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Hump-Back Dolphins

Hump-Back Dolphins

Dolphins of the genus Sousa are commonly characterized by a small dorsal fin arising from a distinct ridge, similar to that of a humpback whale. These ridges may continue along the tail stock of the animal.

Sousa teuszii is the Atlantic species of the hump-back dolphin, residing along the west coast of tropical and sub tropical Africa. The Indo-Pacific hump-backed dolphin, S. chinensis, ranges from northern Australia and southern China, around the coastal rim of the Indian Ocean, to the east coast of southern Africa. Besides being isolated geographically, S. chinensis typically has more teeth (29 to 38 per row) than S. teuszii (26 to 31 per row).

Hump-backed dolphins that live east of Indonesia may lack dorsal ridges altogether and are thus often confused with bottlenose dolphins. They often share each others’ company and have similarly-shaped heads with well-defined beaks. The two genera can be distinguished by the rostrum, which is generally longer in Sousa. Also, the hump-backed dolphins’ melons are less pronounced. Their flippers are broadly attached at their bases and rounded at the tips.

Most hump-backed dolphins are some shade of gray, varying from a deep slate color in the Atlantic to almost white in the waters of Malaysia and northern Australia. Many have a speckled appearance. The population around Hong Kong is noted for its distinctive pink coloration.

Sexual dimorphism is exhibited by the dolphins, with the dorsal ridges exaggerated in males. Males are larger, up to three meters in length and 285 kilograms.

Usually seen in groups of two to ten, hump-backed dolphins do not seem to develop strong social bonds. Researchers speculate, however, that the animals are part of larger groups that maintain their identities over time. A distinct breeding season has not been observed, although calving increases during the summer months.

Unlike TursiopsSousa do not bowride. The surfacing styles of the two genera are different. Hump-backed dolphins fully expose their rostrum and melon, unlike most bottlenose dolphins, then sharply roll and arch their backs to accentuate their humps. To dive, they flex far forward and often expose their flukes, with a distinctive notch between them, before disappearing beneath the surface.

Feeding on nearshore, estuarine, and reef fish, the dolphins are seldom spotted more than one kilometer offshore. Some have even been reported in rivers, though it is not known if they reside in them or were merely visiting.

One of the best known stories of dolphin-human interaction occurs at Cap Timiris in western Africa. There, Atlantic hump-backed dolphins cooperate with Mauritanian fishermen to catch mullet. When the fishermen spot a school of the fish, they beat on the water with sticks to attract the dolphins. The dolphins then drive the mullet into the fishermen’s nets along the beach.

Accidental drowning in fishing nets and occasional hunting for human consumption were historically the greatest threats to the hump-backs. A new concern has arisen with the development of African and Asian countries as mangrove and estuarine habitat is destroyed. These dolphins have not been studied in great detail, so the health of the population is questionable.