Delphinus Delphis

– Short-Beaked Common Dolphin –

The short-beaked common dolphin (Delphinus delphis) is a species of common dolphin. It has a larger range than the long-beaked common dolphin (D. capensis), occurring throughout warm-temperate and tropical oceans, including the Indian Ocean although in smaller numbers than other places they are found.^[4] There are more short-beaked common dolphins than any other dolphin species in the warm-temperate portions of the Atlantic and Pacific Oceans.^[5] It is also found in the Caribbean and Mediterranean Seas. The short-beaked common dolphin is also abundant in the Black Sea, Gulf of Mexico, and Red Sea. They follow the gulf stream up to Norwegian waters. Seldom do any short-beaked dolphin venture near the Arctic.^[4]

Description

The short-beaked common dolphin is a medium-sized dolphin, smaller than the more widely known bottlenose dolphin. Males are generally longer and heavier,^[6] with a sexual size dimorphism of 1.06. Mature males also have prominent postanal humps. 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.^[7] It has a long, thin rostrum with 50–60 small, sharp, interlocking teeth on each side of each jaw.^[8] Juvenile dolphins have a more muted appearance and become more distinguishable when they mature.^[9]

The common dolphin reaches a maximum size of 2.50 m for males and 2.30 m for females . At birth it measures 70 to 80 cm. Its weight is between 70 and 110 kg. The bill (or rostrum) is fine and is clearly demarcated from the melon (head) by a frontal groove. The dorsal is sickle-shaped and measures a third of the body to the right of it. The caudal is brace-shaped with a notch in the hemispherical axis. The coloring of the back varies from dark gray to dark brown passing through black ; it forms a V under the backbone. The coloring is lighter towards the tail. The belly from the head to the anal region is creamy white and then light gray. On the flanks two lines intersect to form an X.

Taxonomy

Early illustration from Transactions of the Zoological Society of London (1944)

The short-beaked common dolphin is a member of the common dolphin genus, Delphinus, within the dolphin family, Delphinidae. Until the mid-1990s, the different forms within Delphinus were not recognized as separate species, but were all considered members of the species D. delphis.^[5][6] Currently, there are two recognized species of Delphinus: the short-beaked common dolphin and the long-beaked common dolphin (D. capensis).^[10] The short-beaked common dolphin is generally smaller than the long-beaked common dolphin and has a shorter rostrum. Differentiation among the short-beaked variety can be attributed to ocean temperature, chlorophyll concentration, and ocean turbidity, rather than zoogeographical distribution.

Habitat

Biotope

Rather offshore and used to pelagic waters, it is an opportunistic animal that does not hesitate to approach the coast to feed. Although it is able to dive to considerable depths, it is often content with shallow immersion.

Distribuition

Short-beaked common dolphins are commonly found in warm tropical waters, they are known to live in areas of the Atlantic, Pacific, and Southeast Indian Ocean. They are believed to have originated in the eastern Indo-Pacific Ocean during the Pleistocene and expanded into the Atlantic Ocean through the Indian Ocean due to climate oscillations.^[15] Short-beaked common dolphins have been shown to reside in the Gulf of Corinth of the Ionian Sea. They were found in groups of striped dolphin; their numbers were very small and believed to be unsuitable for survival. Traces of hybridization were found on Striped dolphins, leading further credence to their inability to survive in such groups.^[16] The number of short-beaked common dolphins has declined elsewhere in Western Greece, specifically the island of Kalamos. Overfishing has been studied to have reduced the number of available prey for dolphins near the island between 1996 and 2007.^[17]

Behavior

Short-beaked common dolphins can live in aggregations of hundreds or even thousands of dolphins.^[5] They sometimes associate with other dolphin species, such as pilot whales.^[5] They have also been observed bow riding on baleen whales, and they also bow ride on boats.^[5] It is a fast swimmer (over 60 kilometres per hour (37 mph)), and breaching behavior and aerial acrobatics are common with this species.^[6] They are very social animals and engage in most necessary activities such as traveling, eating, and breathing together.^[4] They usually associate in groups that can consist of 1000s to 100,000s of individuals.^[11] When some individuals are sick, others in the group assist them by using their fins to help keep them afloat so they survive.^[4]

Biology

Alimentation

Common dolphins can descend as far as 300 m to feed. The diet is mainly made up of small fish living in schools such as herring, pilchard, anchovies, hake, sardines, bonito, and sauries, as well as squid and octopus. The hunting technique consists in bringing up the fish by releasing bubbles or by pulling back. It is of course a collective tactic: some feed, others cut down the food and the last protect the group. To feed, they will be opportunistic and migratory.

The diet of short-beaked common dolphins near the English Channel consists mostly of lanternfish, Atlantic mackerel, sprats, and squids. In the Black Sea, their diet contains sprats, anchovies, and pipefish. In the Mid-Atlantic region near the Northeastern United States, their diet contains Atlantic Mackerel.^[12] They typically eat anywhere from 18 to 20 pounds a day.^[4]

In order to study the long-term diet of short-beak common dolphins, their intake of Cadmium (Cd) was observed in 2004. In oceanic regions short-beaked dolphins diet consist largely of Cephalopods (such as Cranchids and Histioteuthids). In coastal areas, their diet contained up to 10 times less cephalopods.^[13]

The dentition of the common dolphin is formed from 160 to 240 teeth per jaw which are not used for chewing but rather to seize the preys. It is the walls of the stomach that crush the bolus of food.

Reproduction

Females tend to reach sexual maturity when they have grown to 85% of their final size. It is at this point that the distribution of resources in their bodies becomes more focused on their sexual organs, rather than their physical size. This is usually when they are approximately 12 to 15 years of age. Like many other animals, female short-beaked common dolphins are considered sexually mature when they have ovulated at least once.^[14]

Similar to humans, the short-beaked common dolphin is viviparous. The breeding season is variable depending on the water temperature: usually it takes place in spring. It is practically impossible to determine whether they are couples with permanent bonds. Mating takes place with delicacy, it seems that a real love is manifested between the male and the female

Though they have the ability to carry twins, triplets, etc., they usually only carry one baby at a time.^[4] The Short-beaked common dolphin has a gestation period of 10 to 11 months.^[5] The newborn calf has a length of between 70 and 100 centimetres (2.3 and 3.3 ft) and weighs about 10 kilograms (22 lb).^[6] 

Births take place near the surface, the baby dolphin comes out by the tail, the female turns it over, breaks the umbilical cord and helps the baby reach the surface to take its first breath. 

Breastfeeding will last from 15 to 18 months, it will be alternated with a normal diet of this species. At birth, the baby will have two “aunts”, females in general, who will be there to protect it from males and sharks. 

After a baby is born, it immediately becomes part of the family and takes part in daily life. Staying close to its mother, the calf never strays more than a few feet, especially when it is very young. Until the calf is six months old, it obtains its nourishment solely from its mother’s milk. Unlike humans, short-beaked common dolphins have no lips and cannot suck on the teats of its mother. If they did have this ability they would drown. Thus, the female dolphin’s mammary glands have a muscle that when contracted, squirts milk so that it can feed its calf.^[4]

For the Black Sea population, weaning occurs at between 5 and 6 months, but occurs later (up to about 19 months) in other areas.^[5][6] Typical interbirth interval ranges from 1 year for the Black Sea population to 3 years for eastern Pacific Ocean populations.^[5] Age of sexual maturity also varies by location, but can range between 2 and 7 years for females and 3 and 12 years for males.^[5][6] Common short-beaked dolphins have been observed to engage in oral sex, intimating that they – like chimpanzees and humans – engage in sexual activities for pleasure.^[4]

Maximum lifespan is 35 years, but has been estimated at 22 years for the Black Sea population.^[5][6]

Associated Life

During the hunting phase, D elphinus delphis sometimes meets sharks and its main marine predator: the killer whale Orcinus orca . The common dolphin is sometimes parasitized at the ends of the pectorals, its caudal and its dorsal by copepods. It is not uncommon to find in stranded dolphins that internal parasite is the cause of death.

Various Biology

With each breath, the common dolphin renews 90% of the air contained in its lungs while most mammals are only 15%. Yet his breath remains short and barely visible.

Like all mammals, it is homeothermic *, its body temperature is between 35 and 36 ° C.

His vision is as good underwater as on water.

The clicking sounds are emitted on frequencies from 0.2 to 150 khz while the whistle is between 4 and 16 khz, these sounds are emitted by its vent and / or its echolocation system. Its top speed is around 40 km / h. Common dolphins are notorious for riding the bow of ships for several hours.

Its palate has two deep grooves, which makes it possible to unequivocally determine the skull of the common dolphin.

Its longevity is 25 to 30 years.

Similar Species

Delphinus capensis Gray, 1828: the Cape dolphin found mainly in the South Atlantic and the Indian Ocean. It has a short, well-marked beak, a height of 1.80 to 2.60 m and a weight of 75 to 115 kg.

Stenella attenuata (Gray, 1846): the pantropical spotted dolphin has a dress with three shades of gray very demarcated and uniformly spotted. Mainly present in the waters of Réunion.

Stenella clymene (Gray, 1846): the clymene dolphin with three shades of gray.

Stenella longirostris (Gray 1826): the long-billed dolphin has a dark gray back, a very light gray belly and gray sides. The head has a very thin long rostrum * and a tapering melon. The dorsal fin is triangular and of medium size.

Stenella coeruleoalba ( Meyen 1833): the white blue dolphin: It differs from the common dolphin by the absence of an X-shaped or hourglass design. A dark colored stripe runs along the sides.

Conservation

The Mediterranean population of the short-beaked common dolphin Delphinus delphis is listed on Appendix I,^[18] while the North Sea, Baltic Sea, Mediterranean, Black Sea and eastern tropical Pacific populations are listed on Appendix II^[18] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). The regional listing on Appendix I ^[18] means that this population has been categorized as being in danger of extinction and CMS Parties strive towards strictly protecting these animals, conserving or restoring the places where they live, mitigating obstacles to migration and controlling other factors that might endanger them. It was listed on Appendix II^[18] as it has an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.

In addition, the species is also covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS)^[19] and the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS).^[20] The species is further included in the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia^[21][22] and the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MoU).^[23]

Mass Stranding Events

On June 8, 2009, a mass-stranding event (MSE) occurred in Falmouth Bay, Cornwall, United Kingdom. It is believed the MSE was likely caused by naval involvement, as all other factors which cause MSEs in cetaceans currently appear unlikely to have influenced the event. During the event, twenty-six common dolphins washed ashore, and about the same amount floated back out to sea. There were three other MSEs in the UK before this event, all of unknown cause, from the years 1915 to 1938, but with arguably lower counts of stranded dolphins.^[24]

Bycatch

There have been few studies specific to bycatch in relation to the short-beaked common dolphin, however certain models indicate that its effect on the reproduction of the species is greater than expected, and as compared with other species of dolphin.^[25] About 1000 short-beaked common dolphins are bycaught in the North Atlantic each year by either tuna drift, trawling and gillnetting. The regulation is that only cetacean bycatch can not be longer than 15 meters ^[26] and this can lead to be a problem because short-beaked common dolphins are only about 2.7 meters.

Further Information

The common dolphin community is very united; this ranges from caring for newborns, caring for injured companions to problems with infirm and elderly limbs. It is even reported that drowning humans were pushed to shore, supported by dolphins.

It is common to see common dolphins frolic with floating toys, and even animals like turtles.

Offshore species are found in herds of hundreds or even thousands of individuals (500 on average).

The ancestor of the dolphin 50 million years ago was an ungulate animal the size of a wolf. Some dolphins have preserved the remains of ancestral hind legs. Embedded in the flesh, they can only be observed with the dissection of the cetacean.