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Last updated on April 17, 2014 at 17:30 EDT

Bottlenose Dolphin

The Bottlenose Dolphin (Tursiops truncatus) is the most common and well-known dolphin species. It inhabits warm and temperate seas worldwide and may be found in all but the Arctic and the Antarctic Oceans.

Physical description

Bottlenose Dolphins are grey, varying from dark grey at the top near the dorsal fin to very light grey and almost white at the underside. The salt water makes them hard to see both from above and below when swimming. The elongated upper and lower jaws give the animals their name of bottlenose. The real nose however is the blowhole on top of the head, and the nasal septum is visible when the blowhole is open. Their face shows a characteristic “smile”.

Adults range in length from 6 to 13 feet (2 to 4m) and in weight from 330 to 1430 pounds (150 to 650kg) with males being slightly longer and considerably heavier than females on average. The size of the dolphin appears to vary considerably with habitat. Most research in this area has been restricted to the North Atlantic Ocean, where researchers have identified two ecotypes. Those dolphins in warmer, shallower waters tend to have a smaller body than their cousins in cooler pelagic waters. For example a survey of animals in the Moray Firth in Scotland (the world’s northernmost resident population) researchers recorded an average adult length of just under 13 feet (4m). This compares with an 8 feet (2.5m) average in a population off Florida. Those in colder waters also have a fatter composition and blood more suited to deep diving.

The flukes (lobes of the tail) and dorsal fin are formed of dense connective tissue and don’t contain bones or muscle. The animal propels forward by moving the flukes up and down. The pectoral flippers (at the sides of the body) serve for steering; they contain bones clearly homologous to the forelimbs of land mammals (from which dolphins and all other cetaceans evolved some 50 million years ago).

Behavior and life

Bottlenose Dolphins typically swim at a speed of 3-6 miles per hour (5-11 km per hour) for short times, they can reach peak speeds of 21 mph (35 km per hour).

Every 5-8 minutes, the dolphins have to rise to the surface to breathe through their blowhole. (However, on average, they breathe more often – several times per minute.) Their sleep is thus very light; some scientists have suggested that the two halves of their brains take turns in sleeping and waking. It has also been suggested that they have tiny periods of ‘micro sleep’.

Bottlenose Dolphins normally live in groups called pods, containing up to 12 animals. These are long-term social units. Typically, a group of females and their young live together in a pod, and juveniles in a mixed pod. Several of these pods can join together to form larger groups of one hundred dolphins or more. Males live mostly alone or in groups of 2-3 and join the pods for short periods of time.

The species is commonly known for its friendly character and curiosity towards humans immersed in or near water. It is not uncommon for a diver to be investigated by a group of them, and they are often quite receptive to being gently patted or stroked. Occasionally, dolphins have rescued injured divers by raising them to the surface, a behavior they also show towards injured members of their own species. Such accounts have earned them the nickname of “Man’s best friend of the sea”. In November 2004, a more dramatic report of dolphin intervention came from New Zealand. Three lifeguards, swimming 100 m off the coast near Whangarei, were reportedly approached by a 3 m Great White Shark. A group of Bottlenose Dolphins, apparently sensing danger to the swimmers, herded them together and tightly surrounded them for forty minutes, preventing an attack from the shark, as they returned to shore.

Dolphins are predators however, and they also show aggressive behaviors. This includes fights among males for rank and access to females, as well as aggression towards sharks and other smaller species of dolphins. Male dolphins, during the mating season, compete very vigorously with each other through showing toughness and size with a series of acts such as head butting.

Female Bottlenose Dolphins live for about 40 years; the more stressful life of the males apparently takes its toll, and they rarely live more than 30 years.

Diet

Their diet consists mainly of small fish, occasionally also squid, crabs, octopus, and other similar animals. Their peg-like teeth serve to grasp but not to chew food. When a shoal of fish has been found, the animals work as a team to keep the fish close together and maximize the harvest. They also search for fish alone, often bottom dwelling species. Sometimes they will employ “fish whacking” whereby a fish is stunned (and sometimes thrown out of the water) with the fluke to make catching and eating the fish easier.

Senses and communication

The dolphin’s search for food is aided by a form of echolocation similar to sonar: they locate objects by producing sounds and listening for the echo. The broadband burst pulse clicking sounds are emitted in a focused beam towards the front of the animal. They have two small ear openings behind the eyes, but most sound waves are transmitted to the inner ear through the lower jaw. As the object of interest is approached, the echo grows louder; the dolphins adjust by decreasing the intensity of the emitted sounds. (This is in contrast to the technique used by bat echolocation and human sonar where the sensitivity of the sound receptor is attenuated.) As the animal approaches the target, the inter click interval also decreases, as each click is usually produced after the round-trip travel time of the previous click.

They also have sharp eyesight. The eyes are located at the sides of the head and have a tapetum lucidum, which aids in dim light. Their horseshoe-shaped double-slit pupil enables the dolphin to have good vision in both in-air and underwater viewing, despite the differences in density of these media. Underwater, the eyeball’s lens serves to focus light, whereas in the in-air environment, the typically bright light serves to contract the specialized pupil, resulting in sharpness from a small-aperture (similar to a pinhole camera).

By contrast, their sense of smell is very poor, as would be expected as the blowhole, the analog to the nose, is closed in the underwater environment, and opens only voluntarily for respiration. The sense of taste has not been well studied, although dolphins have been demonstrated to be able to detect salty, sweet, bitter (quinine sulfate), and sour (citric acid) tastes. Anecdotally, some animals in captivity have been noted to have preferences for food fish types although it is not clear that this preference is mediated by taste.

Bottlenose Dolphins communicate with body movements and with sounds they produce using six air sacs near their blowhole (they lack vocal cords). Each animal has a characteristic frequency-modulated narrow-band signature vocalization (signature whistle), which is uniquely identifying. Other communication uses about 30 distinguishable sounds, and although famously proposed by John Lilly in the 1950′s, a “dolphin language” has not been found. However, Herman, Richards, & Wolz demonstrated the comprehension of an artificial language by two bottlenosed dolphins (named Akeakamai and Phoenix) in the period of skepticism toward animal language following Herbert Terrace’s critique. See also the article on the dolphin brain for some general information about the intelligence of dolphins.

Natural predators

Large shark species such as the tiger shark, the dusky shark, and the bull shark prey on the Bottlenose Dolphin. However, the dolphin is far from helpless against its predators and it has been known to fight back through charges. The Orca may also prey on it, but this seems rare.

Taxonomy

Scientists have long been aware that the Bottlenose Dolphin might consist of more than one species. The advent of molecular genetics has allowed much greater insight into this previously intractable problem. The consensus amongst scientists is that there are two species:

  • The Common Bottlenose Dolphin (T. truncatus), found in most warm to tropical oceans; color sometimes almost blue; has a dark line from beak to blowhole
  • The Indo-Pacific Bottlenose Dolphin (T. aduncus), living in the waters around India, Australia and South-China; back is dark-gray and belly is white with gray spots.

The following are sometimes recognized as subspecies of T. truncatus:

  • The Pacific Bottlenose Dolphin (T. gillii or T. truncatus gillii), living in the Pacific; has a black line from the eye to the forehead
  • The Black Sea Bottlenose Dolphin (T. truncatus ponticus), living in the Black Sea.

Much of the old scientific data in the field combine data about the two species into a single group – making it effectively useless in determining the structural differences between the two species. Indeed, the IUCN lists both species as data deficient in their Red List of endangered species precisely because of this issue.

Some recent genetic evidence suggests that the Indo-Pacific Bottlenose belongs in the genus Stenella, it being more like the Atlantic Spotted Dolphin (Stenella frontalis) than the Common Bottlenose. The taxonomic situation of these animals is likely to remain in flux for some time to come.

Bottlenose Dolphin