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Common Dolphin

Scientific Name: Delphinus delphis Linnaeus

Common Names: Known in Australia as the Short-beaked Common Dolphin, but by many others names elsewhere.

Protected status: Since 1980, all whales and dolphins have been protected in Australian waters.

Appearance:
Common dolphins have a fine, streamlined body, with a long narrow “beak” and a sharp angle between the beak and the melon (forehead). The dorsal fin is tall and roughly triangular in shape, with a pointed tip. The flippers taper to a pointed tip and the tail flukes are concave with a distinctive median notch on the rear margin. There are 37 to 48 small, sharply pointed teeth in each row.

The colour pattern of the common dolphin is quite elaborate as far as cetaceans go. The back is dark grey to black from the top of the head to the tail dipping to a V on the sides below the dorsal fin. The flanks are light grey behind the dorsal fin and yellowish-tan forward of the dorsal fin, forming an hourglass pattern. The belly is white. The long, well-defined beak is generally black. The dorsal fin ranges in colour from all black to mostly pale grey with a dark border and often has a white patch. The flippers, often with a paler centre, and flukes are dark grey or black above. There are large dark circles around the eyes connected by a dark line that runs across the head behind the beak and a dark stripe runs from the jaw to the flippers. The colour pattern of juveniles is less distinct than that of adults but is nevertheless recognisably the common dolphin pattern

Identifying common dolphins in southern Australia:
Common dolphins are easily identified by their distinctive brown/yellow hourglass pattern along the side of the body, their very dark grey (almost black) colour on the upper parts of the body and by the dark stripe between the beak and the flipper. Several other species, the most common being the bottlenose dolphins, also occur in southern Australia. Bottlenose dolphins are dark grey above and paler below, and lack the obvious patterns described above.

Size:
Common dolphins in southern Australia may range up to 2.3 m in length and about 115 kg, although maximum size for individual adults can be a small as 1.8 m long and about 70 kg. Males tend to be slightly larger than females but this has not been tested statistically.

Lifespan:
The life span of the common dolphin in Australia is not known. However, elsewhere in the world they are known to live to at least 25 years.

Distribution:
The common dolphin maybe one of the most widely distributed species of cetacean. It is found world-wide in temperate, tropical and sub-tropical seas from 40^oN to 45^oS in the coastal waters of the Pacific, and north of 50^o in the Atlantic Ocean. Previously, common dolphins were thought to be entirely pelagic (of the open ocean) but recent information in southern Australia shows that they also occur in shallow, inshore waters. Studies on common dolphins outside Australia suggest that their distribution correlates with water temperature (10 to 28^°C) and bottom topography such as seamounts, escarpments and undersea ridges.

Food:
From studies of a limited number of stomach contents of common dolphins from South Australia, fish and cephalopods appear to be the main food sources, with fish being predominant. Carangids and clupeids are the most often represented fish families. The main cephalopod is the southern calamary (Sepioteuthis australis).

Common dolphins from elsewhere feed from the surface to at least 280 m depth, in the inshore and offshore environments. They sometimes take advantage of human fishing operations to get prey. There is some evidence of competitive interactions for food between spotted and spinner dolphins and tuna. Common dolphins have been observed in frenzied activity when feeding and it is possible that individuals co-operate to frighten fish in order to catch them. On one occasion a common dolphin was observed to throw a fish into the air four times before eating it.

Reproduction:
Almost nothing is known of the reproduction of common dolphins in Australia. In South Australia they become sexually mature at 1.7 to 2.0 m body length. In other parts of the world, mature common dolphins seem to go through seasonal cycles of sexual activity and these differ in duration and timing between populations. In the temperate eastern Pacific, calving occurs in both spring and autumn. Around the UK, juvenile common dolphins are first observed in June, suggesting that breeding takes place in early summer. In the Black Sea, calves are born mainly in summer and autumn, with births peaking in July. In Australian waters, the locations and timing of calving are not yet known.

Based on studies elsewhere, pregnancy lasts for about 9 to 11 months and the young are believed to suckle for about 6 months. Calves eat solid food from 2 to 3 months. Mature females have a calf every 1 to 3 years.

Behaviour:
Common dolphins are very gregarious. Some aggregations observed in Australian waters number thousands, or even 100 000 individuals. They are very acrobatic and ride the bow waves of boats and large whales. They have been seen with other species of dolphin, including bottlenose, as well as larger cetaceans (fin, humpback, blue, southern right whales). Common dolphins are believed to travel great distances although no studies have been conducted in Australian waters to verify this.

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

Sounds
Common dolphins use echolocation as well as audible sounds to communicate, navigate and hunt prey, much like other members of the dolphin family.

Population size:
Population estimates for the northern hemisphere indicate declines in some areas, particularly the eastern tropical Pacific where purse-seine netting results in many incidental deaths. No attempt has been made to estimate numbers in Australian waters but they are considered common based on the number of strandings and sightings.