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Common dolphins
Size comparison against an average human
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Delphinidae
Genus: Delphinus
Linnaeus, 1758
Species

Delphinus capensis
Delphinus delphis
(Delphinus tropicalis)

The Common dolphin is the name given to two species (and possibly a third) 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. Some studies suggest that a third species, the Arabian Common Dolphin (D. tropicalis), can be characterized by an extremely long and thin beak and found in the Red Sea and Indian Ocean.

The common dolphin is not the dolphin of popular imagination despite its name; that distinction belongs to the Bottlenose Dolphin due to its widespread use in marine parks and its appearance in the television series Flipper.[3]

Contents

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 cetaceans, 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

  1. ^ a b Hammond, P.S., Bearzi, G., Bjørge, A., Forney, K., Karczmarski, L., Kasuya, T., Perrin, W.F., Scott, M.D., Wang, J.Y., Wells, R.S. & Wilson, B. (2008). Delphinus delphis. In: IUCN 2008. IUCN Red List of Threatened Species. Downloaded on 7 October 2008.
  2. ^ a b Hammond, P.S., Bearzi, G., Bjørge, A., Forney, K., Karczmarski, L., Kasuya, T., Perrin, W.F., Scott, M.D., Wang, J.Y., Wells, R.S. & Wilson, B. (2008). Delphinus capensis. In: IUCN 2008. IUCN Red List of Threatened Species. Downloaded on 7 October 2008.
  3. ^ "American Cetacean Society - Bottlenose Dolphin". http://www.acsonline.org/factpack/btlnose.htm. Retrieved 2008-08-31. 
  4. ^ "The Common Dolphin". http://library.thinkquest.org/17963/genus-Delphinus.html. Retrieved 2008-07-03. 
  5. ^ Lavery, T. J., Kemper, C., Sanderson, K., Schultz, C. G., Coyle, P., Mitchell, J. G., Seuront, L. (2008). "Heavy metal toxicity of kidney and bone tissues in South Australian bottlenose dolphins (Tursiops aduncus)". Marine Environmental Research 67: 1. doi:10.1016/j.marenvres.2008.09.005
  6. ^ Lavery, T.J., Butterfield, N., Kemper, C.M., Reid, R.J., and Sanderson, K. 2008. Metals and selenium in the liver and bone of three dolphin species from South Australia, 1988 - 2004. Science of the Total Environment, 390: 77 - 85.
  7. ^ Zornetzer H.R.; Duffield D.A. (October 1, 2003). "Captive-born bottlenose dolphin × common dolphin (Tursiops truncatus × Delphinus capensis) intergeneric hybrids". Canadian Journal of Zoology (NRC Research Press) 81 (10): 1755–1762. doi:10.1139/z03-150. http://www.ingentaconnect.com/content/nrc/cjz/2003/00000081/00000010/art00014?crawler=true. 
  8. ^ "The Common Dolphin". http://library.thinkquest.org/17963/genus-Delphinus.html. Retrieved 2008-07-03. 
  • 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-

External links

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