Common dolphin

The common dolphin is the name given to two species of dolphin making up the genus Delphinus. Taxonomists and cetologists usually recognise two species — the short-beaked common dolphin, which retains the systematic name Delphinus delphis, and the long-beaked common dolphin D. capensis. Despite its name, the common dolphin is not the dolphin of popular imagination; that distinction belongs to the bottlenose dolphin (genus Tursiops) due to its widespread use in marine parks and its appearance in the television series Flipper. However, this dolphin was the most frequently represented in the art and literature of ancient Greece and Rome. Both common dolphin species are medium-sized; adults range between 1.9 and 2.5 m (6.2 and 8.2 ft) long, and can weigh between 80–235 kg (176–518 lb), although the range between 80–150 kg (180–330 lb) is more common. Males are generally longer and heavier. The color pattern on the body is unusual. The back is dark and the belly is white, while on each side is an hourglass pattern colored light grey, yellow, or gold in front and dirty grey in back. They have long, thin rostra with up to 50–60 small, sharp, interlocking teeth on each side of each jaw. Despite the historic practice of lumping the entire genus Delphinus into a single species, these widely distributed dolphins exhibit a wide variety of sizes, shapes and colors. Indeed, over the past few decades, over 20 distinct species in the genus have been proposed. Scientists in California in the 1960s concluded 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 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. D. tropicalis is a subspecies of D. capensis. Common dolphins live in both warm-temperate and tropical waters ranging from 40–60°N to 50°S. Long-beaked common dolphins mostly inhabit shallow, warm coastal water. Short-beaked common dolphins are common 'along shelf edges and in areas with sharp bottom relief such as seamounts and escarpments'. Common dolphin have a varied diet consisting of many species of fish and squid. This includes both mesopelagic species and epipelagic schooling species. They have been recorded to make dives up to 200 metres (660 ft) deep. Common dolphins can live in aggregations of hundreds or even thousands of dolphins. They sometimes associate with other dolphin species, such as pilot whales. They have also been observed bow riding on baleen whales, and they also bow ride on boats. They are fast swimmers (up to 60 km/h), and breaching behavior and aerial acrobatics are common with this species. They are also known to display altruistic behaviors to support injured members. The short-beaked common dolphin has a gestation period of 10 to 11 months. The newborn calf has a length of 70 to 100 centimetres (2.3 to 3.3 ft) and weighs about 10 kilograms (22 lb). For the Black Sea population, weaning occurs at between five and six months, but occurs later (up to about 19 months) in other areas. Typical interbirth interval ranges from one year for the Black Sea population to three years for eastern Pacific Ocean populations. Age of sexual maturity also varies by location, but can range between two and seven years for females and three and 12 years for males. No evidence exists of any major reproductive differences between the two species. In captivity, the long-beaked common dolphin has hybridized with the common bottlenose dolphin (Tursiops truncatus). One of the hybrids has been bred back to a bottlenose dolphin, demonstrating such hybrids are fertile. Common dolphins face a mixture of threats due to human influence. Moderate levels of metal pollutants, which are thought to negatively impact dolphin health, have been measured in some populations. 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 US, 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.