Tursiops truncatus

Tursiops truncatus

The common bottlenose dolphin or Atlantic bottlenose dolphin (Tursiops truncatus, Montagu, 1821) is a cetacean belonging to the Delphinidae family.

Systematic –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Chordata,
Subphylum Vertebrata,
Infraphylum Gnathostomata,
Superclass Tetrapoda,
Mammalia class,
Subclass Theria,
Infraclass Eutheria,
Superorder Laurasiatheria,
Order Artiodactyla,
Suborder Odontoceti,
Delphinidae family,
Genus Tursiops,
Species T. truncatus.
The term is basionym:
– Delphinus truncatus Montagu, 1821.
The terms are synonymous:
– Delphinus compressicauda Lesson, 1828;
– Delphinus (Tursio) cymodoce Burmeister, 1867;
– Delphinus erebennus Cope, 1865;
– Delphinus metis Gray, 1846;
– Delphinus nesarnack Lacépède, 1804;
– Delphinus nesarnak Reichenback, 1846;
– Delphinus parvimanus Lutken, 1887;
– Delphinus troncatus F.Cuvier, 1836;
– Delphinus tursio Bonnaterre, 1789;
– Delphinus tursio Gunnerus, 1768;
– Delphinus tursio subsp. obtusus Schlegel, 1862;
– Phocoena compressicauda (Lesson, 1828);
– Tursio compressicauda (Lesson, 1828);
– Tursio cymodice Figueira, 1894;
– Tursio cymodoce Gray, 1868;
– Tursio metis Gray, 1868;
– Tursio truncatus Gray, 1843;
– Tursiops coerulescens Giglioli, 1889;
– Tursiops communis Fitzing, 1846;
– Tursiops compressicauda (Lesson, 1828);
– Tursiops dawsoni Lydekker, 1909;
– Tursiops eurynome Gray, 1846;
– Tursiops gillii Dall, 1873;
– Tursiops gillii Scammon, 1874;
– Tursiops maugeanus Iredale & Troughton, 1934;
– Tursiops nesarnack (Lacépède, 1804);
– Tursiops nuuanu Andrews, 1911;
– Tursiops parvimanus Van Beneden, 1886;
– Tursiops ponticus Barabash-Nikiforov, 1940;
– Tursiops subridens Flower, 1884;
– Tursiops truncantus Montagu, 1821;
– Tursiops truncatus (Gray, 1866);
– Tursiops truncatus subsp. gillii Dall, 1873;
– Tursiops truncatus subsp. gillii Tomilin, 1957;
– Tursiops tursio Bonnaterre, 1789;
– Tursiops tursio Gervais, 1855;
– Tursiops tursio Ihering, 1892;
– Tursiops tursio subsp. obtusus (Schlegel, 1862).
The following subspecies are recognized within this species:
– Tursiops truncatus subsp. gephyreus Lahille, 1908;
– Tursiops truncatus subsp. ponticus Barabash-Nikiforov, 1940;
– Tursiops truncatus subsp. truncatus;
– Tursiops tursio subsp. cymodoce Trouessart, 1898;
– Tursiops tursio subsp. tursio.

Geographic Distribution and Habitat –
The Tursiops truncatus is a dolphin that lives in temperate, subtropical and tropical oceans around the world. The world population has been estimated at 600,000. Some bottlenose dolphin populations live closer to shore (coastal populations) while others live further offshore (offshore populations). Generally, offshore populations are larger, darker, and have proportionately shorter fins and bills. Offshore populations can migrate up to 4,200 km in a season, but coastal populations tend to move less. However, some coastal populations undertake long migrations in response to El Niño events.
The species is present up to 50° north in the waters of the eastern Pacific, probably as a result of warm water currents. Coastal dolphins appear to adapt to warm, shallow waters. It has a smaller body and larger fins, for maneuverability and heat dispersion. They can be found in harbors, bays, lagoons and estuaries. Deep-sea dolphins, however, are adapted to cooler, deeper waters. Certain qualities in their blood suggest that they are better suited to deep diving. Their considerably larger body protects them from predators and helps them retain heat.
This cetacean is widespread in all the seas of the world, with the exception of the Arctic and Antarctic areas and there are, as mentioned, two distinct populations, one coastal and one open sea.
Populations have been recorded in the Pacific Ocean, where it is widespread from Japan to the Philippines and from the Gulf of California to New Zealand and Chile; in the Atlantic Ocean, where it is found from Scotland and Norway to Patagonia; in the Indian Ocean, starting from the eastern coast of Africa up to Australia.
In the western Atlantic it is common along the coasts of the United States up to the Gulf of Mexico; it is also widespread in the Black Sea and the Mediterranean Sea, where it is the most common cetacean. In Italy they are frequent along the Sicilian coasts, in the Adriatic and in some portions of the Cetacean Sanctuary. The latter, now called the Pelagos Sanctuary, is a stretch of sea between Liguria, Tuscany, northern Sardinia and southern France where there are twelve species of cetaceans, eight regular and four occasional. In this marine area the bottlenose dolphin is especially frequent along the coasts of Tuscany, the Tuscan Archipelago and eastern Liguria.
As regards the marine habitat, some populations live in a pelagic environment, especially those found near oceanic islands, while others live in coastal areas, in warm waters that do not exceed a depth of 30 m. There are therefore two ecotypes of bottlenose dolphins. The bottlenose dolphin can perform migrations which appear to be sometimes due to changes in water temperature and the abundance and distribution of prey. It has been observed that coastal waters are sometimes used as a nursery area.
However, despite being a mostly coastal species, it can also be found in other habitats, from the waters of the continental shelf, lagoons and closed seas, to waters surrounding islands and archipelagos. Less frequent, but still present, in deeper waters and pelagic areas (Bearzi et al. 2009).

Description –
The Tursiops truncatus is a dolphin, like all Cetaceans, almost completely hairless and measures from 2.5 to 3.8 meters in length, weighing up to 650 kg.
It has around 100-120 teeth all the same which are used only to grab prey but not to chew it because they swallow it whole by sliding it on the tongue; there are 18 to 26 pairs of conical teeth on each jaw.
As a result of evolutionary convergence, bottlenose dolphins have a fusiform body similar to that of fish which ensures them great hydrodynamics, reducing friction with the water.
Pelagic bottlenose dolphins have larger and more robust bodies than coastal ones and, as mentioned, there are also differences in the composition of blood hemoglobin. These differences appear to be due to the fact that pelagic bottlenose dolphins dive deeper than coastal ones.
They have, however, almost identical coloring in both populations and appear gray with various shades on the back and white on the belly. On the sides the gray becomes lighter. This coloration makes dolphins difficult to identify both when observed from bottom to top and from top to bottom.
On the head of this cetacean there is a pronounced melon (oval-shaped organ made of adipose tissue) and the elongated maxilla and mandible form a short and stocky rostrum, about 8 cm long.
The muzzle is characterized by the presence of a kind of “smile” due to the fact that the animal is unable to move its jaws to another position. On the apical portion of the head there is a blowhole, through which the bottlenose dolphin expels the breathed air and whose opening and closing is due to voluntary muscles. When the breather is open, you can observe the nasal septum.
The dorsal fin, triangular and curved in shape, is about 23 cm high while the pectoral fins, called flippers, are about 30–50 cm long. The caudal fin, divided into two lobes (flukes), is about 60 cm wide. The dorsal fin and caudal fin are both made up of connective tissue, and there are no bones or muscles inside them. The pectoral fins instead have bones homologous to those of the terrestrial mammals from which the Cetaceans evolved about 50 million years ago.
However, it is not a particularly easy species to identify. It can be confused with the common dolphin and the striped dolphin, two other cetaceans that do not exceed 4 m. They can be distinguished thanks to their coloration; in fact, the bottlenose dolphin does not have white streaks on its sides. Furthermore, dolphins are smaller on average.
It can be confused with the coastal species Sotalia guianensis in the southern Atlantic Ocean.
To carry out studies on these animals and be able to easily recognize them, researchers use the photoidentification technique, which consists of photographing the dorsal fin of Cetaceans and then noting down the information regarding the presence of scars or other distinctive signs on it.

Biology –
The reproduction of the Tursiops truncatus is quite well known.
Males have two openings in the lower part of the body, one hides the penis and the other forms the anus. The female, however, has only one genital opening, which accommodates both the vagina and the anus. Next to each side of the genital opening, there are lateral mammary slits that hide the mammary glands for breastfeeding the little ones.
Females reach sexual maturity around 6-12 years, while males around 10-13 years.
Gestation lasts 12 months and births occur in summer. Bottlenose dolphins normally give birth to a single calf, about 1 m long, which will remain in contact with the mother for about 6 years. Complete weaning occurs after approximately 18 months and in any case ends before the birth of a second baby. In bottlenose dolphins we witness the phenomenon of babysitting: the babies are looked after by a single female, while the other mothers go hunting. They reproduce every 2 or 3 years, changing partners each time, but if the first cub dies at birth, the female can reproduce after a year.
As in all Cetaceans, the young are born from the tail and are already able to swim to follow the mother who, after birth, accompanies the young towards the surface to make it breathe and in this operation is sometimes assisted by other females, generally related to her and who were called “aunts” (Herman and Tavolda, 1988).
During the mating season, males fight among themselves for females and usually establish a hierarchy based on size. Pairs are formed when a male shows a certain preference for swimming next to a female and stays with her for a given period of time. Subsequently, the male faces the female by arching the back of her body, “stroking” her and rubbing himself on her. The sexual act is rapid, lasts about 10-30 seconds, but is repeated several times with an interval of a few minutes between each one and takes place underwater: the dolphins swim belly to belly, with the female facing her back downwards ; the male extends his penis, which is inserted inside the female’s vagina.
It should also be remembered that hybrids have been born in captivity from crosses between bottlenose dolphins and other delphinids.

Ecological Role –
The Tursiops truncatus is a social animal that lives in schools, called pods, generally composed of 2-6 individuals. However, it is not uncommon to observe solitary individuals, generally males. In fact, the pods are made up of a group of females with their young and the males join them only for a short time. Some bottlenose dolphins live together with other cetacean species.
Studies carried out by Wells in Sarasota, Florida, and by Smolker in Shark Bay, Australia, have shown how the females of the community are connected either directly or through mutual associations according to a social pattern of “fission-fusion”. Groups of stronger associations are called “gangs” and their composition can remain stable for years. There is genetic evidence that gang members may be related, but these gangs are not necessarily limited to a single matrilineal line. Females associate mainly to protect their young from predators and conspecifics and there is no evidence that the bands compete with each other.
In the same research areas, as well as in the Moray Firth in Scotland, males form strong associations of two or three individuals. These groups of males are known as “alliances” and their members display synchronized behaviors, such as breathing, jumping and breaching. The composition of the alliance is stable for decades and can bring benefits in the search for females for mating. Once the female is found, the males surround or chase her, and cases of aggression towards her are not uncommon. Connor reports that alliances can temporarily join other alliances, forming “super-alliances” or “second-order alliances” primarily for the purpose of obtaining females that are undermined by other alliances. For example, alliance A consists of two individuals, alliance B consists of three, and alliance C also consists of three individuals. All three alliances compete for the same female, so the formation of the super-alliance between A and B, made up of five individuals, gives a numerical advantage in the competition with C, made up of only three individuals.
Researchers from the Institute for Applied Dolphin Research – BDRI have instead demonstrated how the social structure of bottlenose dolphins resident on the north-eastern coast of Sardinia varies according to their trophic behavior and the need to collaborate with each other. The distribution and presence of bottlenose dolphins has been correlated to opportunistic feeding in the vicinity of a fish farm which has caused changes in the distribution and concentration of prey, thus facilitating the feeding of bottlenose dolphins and making cooperation for hunting useless.
In these conditions the associations do not vary according to the sex of the individuals, but in relation to trophic behavior. It has been shown how dolphins associate, regardless of sex, with specimens with which they share the same food preferences. Aquaculture systems are therefore able to interfere in the behavior and social structure of these mammals.
Regarding their movements and acrobatics, these dolphins are capable of performing acrobatics out of the water, the meaning of which is not yet clear. Among these the most common are:
– leaping: jumping completely out of the water;
– tailspinning: “walking” backwards on the water using the tail as a pivot;
– lobtailing: flapping the caudal fin on the surface of the water;
– bow: jump vertically completely out of the water;
– bowriding: swimming on the waves left by the bow of the boats;
– breaching: making “dives” out of the water.
In reference to their dietary characteristics, they are predatory animals and often show aggressive behavior which includes fighting between males for females and aggression towards other small dolphins.
However, it is an opportunistic species, feeding mainly on fish (anchovies, mackerel, mullet, etc.) and cephalopods, in particular coastal species (squid, cuttlefish and octopus), but does not disdain crustaceans when necessary. Studies carried out on stomach contents have shown that in the Mediterranean, bottlenose dolphins feed mainly on hake, scabbard fish, conger eels and squid. Conical teeth are used for grasping food, not chewing it.
Bottlenose dolphins often cooperate with each other to hunt and cooperation between dolphins and fishermen is also known. Furthermore, bottlenose dolphins frequently follow fishing boats to feed on fishermen’s waste or waste.
A particular hunting technique used by some bottlenose dolphins is strand-feeding or beach hunting, a tactic similar to that used by killer whales: the dolphins first gather the fish towards the sandy coasts, swimming parallel to them, then push them onto the sand and they feed on it by beaching themselves partially or completely. Finally they return to the sea, with a U-shaped movement. Strand-feeding is a rather dangerous technique, which is why it is generally practiced during periods of high tide. It can be practiced by solitary individuals or by groups of dolphins that collaborate to catch fish. It is a behavior that develops only in bottlenose dolphins born to other beach hunters, but studies carried out on mitochondrial DNA have led to the exclusion of genetic transmission of this hunting technique, so it is thought that dolphins learn it by observing their mothers.
Furthermore, it is certainly the best known and most studied dolphin; some abilities of this animal have proven to be very useful. For example, the armies of the United States of America and Russia train bottlenose dolphins to search for mines and enemy divers.
Furthermore, direct interaction with bottlenose dolphins is used in the therapy of many disabled adults and children and many believe that this has highly positive effects, especially in the treatment of depression or autism.
However, bottlenose dolphins can also be harmful, particularly to fishermen, whose nets they tear to feed on the fish trapped in them.
It is also one of the rare species of dolphins that tolerate captivity; also due to this it is the most studied and the most common in dolphinariums.
The production of sounds is also important.
Without a doubt, the most developed sense of bottlenose dolphins is hearing, combined with the great ability to emit sounds of different frequencies, divided into three categories:
– Clicks, consisting of a series of high frequency sounds;
– Whistles;
– Creaks (Barks), similar to the barking of dogs.
Clicks are used for echolocation, while other sounds are used for communication. Each bottlenose dolphin has its own characteristic whistle, a sort of “signature whistle”, which makes it immediately identifiable by its conspecifics. They lack vocal cords, so low-frequency sounds are thought to be generated through the larynx and by means of six air sacs located near the blowhole.
Since sound propagates better in water than in air, bottlenose dolphins, like all cetaceans, lack ears. This allows them to have greater hydrodynamics. The inner ear is inserted into a separate bone from the skull while the middle ear is highly vascularized. When the cetacean dives, this tissue helps it compensate for the underwater pressure and therefore avoid injury to the eardrum.
As regards echolocation, bottlenose dolphins are able to recognize obstacles and search for food by means of echolocation. When the sound waves produced by the animal, the clicks, reach an obstacle or prey, they bounce and come back. Clicks are produced by three air sacs on the head. The air, due to the contraction of the blowhole muscles, passes first into the upper bag, then into the intermediate one and finally into the lower one, producing a pop which is then amplified by the melon, a mass of adipose tissue present on the head. The returning echo is picked up by the dolphin through its lower jaw and is transferred through a kind of oil to the inner ear.
The study of the behaviors of this cetacean is also important to determine its level of intelligence.
The brain of bottlenose dolphins is quite large and reaches dimensions comparable to that of an anthropomorphic monkey. As in humans, it consists of two hemispheres, but has a thinner cortex, although 40% larger and with a complexity almost equivalent to that of humans. Its development is completed in about 10 years.
All mammals, including dolphins, go through a phase called REM during sleep. The dolphin is a voluntary breather, even while sleeping, and this makes it impossible for veterinarians to give it anesthesia, which would lead to death by asphyxiation. The electroencephalogram showed that dolphins use only one cerebral hemisphere at a time for sleep, probably to control the voluntary breathing system.
According to some authors, the size of the dolphin’s brain is synonymous with intelligence (even if to date there is no generally accepted definition of intelligence) and potential language abilities, while according to others the majority of the brain is used by the bottlenose dolphin for swimming and for hearing.
Among the predators of Tursiops truncatus there are large species of sharks, such as Galeocerdo cuvier, Carcharodon carcharias and Carcharhinus leucas.
Some orcas feed on dolphins, but this appears to be quite rare. In fact, while some orcas that feed on other mammals prey on dolphins, others have been observed swimming alongside dolphins. Swimming in a school allows dolphins to better defend themselves from predators, and they also use complex evasive strategies to escape them. Bottlenose dolphins help their injured counterparts by holding them out of the water to breathe, a behavior sometimes also observed towards divers in difficulty.
Regarding the conservation status of this species, it is classified as “least concern” (LC) by the IUCN red list because it is very common and has a very large range. However, there are several dangers that especially threaten local populations.
The North Sea, Baltic, Mediterranean and Black Sea populations of the common bottlenose dolphin are listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS) of the Bonn Convention, as they have a conservation character unfavorable status or would benefit significantly from international cooperation organized through tailor-made agreements.
The species is included in Appendix II of the Convention on International Trade in Endangered Species (CITES), meaning that international trade (including in parts/derivatives) is regulated.
The species is covered by the Agreement on Small Cetaceans of the Baltic, North-East Atlantic, Irish and North Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, the Mediterranean Sea and the Contiguous Atlantic Area (ACCOBAMS), the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region, and the Memorandum of Understanding Concerning the Conservation of Manatees and Small Cetaceans of West Africa and Macaronesia.
One of the main dangers is hunting: in Japan and the Faroe Islands, bottlenose dolphins, together with other dolphin species such as dolphins, are hunted for food or for sale to dolphinariums. The dolphins are surrounded with boats and fishermen bang steel bars underwater in order to create a sound barrier that pushes the bottlenose dolphins towards the beach, where they are then trapped in nets. Some dolphins are selected for sale to dolphinariums, while the others are brought ashore where they are killed and sent to food markets. Even in Italy, before their hunting was prohibited, bottlenose dolphins were killed to prepare “musciame”, a dish consisting of dolphin fillets dried in the sun. Today dolphin meat has been replaced by tuna belly. However, accidental catches are still quite common.
The number of bottlenose dolphins in the Black Sea is decreasing, as is the case in the Mediterranean, although they represent the most abundant cetacean species in the latter sea.
The other threats are partly due to fishing, which leads to a decrease in food available for bottlenose dolphins, and partly to other human activities, which cause noise pollution and water pollution, which is particularly harmful to younger specimens. Furthermore, pollution of the seas and the presence of discharges of toxic substances and heavy metals appear to be among the potential causes of threat to this and other marine species.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Information Facility on Biodiversity.
– Gordon Corbet, Denys Ovenden, 2012. Guide to the mammals of Europe. Franco Muzzio Editore.
– John Woodward, Kim Dennis-Bryan, 2018. The great encyclopedia of animals. Gribaudo Publisher.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/258301921/original.png