Delphinus delphis

Delphinus delphis

The common dolphin (Delphinus delphis Linnaeus, 1758) is a marine mammal belonging to the Delphinidae family.

Systematics –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Subkingdom Eumetazoa,
Superphylum Deuterostomia,
Phylum Chordata,
Subphylum Vertebrata,
Infraphylum Gnathostomata,
Superclass Tetrapoda,
Mammalia class,
Subclass Theria,
Infraclass Placentalia,
Superorder Laurasiatheria,
Order Artiodactyla,
Suborder Odontoceti,
family Delphinidae,
Genus Delphinus,
D. delphis species.
The terms are synonyms:
– Delphinus albimanus Peale, 1848;
– Delphinus algeriensis Loche, 1860;
– Delphinus bairdi Norris & Prescott, 1961;
– Delphinus capensis Gray, 1828;
– Delphinus capensis subsp. capensis Gray, 1828;
– Delphinus delphus Linnaeus, 1758;
– Delphinus fluvofasciatus Wagner, 1853;
– Delphinus forsteri Gray, 1846;
– Delphinus frithii Blyth, 1859;
– Delphinus fulvofasciatus True, 1889;
– Delphinus loriger Wiegmann, 1846;
– Delphinus major Gray, 1866;
– Delphinus marginatus Lafont, 1868;
– Delphinus microps Burmeister, 1866;
– Delphinus moorei Gray, 1866;
– Delphinus novaezealandiae Gray, 1850;
– Delphinus novaezeelandiae Wagner, 1846;
– Delphinus novaezelandiae Quoy & Gaimard, 1830;
– Delphinus pliocaenicus (Gervais, 1859);
– Delphinus pliocaenus (Gervais, 1859);
– Delphinus ponticus Barabash, 1935;
– Delphinus sao Gray, 1850;
– Delphinus zelandae Gray, 1853;
– Eudelphinus delphis (Linnaeus, 1758);
– Eudelphinus pliocaenus (Gervais, 1859);
– Lagenorhynchus decastelnau Van Beneden, 1873.
Within this species the following subspecies are recognised:
– Delphinus delphis subsp. bairdii Dall, 1873;
– Delphinus delphis subsp. bairdii Tomilin, 1957;
– Delphinus delphis subsp. capensis Gray, 1828;
– Delphinus delphis subsp. curvirostris Riggio, 1883;
– Delphinus delphis subsp. delphis Linnaeus, 1758;
– Delphinus delphis subsp. ponticus Barabash, 1935;
– Delphinus delphis subsp. tropicalis van Bree, 1971.

Geographic Distribution and Habitat –
The Delphinus delphis, thanks to its numerous subspecies, has adapted to live in the temperate and tropical waters of the whole world. This species is found more frequently in the Atlantic Ocean, in the Red Sea and in the Black Sea, while in the Mediterranean Sea, where it was very abundant at least until the Second World War (based on what can be observed from the numerous exhibits scattered throughout the museums of all the Mediterranean countries), the species seems to have undergone a fairly drastic decline and is found with relative frequency only in the Tyrrhenian, Ionian and Aegean seas and near the Strait of Gibraltar.
It is a pelagic species that can be found both near the coast and offshore (even in large groups of 30-50 individuals) and its preferential habitat is that of deep waters and with a temperature above 10 °C; the optimal temperature range is between 10 and 28 °C, below which the species tends to make migratory movements: during the warm months, it also goes into the subpolar areas. Its environment of choice is the continental shelf, where it tends to remain almost always near the coasts.

Description –
The Delphinus delphis is a mammal 170 to 240 cm long and weighing 100-135 kg; this species is among the smallest dolphins.
It is distinguished by the presence of an hourglass-shaped patch on the sides of the body that is light gray towards the tail and creamy-white towards the head.
The back is gray and the belly tends instead to white, to blend in better both against predators or prey coming from below (the white blends in with the clear waters, illuminated by the sun) and from above (the gray helps the animal to blend in with the rest of the water).
The head also has two dark stripes around the eyes and mouth. The pectoral fins are dark.
The mouth is elongated in a rostrum inside which there are up to 110 pointed and sharp teeth: the rostrum of the common dolphin differentiates it from another species, the Delphinus capensis, very similar to the common dolphin, so much so that it was once considered a subspecies of the latter (Delphinus delphis capensis).
When he sleeps, he never closes both eyes at the same time but alternates them every 5-10 minutes. In this way the animal rests each eye for 3 or 4 hours and always remains alert.

Biology –
In the Delphinus delphis, mating takes place during the summer period; it deals with a rather promiscuous event, in the sense that each female mates with numerous males, who in turn mate with the greatest number of females possible and sometimes also with other younger males: the common dolphins, together with other species of dolphin and some primates, are among the few animals which mate even without a reproductive aim, that is, only for amusement.
Sexual maturity is reached late, on average at 6-7 years, and life expectancy is just over 20 years.
The reproductive act is preceded by a courtship during which the male rubs his pectoral fins with those of the female and swims perpendicular to her, until the latter turns sideways and allows herself to be covered.
Gestation lasts about eleven months; the females give birth to a single young, rarely two, which they nurse for a year and a half.
Delivery is breech and the newborn puppy is helped by the mother to reach the surface in order to breathe. The female and the pup return to the original group immediately after giving birth, and the mother, also helped by the other females of the group, takes care of the pups for more than three years, even if the weaning of the pup can be said to be completed when the baby is, as mentioned, about a year and a half old. Note that the puppy cannot suck milk from the teat like other mammals, as it is underwater. For this reason, it is the mother herself who inoculates the milk into the puppy’s mouth by spraying it out through the contraction of special muscles.
The puppies learn to hunt and behave by observing the adults, who in turn tease the undisciplined pupils. Sexual maturity is reached after the age of five.

Ecological role –
Delphinus delphis is a very sociable animal that eats, swims and breathes together with other specimens in unison. The size of the group varies from a few individuals up to more than 100,000, depending on the availability of food.
At high latitudes, dolphins tend to live in sex-based groups, with females moving in large schools especially during the cold months.
Dolphins are very intelligent animals and enjoy playing with each other, jumping and splashing the water, as well as exploiting the waves generated by the keel of boats or by large whales to be able to move effortlessly: however, they are great swimmers, so much so that , with their maximum speed of 40 km/h and more, they are considered among the fastest cetaceans.
These animals communicate through a wide range of sounds and show affection and solidarity towards those who are sick or in difficulty. Furthermore, they are playful acrobats who never miss the opportunity to “surf” in the wake of the ships and perform spectacular somersaults out of the water. They are excellent predators and raid squid, octopus and small fish such as sardines and anchovies. The greatest dangers for these cetaceans come from fishing nets and pollution, while in the wild they only have to fear sharks and killer whales.
Compared to the bottlenose dolphin “cousin”, the common dolphin is much less known, mainly because bottlenose dolphins, much easier to raise and train, are much more common in water parks: the common dolphin, on the other hand, proves to be extremely shy in captivity.
Common dolphins feed mainly on small fish, cephalopods and crustaceans, which they swallow whole. Foraging is a community action, during which the whole group collaborates, for example, in gathering a large school of fish to then make it easier for the various specimens to throw themselves in turn into the fray and swallow as much nourishment as possible. Each dolphin eats an average of 5–6 kg of food per day.
From an ecological point of view, the species is included in Annex IV of the Habitats Directive and in Annex II of the Bern Convention, in Annex II of the SPA/BIO Protocol of the Barcelona Convention and in Annexes I and II of the Bonn Convention . The species is included in the CITES Convention and in the ACCOBAMS Agreement.
The population of the species is under various pressures and threats. In the Mediterranean in recent decades it has undergone a strong decline (about 50% in the last 30-45 years), especially in the central-western sector.
The main pressures are voluntary and accidental catches with fishing gear (F02), habitat degradation, climate change (M). In the Mediterranean, entanglement in fishing gear has been declining in recent years thanks to the entry into force of new EU regulations that limit the use of driftnets. A threat to the health of the species is the high level of contaminants and heavy metals in the tissues. The increase in oil and gas (C02) exploration and extraction expected in the near future could pose a threat to the species.
This species has been legally protected nationally since the 1980s and internationally and included in numerous protected areas (Reeves & Notarbartolo di Sciara 2006).

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Facilidad Global de Información sobre Biodiversidad.
– Gordon Corbet, Denys Ovenden, 2012. Guide to mammals of Europe. Franco Muzzio Publisher.
– John Woodward, Kim Dennis-Bryan, 2018. The Great Encyclopedia of Animals. Gribaudo Publisher.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/266930797/original.jpg