Common dolphin
The common dolphin is the most abundant cetacean in the world, with an estimated global population of about six million. It is currently the only member of the genus Delphinus, the type genus of the subfamily Delphininae. This places it as a close relative of the bottlenose dolphin, humpback dolphin, striped dolphin, spinner dolphin, Clymene dolphin, spotted dolphin, Fraser's dolphin, the tucuxi, and the Guiana dolphin.
The common dolphin was once divided into two different species, the short-beaked common dolphin and the long-beaked common dolphin. These are now generally regarded as ecotypes. Recent research shows that many long-beaked populations worldwide are not closely related to one another, often originating from short-beaked ancestors, and they do not consistently share the same derived traits. For this reason, these forms are no longer classified as separate species.
Despite its name, the common dolphin is not widely considered the "archetypal" dolphin; that role is more often associated with the bottlenose dolphin, largely because of its frequent appearances in aquaria and the media. Nevertheless, the common dolphin held cultural significance in antiquity and frequently appeared in Greek and Roman art, such as in murals created by the Minoan civilization.
Physical characteristics
Common dolphin are medium-sized dolphins; adults range between long, and can weigh between, although the range between is more common. Males are generally longer and heavier. The dolphin's coloration is unusual, with a dark back and white belly, 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.Taxonomy
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. The long-beaked common dolphin was thought to have a disjointed range in coastal areas in tropical and warmer temperate oceans; this range included parts of western and southern Africa, much of western South America, central California to central Mexico, coastal Peru, the seas around Japan, Korea and Taiwan, and possibly near Oman. Vagrants have been recorded as far north as Vancouver Island.This analysis was seemingly confirmed by a more in-depth genetic study in the 1990s. This study also suggested a third species, 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.
Recent evidence has demonstrated that different populations of long-beaked common dolphins around the world are not closely related to one another and are often descendant from a short-beaked ancestor. Therefore, all long-beaked and short-beaked common dolphin populations are currently listed as the same species; they are all subspecies of Delphinus delphis. Currently, the common dolphin is divided into four subspecies:
- D. d. delphis, the nominate subspecies
- D. d. bairdii, the Eastern North Pacific long-beaked common dolphin
- D. d. ponticus, the Black Sea common dolphin
- D. d. tropicalis, the Arabian or Indo-Pacific common dolphin
Fossil record
Biology
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". In general, they are a widely distributed species. They can be found around the world in both offshore and coastal waters. In the Northwest Atlantic, they can be found from Cape Hatteras to Newfoundland and are strongly associated with the Gulf Stream. Dolphins in this region can often be found in areas that have certain geological factors like underwater canyons and ridges because this is where up-welling occurs which result in greater nutrients. In the Eastern Pacific, common dolphins are found along the coast of California and are associated with the California current. Short-beaked common dolphins can also be found in Europe, particularly the Scotian Shelf, the Black Sea and the Mediterranean. In the Southern Hemisphere, they are abundant in the southwestern Pacific, around New Zealand and southern Australia. They are generally a pelagic species that are often found in waters deep, with the short-beaked type preferring deeper waters than the long-beaked type Temperature also plays a large role in the congregation of dolphins. For example, in the western North Atlantic, almost all sightings of common dolphins took place in waters from. However, common dolphins were sighted in waters as cold as.Sociality
Common dolphins can live in aggregations or "pods" of hundreds or even thousands of dolphins. Common dolphins are often seen in groups numbering several hundred individuals. Occasionally, different groups will come together to form mega-pods which can consist of over 10,000 dolphins. Genetic studies in the Northeast Atlantic suggest that common dolphin pods generally do not consist of close kin, but rather of members that are not closely related. Unlike many delphinids, common dolphins do not live in a matriarchal society. That being said, closely related individuals are usually found in similar geographical locations fairly consistently, providing evidence that this species displays site fidelity. Male common dolphins display greater site fidelity in relation to their kin than females. Common dolphin pod structure often consists of nursery pods, bachelor pods and mixed groups of males and females, including sub-adults and calves. Genetic evidence seems to indicate that common dolphins live in fission-fusion societies, where dolphins form pods that are not necessarily stable and do not necessarily consist of related individuals. It is not known if common dolphins form lifelong bonds with other individuals like the long-term male alliances seen in bottlenose dolphins.There is some evidence that common dolphins use signature whistles, similar to that of the bottlenose dolphin. These whistles are believed to serve as an acoustic label, and provide identification information similar to that of a name. It takes approximately 1 year for a calf to learn its signature whistle after which it remains stable for the rest of a dolphin's life. Off South Africa, as many as 29 common dolphin signature whistle types were detected. However, it was difficult to determine if each dolphin had its own signature whistle due to the vast number of dolphins present and anthropogenic background noise. Additionally, considering the vast number of dolphins present and taking into account their feeding and diving behavior, it appears that common dolphin signature whistles are also used for group cohesion. Another hypothesis for the function of signature whistles is that they serve as a beacon for lost individuals.
Interspecific relations
Common dolphins sometimes associate with other dolphin species, such as pilot whales. In the Gulf of Corinth, common dolphins frequently display mixed species association, especially with striped and Risso's dolphins. Over one third of all dolphin sightings in this Gulf consisted of mixed species associations that partially consisted of common dolphins. In mixed species associations, the ratio of striped to common dolphins ranged from 6:1 to 11:1. When Risso's dolphins were present, it appeared that many of their scars were the result of interactions with striped and spinner dolphins. In many of the interactions, the Risso's dolphins would chase and herd the common dolphins toward the boat, while the common dolphins would try and swim under the Risso's dolphins. When groups of common and striped dolphins would charge at each other, the Risso's dolphin would chase the striped dolphins. Sometimes these interactions appeared to be playful, and at other times aggressive. Synchronized swimming and surfacing were commonly observed. These interactions take place in the deepest part of the Gulf, furthest from shore, and usually consist of a total of about 60 dolphins from all three species.There have been confirmed cases of hybridization between striped and common dolphins in this region, with 15 confirmed cases of such hybrids. Genetic and observational evidence has demonstrated that the hybrids are fertile and are capable of not only reproducing with other hybrids, but are capable of reproducing with each of the parent species. Striped dolphins have been known to mate with other dolphins, as the Clymene dolphin is the result of hybrid speciation between striped and spinner dolphins. However, this is unlikely to happen with common dolphins, as their population in the Gulf of Corinth is too low. There is one confirmed case of a wild hybrid between a bottlenose and common dolphin in southern Spain, an important feeding ground for both species. The mother was a female bottlenose dolphin who had spent 10 years in a common dolphin pod. Billie was observed assisting common calves reach the surface three different times and would babysit the calves after the mother went through labor. The length of the calf was similar to that of a bottlenose dolphin calf, with the lateral stripes and coloration of a common dolphin. The calf was seen with its mother almost daily on dolphin-watching tours among a nursery pod of common dolphins which also contained some immature striped dolphins. Sightings of the calf took place when temperatures were between. The calf was mainly observed swimming alongside the mother and was seen rubbing its head on its mother, jumping backwards over its mother and engaging in flipper-to flipper, belly-to flipper and belly-to belly contact. Common dolphins and bottlenose dolphins are also known to interbreed in captivity. One of these hybrids has been bred back to a bottlenose dolphin, demonstrating such hybrids are fertile.
Common dolphins have been observed bow riding on both baleen whales and boats. They are fast swimmers, and breaching behavior and aerial acrobatics are common with this species. They are known to display altruistic behaviors to support injured members.
Common dolphins 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 deep.