The dusky dolphin (Lagenorhynchus obscurus) is a marine mammal of the family Delphinidae and infraorder Cetacea. It is a small dolphin which inhabits the temperate ocean waters of the southern hemisphere, especially near the coastal regions of South America, southern portion of Africa, and New Zealand. It is always encountered in large number of individuals, which swim together in pods. It was first described as Delphinus obscurus in 1828 by John E. Gray, who had seen it off the coast of South Africa. The dusky dolphin and the Pacific white-sided dolphin are related species. These two sister species began to diverge at around 2.5 million years ago. Dusky dolphins from Argentina and southwest Africa separated 2000 generations ago from an ancestral Atlantic population and subsequently diverged without much gene flow.
Physical Characteristics
The adult individual of the dusky dolphin can reach a maximum body length of 2.10 m, with an average length of 1.8 m. It can weigh up to 100 kg (210 pounds). Thus, compared to other species of Delphinidae, it is relatively small, yet robust and strong. The rostrum (snout) of the dusky dolphin is short and it is clearly demarcated from the melon (forehead). The conspicuous dorsal fin is moderately falcate (curved) and pointed. The flippers are also curved on the leading edge, with a blunt tip. The body coloration is complex, ranging from dark grey to black on the back ( all along the dorsal portion of body), and white on the belly. The sides are marked with blazes and patches of pale or pearly grey. Each jaw has between 27 and 36 small, pointed teeth.
Biology and Behavior
Dusky dolphins are highly social and gregarious animals. They sometimes form impressive pods of over 1,000 individuals. However, they are more likely to form groups of 30 to 500 individuals. The dusky dolphin is one of the most acrobatic dolphins, frequently leaping high out of the water, sometimes twisting and tumbling in the air. They usually approach vessels to engage in bow-riding, swimming along the ship. Many species of cetaceans have been observed in association with dusky dolphins. Off the coast of Argentina and New Zealand, calving is believed to peak in Summer (November and February). Dusky dolphins feed on a wide variety of small fish and sea creatures, such as squid; southern anchovy, sardine, mackerel, and laternt fish are included in the range of prey they hunt and eat.
Reproduction
During the mating season, a group of around eight males chase a single female. The males swim swiftly after a single female at high-speed. Females seem to prefer males with great speed and agility rather than size, strength, or aggression. Females may extend the chase as long as possible so only the fastest and fittest male remains. Unlike male bottlenose dolphins, male dusky dolphins do not monopolize a female, which lets other males to copulate with her after she has mated with the fastest one. Thus, the female copulate with multiple partners. Females with calves gather in nursery groups in shallow waters rather than in deep waters. Thus, these groups likely provide mothers and calves more time to rest. The formation of nursery groups in shallow waters also allows members to hunt prey species that inhabit in these waters. Both adults and calves have been observed to chase and catch fish, and the adults may be teaching the calves how to hunt.
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| Above, a dusky dolphin leaping out of the water in the South Atlantic Ocean. |
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| The world map shows the coastal areas where the dusky dolphins are usually sighted. |
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| They swim in large numbers. A group of dolphins breaching off the coast of New Zealand. |