Bottlenose Dolphin

2008/9 Schools Wikipedia Selection. Related subjects: Mammals

Bottlenose Dolphin
Bottlenose Dolphin breaching in the bow wave of a boat
Bottlenose Dolphin breaching in the bow wave of a boat
Size comparison against an average human
Size comparison against an average human
Conservation status

Data Deficient ( IUCN 2.3)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Delphinidae
Genus: Tursiops
Species: T. truncatus
Binomial name
Tursiops truncatus
Montagu, 1821
Bottlenose Dolphin range (in blue)
Bottlenose Dolphin range (in blue)

The Bottlenose Dolphin is one of the most common and well-known dolphins. Recent molecular studies show it is in fact two species, the Common Bottlenose Dolphin (Tursiops truncatus) and the Indo-Pacific Bottlenose Dolphin (T. aduncus). Bottlenose dolphins inhabit warm and temperate seas worldwide.

Description

The Bottlenose Dolphin is perhaps the archetypal dolphin. It is gray, varying from dark gray at the top near the dorsal fin to very light gray and almost white at the underside. This makes it hard to see, both from above and below, when swimming. Its elongated upper and lower jaws form what is called a rostrum, or beak-like snout, which gives the animal its common name, the Bottlenose dolphin. The real, functional nose is the blowhole on top of its head; in fact the nasal septum is visible when the blowhole is open.

Adults range in length from 2 to 4  meters (6 to 13  ft) and in weight from 150 to 650 kilograms (330 to 1430  lb) with males being on average slightly longer and considerably heavier than females; however, in most parts of the world the adult's length is about 2.5 m (8 ft) with weight ranges from 200 to 300 kg (440 to 660 lb). The size of a dolphin appears to vary considerably with habitat. 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 second northernmost resident dolphin population, recorded an average adult length of just under 4 m (13 ft) compared with a 2.5 m (8 ft) average in a population off the coast of Florida. Those in colder waters also have a fattier composition and blood more suited to deep-diving. Bottlenose Dolphins typically have 18%-20% of their bodyweight made up of blubber. Most research in this area has been restricted to the North Atlantic Ocean, where researchers have identified two ecotypes.

The flukes (lobes of the tail) and dorsal fin are formed of dense connective tissue and do not contain bones or muscle. The animal propels itself forward by moving the flukes up and down. The pectoral flippers (at the sides of the body) are 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). In fact a Bottlenose Dolphin was recently discovered in Japan that has two additional pectoral fins, or "hind legs," at the tail, appearing to be about the size of a human's pair of hands. Scientists believe that a mutation must have caused the ancient trait to reassert itself as a form of atavism.

Female Bottlenose Dolphins live for about 40 years, whereas males rarely live more than 30 years.

Bottlenose calves are born with 88 teeth.

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; colour sometimes almost blue; has a dark line from beak to blowhole, and
  • the Indo-Pacific Bottlenose Dolphin (T. aduncus), living in the waters around India, Australia and South-China; back is dark-grey and belly is white with grey spots.

The following are sometimes also 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.

Behaviour

An adult female Bottlenose Dolphin with her young, Moray Firth, Scotland
An adult female Bottlenose Dolphin with her young, Moray Firth, Scotland
A Bottlenose Dolphin attacks and kills a Harbour Porpoise at Chanonry Point, Scotland
A Bottlenose Dolphin attacks and kills a Harbour Porpoise at Chanonry Point, Scotland

The Bottlenose Dolphin normally lives in small groups, usually containing up to 12 animals. However, group size may be highly variable since they live in fission-fusion societies within which individuals associate in small groups that change in composition, often on a daily or hourly basis. Typically, a group of adult 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.

Bottlenose Dolphins studied by researchers of the Bottlenose Dolphin Research Institute BDRIoff Sardinia island show non-random social behaviour during feeding activities and their social behaviour differs depending on the feeding activity in which they are engaged. In Sardinia the presence of a floating marine fin-fish farm, has been linked to a change in Bottlenose Dolphin distribution as a result of high fish density around the floating cages in the farming area.

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. Occasionally, dolphins have rescued injured divers by raising them to the surface, a behaviour 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. Four lifeguards, swimming 100 m (328 ft) off the coast near Whangarei, were reportedly approached by a 3 m (10 ft) 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 have also been documented exhibiting altruistic behaviour toward other sea creatures. On Mahia Beach, New Zealand on March 10, 2008 two Pygmy Sperm Whales — a female and calf — became stranded on the beach. Rescuers, including Department of Conservation officer Malcolm Smith, attempted to refloat the whales, however their efforts failed four times. Shortly before the whales were to be euthanized a playful Bottlenose Dolphin known to local residents as Moko arrived and, after seemingly communicating with the whales, led them 200 meters along a sandbar to the open sea.

Dolphins are predators however, and they also show aggressive behaviours frequently. This includes fights among males for rank and access to females, as well as aggression towards sharks, certain Orcas, and other smaller species of dolphins. During the mating season male dolphins compete very vigorously with each other through displays of toughness and size with a series of acts such as head butting. At least one population, off Scotland, has been observed to practice infanticide, and has also been filmed attacking and killing Harbour Porpoises. Researchers at the University of Aberdeen in Scotland have discovered that the local Bottlenose Dolphins attack and kill harbour porpoises without eating them due to competition for a decreasing food supply.

Diet

Their diet consists mainly of small fish with occasional squid, crabs, shrimp, and other smaller animals. Their cone-like teeth serve to grasp but not to chew food. When a shoal of fish is found dolphins 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 dolphins 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.

Perceived conflict of interactions between common Bottlenose Dolphins and coastal, small scale commercial fisheries has been reported in a number of Mediterranean areas. Common Bottlenose Dolphins are probably attracted to fishing nets activities because they make it easier for the dolphins to exploit a concentrated food source.

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. A broadband burst pulse of clicking sounds is emitted in a focused beam in front of the dolphin. To hear the returning echo 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, and 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 interclick interval also decreases, as each click is usually produced after the round-trip travel time of the previous click is completed. Details of the dolphin's echolocation, such as signal strength, spectral qualities, discrimination abilities, etc., have been well investigated by researchers. Also, Pack & Herman demonstrated that Bottlenose Dolphins are able to extract shape information from their echolocative sense, suggesting that they are able to form an "echoic image" of their targets.

Dolphins also have sharp eyesight. The eyes are located at the sides of the head and have a tapetum lucidum, or reflecting membrane at the back of the retina, which aids vision 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. When 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 smaller aperture (similar to a pinhole camera).

By contrast their sense of smell is very poor, as would be expected since the blowhole, the analogue to the nose, is closed in the underwater environment, and opens only voluntarily for breathing. The olfactory nerves as well as the olfactory lobe in the brain are missing. Their sense of taste has not been well-studied, although dolphins have been demonstrated to be able to detect salty, sweet, bitter (quinine sulphate), 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 one another through squeaks, whistles, and body language. Examples of body language include leaping out of the water, snapping jaws, slapping tails on the surface of the water, and butting heads with one another. All of these gestures are a way for the dolphins to convey messages. The sounds and gestures that Bottlenose Dolphins produce help keep track of other dolphins in the group and alert other dolphins to possible dangers and nearby food. They produce sounds using six air sacs near their blow hole (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 1950s, a "dolphin language" has not been found. However, Herman, Richards, & Wolz demonstrated the comprehension of an artificial language by two Bottlenose Dolphins (named Akeakamai and Phoenix) in the period of skepticism toward animal language following Herbert Terrace's critique.

Respiration and Sleep

The Bottlenose dolphin has a single blowhole located on the dorsal surface of the head consisting of a hole and a muscular flap. The flap is closed during muscle relaxation and opens during contraction. A dolphin is able to exchange 80% or more of its lung air with each breathe; constrastly, humans are only able to exchange 17%. The exhale-inhale cycle lasts approximately 0.3 seconds. The Bottlenose dolphin typically rises to the surface to breathe through its blowhole 2-3 times per minute; if necessary, it has the ability to remain submerged for up to 20 minutes. As a direct result of the voluntary breathing requirement scientists have determined that during the sleeping cycle one brain hemisphere remains active while the other hemisphere shuts down. The sleeping cycle lasts for approximately 8 hours during each 24 hour period, in increments of several minutes (or less) to several hours. During the sleeping cycle dolphins remain near the surface swimming slowly or "logging", occasionally closing one eye.

Intelligence

Cognition

Cognitive abilities investigated in the dolphin include concept formation, sensory skills, and the use of mental representation of dolphins. Such research has been ongoing from the late 1970s through to the present, and include the specific areas of: acoustic mimicry, behavioural mimicry (inter- and intra-specific), comprehension of novel sequences in an artificial language (including non finite state grammars as well as novel anomalous sequences), memory, monitoring of self behaviours (including reporting on these, as well as avoiding or repeating them), reporting on the presence and absence of objects, object categorization, discrimination and matching (identity matching to sample, delayed matching to sample, arbitrary matching to sample, matching across echolocation and vision, reporting that no identity match exists, etc.), synchronous creative behaviours between two animals, comprehension of symbols for various body parts, comprehension of the pointing gesture and gaze (as made by dolphins or humans), problem solving, echolocative eavesdropping, attention, mirror self-recognition, and more. Recent research has shown that dolphins are capable of comprehending numerical values. In an experiment where a dolphin was shown two panels with a various number of dots of different size and position, the dolphin was able to touch the panel with a greater number of dots, much more rapidly than many human beings could do.

Tool use and culture

In 1997, tool use was described in Bottlenose Dolphins in Shark Bay. A dolphin will stick a marine sponge on its rostrum, presumably to protect it when searching for food in the sandy sea bottom. The behaviour has only been observed in this bay, and is almost exclusively shown by females. This is the only known case of tool use in marine mammals outside of Sea Otters. An elaborate study in 2005 showed that mothers most likely teach the behaviour to their daughters. Subsets of populations in Mauritania are known to engage in interspecific cooperative fishing with human fishermen. The dolphins drive a school of fish towards the shore where humans await with their nets. In the confusion of casting nets, the dolphins catch a large number of fish as well. Intraspecific cooperative foraging techniques have also been observed, and some propose that these behaviours are transmitted through cultural means. Rendell & Whitehead have proposed a structure for the study of culture in cetaceans, although this view has been controversial (e.g. see Premack & Hauser).

Natural predators

Large shark species such as the tiger shark, the dusky shark, and the bull shark prey on the Bottlenose Dolphin, especially the young. However, the dolphin is far from helpless against its predators and it has been known to fight back through charges; indeed, dolphin 'mobbing' behaviour of sharks can occasionally prove fatal for the shark. Even a single adult dolphin is dangerous prey for a shark of similar size. Certain (but not all) orcas may also prey on dolphins, but this seems very rare. While certain orcas that eat other mammals prey on the dolphin, other non-mammal eating orcas have been seen swimming with dolphins.

Swimming in pods allows dolphins to better defend themselves against predators. Bottlenose Dolphins either use complex evasive strategies to outswim their predators or they will batter the predator to death. Bottlenose Dolphins will also aid their injured by holding injured dolphins above water for air.

Conservation

Bottlenose Dolphins are not endangered. Their future is currently foreseen to be stable because of their abundance and high adaptability. However, some specific populations are threatened due to various environmental changes. For example, the population in the Moray Firth in Scotland is estimated to consist of around 150 animals and to be declining by around 6% per year due to the impact of harassment and traumatic death, water pollution and reduction in food availability. Less local climate change such as increasing water temperature may also play a role.

In U.S. waters, hunting and harassing of marine mammals is forbidden in almost all circumstances. The international trade in dolphins is also tightly controlled.

High levels of metal contaminants have been measured in Bottlenose Dolphin tissues in many areas of the globe. A recent study found very high levels of cadmium and mercury in Bottlenose Dolphins from South Australia.

Bottlenose Dolphins and humans

K-Dog, trained by the US Navy to find mines and boobytraps underwater, leaping out of the water
K-Dog, trained by the US Navy to find mines and boobytraps underwater, leaping out of the water

Bottlenose Dolphins are still occasionally killed in dolphin drive hunts for their meat or because they compete for fish. Bottlenose Dolphins (and several other dolphin species) often travel together with tuna, and since the dolphins are much easier to spot than the tuna, fishermen commonly encircle dolphins to catch tuna, sometimes resulting in the death of dolphins. This has led to boycotts of tuna products and a "dolphin-safe" label for tuna caught with methods that do not endanger dolphins.

Bottlenose Dolphins (as well as other dolphins) are often trained to perform in dolphin shows. Some animal welfare activists claim that the dolphins there are not adequately challenged and that the pools are too small; others maintain that the dolphins are well cared for and enjoy living and working with humans.

Eight Bottlenose Dolphins that were washed out of their aquarium pool during the devastating August 2005 strike of Hurricane Katrina were later found alive by rescue forces, huddled together in coastal waters near their former home in Gulfport, Mississippi, USA.

Direct interaction with dolphins is used in the therapy of severely handicapped children and adults, and many report it as having a highly positive effect.

The military of the United States and Russia train Bottlenose Dolphins as military dolphins for wartime tasks such as locating sea mines or detecting and marking enemy divers. The USA's program is the U.S. Navy Marine Mammal Program, located in San Diego, California.

In the town of Laguna in south Brazil, a pod of Bottlenose Dolphins is known to drive fish towards fishermen who stand at the beach in shallow waters. One dolphin will then roll over, which the fishermen take as sign to throw out their nets. The dolphins feed on the escaping fish. The dolphins were not trained for this behaviour; the collaboration has been going on at least since 1847. Similar cooperative fisheries also exist in Africa, and have been reported through recorded history.

A dolphin with an extra set of fins was found in Wakayama, Japan on October 28 2006. Scientists are researching and have found that they may be the remains of hind legs. The dolphin is alive and will go through X-Ray and DNA tests.

In New Zealand there was a very famous dolphin by the name of Opo or her long name Opo the Dolphin. She was friendly to the locals and when she died they honoured her with a statue of her, including a child on her back and she was also honoured with a public funeral. There is also a NZ folk song about her. Maori believed that she was a messenger from one of the first people who discovered New Zealand.

Bottlenose Dolphins in popular culture

  • The popular television show Flipper, created by Ivan Tors, portrayed a Bottlenose Dolphin in a friendly relationship with two boys, Sandy and Bud; a kind of seagoing Lassie, Flipper understood English unusually well and was a marked hero: "Go tell Dad we're in trouble, Flipper! Hurry!" The show's theme song contains the lyric no one you see / is smarter than he. The television show was based on a 1963 film, and remade as a feature film in 1996 starring Elijah Wood and Paul Hogan, as well as a television series running from 1995-2000 starring Jessica Alba.
  • Ensign Darwin was a Bottlenose Dolphin crew member of seaQuest on the television series seaQuest DSV. Thanks to an invention by Lucas Wolenczak ( Jonathan Brandis), Darwin could communicate verbally with the crew. Darwin was not played by a real dolphin; it was an animatronic.
  • Bottlenose Dolphins have appeared in the film adaptation of The Hitchhiker's Guide to the Galaxy as well as the novel and one of its sequels, So Long, and Thanks For All the Fish. The dolphins are very intelligent creatures who tried in vain to warn humans of the impending destruction of Earth before making their own escape. However, their behaviour was misinterpreted as playful acrobatics. In particular, dolphins are noted to be the second most intelligent species on the planet Earth, ahead of humans, who ranked third.
  • The science fiction video game series Ecco the Dolphin, published by Sega, stars Ecco, a young adult male Bottlenose Dolphin. The series also features societies of sapient cetaceans, time travel, and malevolent space aliens.
  • Zeus and Roxanne, an HBO TV movie about a female Bottlenose Dolphin (Roxanne) and a male dog (Zeus) becoming friends. The film stars Steve Guttenberg and Kathleen Quinlan.
  • In David Brin's series of Uplift Universe novels, one of the two species that humans have uplifted to sentience are the Bottlenose Dolphin (the other species is the chimpanzee). Numerous Bottlenose Dolphins are central characters in his book Startide Rising.
  • Dolphins are some of the primary characters in Anne McCaffrey's " The Dolphins of Pern" book, one of the books in the " Dragonriders of Pern" series (published by Del Rey). In the novel, Dolphins have been treated by a process to enhance intelligence slightly and allow them to communicate vocally with humans before coming with the colonists to the planet Pern. Dolphins are also mentioned the books "Dragonsdawn" and "Chronicles of Pern: First Fall" in the same series.
  • In Treehouse of Horror XI, Segment 3 in The Simpsons, Dolphins reveal to humans that they could talk and that they once lived on land, but were driven into the sea by humans. They later successfully drive the humans (the townsfolk of Springfield) into the sea while they took over dry land.

Factual descriptions of the Bottlenose Dolphin date back into antiquity - the writings of Aristotle, Oppian and Pliny the Elder all mention the species.

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