Common dolphin

Common dolphins
Size comparison against an average human
Conservation status
Least Concern (IUCN 3.1)[1][2]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Cetacea
Family:	Delphinidae
Genus:	Delphinus Linnaeus, 1758
Species
Delphinus capensis Delphinus delphis

The Common dolphin is the name given to up to three species of dolphin making up the genus Delphinus.

Prior to the mid-1990s, most taxonomists only recognised one species in this genus, the Common Dolphin Delphinus delphis. Modern cetologists usually recognise two species - the Short-beaked Common Dolphin, which retains the systematic name Delphinus delphis, and the Long-beaked Common Dolphin Delphinus capensis. Despite its name the common dolphin is not the dolphin of popular imagination; that distinction belongs to the Bottlenose Dolphin, largely due to its widespread use in marine parks, as well as its appearance in the television series Flipper.^[3]

Differentiating species

Despite the historic practice of lumping the entire Delphinus genus into a single species, these widely distributed dolphins exhibit a wide variety of size, shape and colour. Indeed over the past few decades over 20 distinct species in the genus have been proposed. Scientists in California in the 1960s concluded that there were two species - the long-beaked and short-beaked. This analysis was essentially confirmed by a more in-depth genetic study in the 1990s. This study also suggested that a third species (D. tropicalis, common name usually Arabian Common Dolphin), characterized by an extremely long and thin beak and found in the Red Sea and Indian Ocean, might be distinguished from the long-beaked species. The current standard taxonomic works recognize this as just a regional variety.

Behavior

Common dolphins travel in groups of around 10-50 in number and frequently gather into schools numbering 100 to 2000 individuals. These schools are generally very active - groups often surface, jump and splash together. Typical behaviour includes breaching, tail-slapping, chin-slapping, bow-riding and porpoising. Common dolphins are among the fastest swimming caetaceans, possibly reaching speeds of over 40 km/h.^[4]

The dolphins have been seen to mix with other cetaceans such as other dolphins in the Yellowfin tuna grounds of the eastern Pacific and also schools of Pilot Whales. An intriguing theory suggests that dolphins 'bow-riding' on very large whales was the origin of bow-riding on boats.

The gestation period is about 11 months and the calving period is between one and three years. Sexual maturation occurs at five years and longevity is twenty to twenty-five years. These figures are subject to large variation across different populations.

Conservation

Common dolphins face a mixture of threats due to human influence. Moderate levels of metal pollutants, which are thought to negatively impact dolphin health ^[5], have been measured in some populations ^[6]. Populations have been hunted off the coast of Peru for use as food and shark bait. In most other areas the dolphins have not been hunted directly. Several thousand individuals have been caught in industrial trawler nets throughout their range. Common dolphins were abundant in the western Mediterranean Sea until the 1960s but occurrences there have tailed off rapidly. The reasons are not well understood but are believed to be due to extensive human activity in the area. In the U.S. they are a protected species and sometimes are caught by accident in some trawler nets as bycatch, though despite this they are still quite common throughout their range. Despite these potential threats, the Short-beaked Common Dolphin is considered to be Least Concern by the IUCN Red List, and the Long-beaked Common Dolphin is listed as Data Deficient.^[1][2]

Captivity

Common dolphins are not common in captivity. But on at least three occasions, a beached common dolphin in California was nursed back to health at SeaWorld, San Diego but deemed unfit to release back to the ocean. These common dolphins remained at SeaWorld with the bottlenose dolphin exhibit. On one occasion a male common dolphin managed to impregnate the female bottlenose dolphins in his exhibit, leading to four hybrid births.^[7] One of the resulting common dolphin/bottlenose dolphin hybrids remained at SeaWorld, San Diego (alternately under the name Cindy or Bullet) while the other was kept at Discovery Cove. They also participate in shows with Bottlenose Dolphin and Pilot Whale at Sea World.

Other than at SeaWorld, at least 90 common dolphins are known to have been captured from the wild and kept in captivity. Captured common dolphins are said to be difficult to keep in captivity.^[8]

References

• Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp.
• National Audubon Society Guide to Marine Mammals of the World ISBN 0-375-41141-0
• Encyclopedia of Marine Mammals ISBN 0-12-551340-2
• Whales, Dolphins and Porpoises, Mark Carwardine, ISBN 0-7513-2781-

Also in New Zealand they had 4 common dolphins in Marine World the last of the Dolphins Kelly lived to be 39 and died in 2008

External links

• Whale and Dolphin Conservation Society
• An earthOCEAN documentary film about common dolphin decline in the eastern Mediterranean Sea