Common Bottlenose Dolphin ( Tursiops truncatus )

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Description

The bottlenose dolphin is the largest of the beaked dolphins with the males are much larger than the females. Adults range from 190-380cm in length and weigh 220-500kg (1), although this is varies extremely geographically. Body size also seems to vary inversely with water temperature in many parts of the world (1). Newborn calves are usually 98-126cm long and weigh 9-11kg (2). The pectoral fin is 30-50cm in length; the tall and falcate dorsal fin is approximately 23cm high and the tail fluke has a width of around 60cm (2).

The genus Tursiops is characterised by its characteristic short, well-defined snout; the snout is distinctly set off from the melon by a crease. The bottlenose dolphin has 20-28 sharp, robust teeth. In older individuals, these teeth will often be broken or worn down.

There are two species in the genus Tursiops, T. truncatus and T. aduncus, the Indian Ocean bottlenose dolphin. T. truncates appears to have two main variations; a robust form that lives mainly offshore, and a smaller, inshore form (3).

The dark colouration of the bottlenose dolphin varies from a light grey, to a near black, on both the back and sides, fading to white on the stomach. A sharp demarcation can be found between the melon and the short rostrum. A dark stripe also runs from the eye to the flipper.

Habitat

T. truncatus is found in warm temperate and tropical oceans and seas worldwide (2). Despite tending to be primarily found in coastal regions, they have also been found out in pelagic waters (6). Bottlenose dolphins have an extensive range of habitats; the inshore form is found frequenting river mouths, bays, lagoons and other shallow coastal regions (4), they will usually be found as far offshore as the continental shelf edge (2). The offshore form is apparently less restricted in range and movement, and can be found in many productive areas, particularly in the tropics. Some offshore populations are residents around oceanic islands (1).

Life Cycle

The breeding period is around March and April; the males will fight aggressively for the females and a hierarchy based on size is established within the group of males (2). The pair bonding will take place by a male showing a preference for a female, he will then remain with her for extended periods of time. The mating is swift, usually about 10 seconds, but will often be repeated.

Gestation takes approximately 12 months with a 2-3 year calving interval. Females become sexually mature at 5-12 years of age, while males are mature at 9-13 years (2). Lactation will continue for 12-18 months and the calf will remain close to the mother until it is around 4-5 years old.

Distribution

T. truncatus is distributed worldwide in coastal and inshore regions of tropical and temperate waters of the world (7), and population density seems to be higher near shore. There are also pelagic populations, such as those in the eastern tropical Pacific and around the Faroe Islands. Except for their occurrence around the United Kingdom and northern Europe, they generally do not range poleward of 45° in either hemisphere. The bottlenose dolphins occurring around the Faroe Islands (62°N 7°W) seems to be the most northerly of the North Atlantic offshore populations (7).

Migration

Most populations of T. truncatus do not migrate; instead, they travel widely to discover waters of a preferable temperature or in search of food (2). Wells and Scott (1999) (7) found that although little is known about the ranging patterns of pelagic bottlenose dolphins, the coastal dolphins exhibited a full spectrum of movements, including; seasonal migrations, year-round home ranges, periodic residency, and a combination of occasional long range movements and repeated local residency (1).

It has been thought that bottlenose dolphins living in extremely cold conditions or at high latitudes may display seasonal migrations or a seasonal residency pattern (spending winter in one location and summer in a different location). Any long-term residency may be shown by either; a relatively permanent home range, or by repeated sightings in a given area over many years (1).

Behaviour

T. truncatus is an extremely social species which usually travelling in groups of up to 12, although aggregations of several hundred have been spotted (2). Bottlenose dolphins commonly associate with many other cetaceans, including both large whales and other dolphin species (4). The bottlenose dolphin is an energetic creature which engages in various behaviours; breaching, lobtailing and bow-riding (5).

Each dolphin seems to have its own individual whistle to communicate a limited amount of information with. Nasal sacs in the forehead also produce “click” pulses which is used for echolocation (2).

Food

T. truncatus are opportunistic carnivores; they appear to make use of whichever suitable prey is most abundant. Their feeding behaviour is variable; ranging from cooperative foraging on schooling fish, to individually chasing fish onto mud banks, to feeding behind shrimp trawlers and other fishing operations (8).

The coastal and pelagic bottlenose dolphins also have differences in their feeding habits; coastal dolphins feed on fish and invertebrates in the littoral and sub-littoral zones, whereas the pelagic form feeds on mesopelagic fish and squids (1).

Conservation status
  • Listed as of least concern on IUCN Red List (4)
  • Appendix I of CITES
  • Annex II of the European Union’s Habitats Directive.
  • Appendix II of the Convention on Migratory Species (Bonn Convention)
  • Appendix II of the Bern Convention
  • Annex A of EU Council Regulation 338/97
References

Description written by Ben Harvey (2009)

(1) CMS, 2004. CMS: Tursiops truncatus, Bottlenosed Dolphin. [Online] Available at: http://www.cms.int/reports/small_cetaceans/data/t_truncatus/t_truncatus.htm [Accessed 2009 June 02].
(2) Ballenger, L. and T. Lindsley. 2003. “Tursiops truncatus” (On-line), Animal Diversity Web. Accessed June 26, 2009 at http://animaldiversity.ummz.umich.edu/site/accounts/information/Tursiops_truncatus.html.

(3) Cawardine, M. 1995. Whales, Dolphins and Porpoises. Dorling Kindersley, London.

(4) 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. Tursiops truncatus. In: IUCN 2009. IUCN Red List of Threatened Species. Version 2009.1. <www.iucnredlist.org>. Downloaded on 26 June 2009.

(5) MacDonald, D. 2001. The New Encyclopedia of Mammals. Oxford University Press, Oxford.)

(6) Wells RS, Scott MD. 2002. Bottlenose dolphins. In: Encyclopedia of marine mammals (Perrin WF, Würsig B, Thewissen JGM, eds.) Academic Press, San Diego, 122-127

(7) Wells RS, Scott MD. 1999 Bottlenose dolphin – Tursiops truncatus (Montagu, 1821) In: Handbook of Marine Mammals (Ridgway SH, Harrison SR Eds.) Vol. 6: The second book of dolphins and porpoises. pp. 137 – 182

(8) Jefferson, T.A., S. Leatherwood, and M.A. Webber. 1993. FAO species identification guide. Marine mammals of the world. Rome, FAO. 320p. 587 figs.