Common dolphin (Delphinus delphis Linnaeus, 1758)

Order: Cetacea
Suborder: Odontoceti. Family: Delphinidae

Identification: The most striking feature of the common dolphin is its characteristic hourglass pattern on the flanks; created by overlapping of the contrasting dorsal and ventral patches on the sides creating a V-shaped saddle below the dorsal fin. This overlapping results in a greyish green or bronze tan colouration of the thoracic patch on each side of the animal. The body is fine and streamlined, with a long narrow rostrum and sharp angle between the beak and forehead. The dorsal fin is tall and falcate, the fins taper to a pointed tip, and the tail flukes are concave with a distinctive median notch on the rear margin.

The long, well-defined beak is generally black, but often tipped with white. The dorsal fin ranges in colour from all black to mostly white with a dark border; there is often a greyish area in the middle of an otherwise dark fin. The flippers and flukes are dark grey or black, but in the eastern North Atlantic, the flippers are commonly pale - light grey to white. There is sexual dimorphism in the colouration of the areas around the genitals: males have a distinctive black blaze above the genital opening, and females have a narrower band of black with grey counter-shading.

More than 20 species of Delphinus delphis have been proposed because of the great variation in appearance. However, two species are currently recognised the short-beaked common dolphin (Delphinus delphis), found in British waters, and the long-beaked form (Delphinus capensis).

Common dolphins are often active at the surface; jumping and splashing can be often seen and heard at great distances. They frequently breach (and may even somersault), and slap the water surface with their chins and flippers. Several members of a group will often surface together. They are fast swimmers and commonly bow-ride with boats and large whales such as fin whales, gray whales, humpbacks and blue whales.

Distribution & Habitat: The common dolphin may be one of the most widely distributed species of cetacean, since it is found world-wide in temperate, tropical and sub-tropical seas. These are pelagic, offshore dolphins that are most likely to be encountered along, or seaward of, the continental shelf, although they do occur close to shore in some areas. Distribution has been reported to correlate with water temperature and bottom topography (such as prominent underwater topography including sea mounts and escarpments and even the mid-Atlantic ridge). They apparently prefer water temperatures ranging from 10 to 28 degrees centigrade.

The long-beaked form is mainly found in the Gulf of California or western Baja California and there is possibly a population of this type in the eastern South Pacific. Other studies have documented them from Japan, Korea and China. There are also local forms recognised in the Mediterranean, Black Sea, Indian Ocean and both European and African Atlantic coasts.

Well-defined migrations are not known, and common dolphins are present year round in some parts of their range. In some locations, seasonal shifts in distribution are observed, for example, off Southern California where peaks of abundance are observed in June, September to October, and January. Seasonal shifts also occur off the British Isles, and it is thought that the movements may be correlated with shifting prey distribution and abundance.

Natural History & Ecology: Common dolphins may range up to 2.6 m in length, and over 130 kg in weight, although most adults are less than 2.3 m long and weigh around 80 kg. Males tend to be slightly larger than females, and mean body length may vary geographically. Lengths at sexual maturity vary geographically, but sexual maturity is generally reached at 150 to 180 cm in length and an age of 3 years, while females were 150 to 200 cm and between 5 to 7 years of age. This corresponds to at least 6 or 7dentinal growth layers.

Around the UK, juvenile common dolphins are first observed in June, suggesting that breeding takes place in early summer. Gestation lasts approximately 10 months and lactation lasts for approximately 6, but solid food is taken anywhere from 2 to 3 months of age.

Many species of small shoaling fish, several neritic species and occasionally oceanic cephalopods have all been identified as prey items. The wide distribution, abundance and lethargic behaviour of deep scattering organisms make many species ideal prey for these pelagic delphinids.

Off SW England mackerel, sprats and squid known prey items, and off southern Scotland, Norway pout, cod, herring, sprat, whiting, mackerel and squid have been found in the stomachs of common dolphins.

Studies using radio-telemetry and analysis of stomach contents suggest that common dolphins off Southern California start feeding at dusk and feed throughout the night, primarily on organisms in the migrating deep scattering layer, regularly diving to depths of 50 m. Off the British Isles, common dolphin movements may be related to prey migrations such as the inshore movements of mackerel in winter, spawning herring off northern Scotland in August, and sprat, sand eel and mackerel movements off Ireland and England in the summer. Common dolphins have been observed to exhibit frenzied activity when feeding, and it is possible that individuals co-operate to panic fish in order to catch them more easily.

Social behaviour: Common dolphins occur in herds or schools of 100's to 1000's of individuals, but the basic pod size may be less than 30. In British waters, most schools are comprised of less than ten individuals, and animals are mainly observed in singles or pairs; although large schools of more than 100 dolphins have been seen on occasion. The school size generally increases from July to September and again in December and February, possibly associated with inshore movements of prey.

Young adult and/or sub-adult females may segregate from their herd proper for periods of time.

Altruistic and epimeletic behaviour (such as females assisting others with birth, and baby-sitting behaviour) have been observed in common dolphins, both in captivity and in the wild.