphins (Klinowska, 1991). As a result, the local fishermen are very protective of "their" dolphins, although they will utilize stranded or accidently killed dolphins.

Feeding

The hump-backed dolphin feeds mainly on fish species. The species taken depend on the area, but include clupeids (herring family) and mullet, as well as small reef fishes.

A sociable hump-backed dolphin In Tin Can Bay, Queensland, Australia, a female hump-backed dolphin appears daily to interact with bathers. In 1996, she had a calf and took the calf with her on her visits to the Bay. Her visits probably started when people offered her fish. Recently feeding her has been discontinued and is actively discouraged, but she continues to visit the Bay. (see also the Photo feature on this dolphin)

References

Evans, P.G.H. (1987)

The natural history of whales and dolphins. Christopher Helm, London.

International Whaling Commission (1996)

Report of the sub-committee on small cetaceans. Rep. Int. Whal. Commn. 46:160-179

Klinowska, M. (1991)

Dolphins, Porpoises and Whales of the World. The IUCN Red Data Book. IUCN, Gland, Switzerland.

Ross, G.J.B., Heinsohn, G.E. and Cockcroft, V.G. (1994)

Humpback dolphins Sousa chinensis (Osbeck, 1765), Sousa plumbea (G. Cuvier, 1829) and Sousa teuszii (Kukenthal, 1892). In: S.H. Ridgway and R. Harrison: Handbook of Marine Mammals. Volume 5: The first book of dolphins, pp. 23-42. Academic Press, San Diego

Note on the photographs

The pictures of the dolphin out of the water were taken at Sea World, Gold Coast, Australia in November 1996. The other pictures were taken in Tin Can Bay, Australia, also in November 1996.

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