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.
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 caetaceans, possibly reaching speeds of over 40 km/h.
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.

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