Bottlenose Dolphin

Bottlenose Dolphin

Conservation status: Data deficient

A Bottlenose Dolphin

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)

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 2 to 4m (6 to 13 feet) and in weight from 150 to 650kg (330 to 1430 pounds) 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 (Hersh and Duffield, 1990) 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, recorded an average adult length of just under 4m (13 feet). This compares with a 2.5m (8 feet) average in a population off Florida. Those in colder waters also have a fattier 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

A wild Bottlenose Dolphin playing in the wake of a boat in Florida.

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

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 ' microsleep'.

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 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. Three lifeguards, swimming 100m off the coast near Whangarei, were reportedly approached by a 3m 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. (Thomson, 2004)

Dolphins are predators however, and they also show aggressive behaviors. This includes fights among males for rank and access to females, as well as aggressions 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 and 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: here the sensitivity of the sound receptor is attenuated.) Also, 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 sounds. Details of the dolphin's echolocation, such as signal strength, spectral qualities, discrimination abilities, etc., have been well investigated by researchers such as Whitlow Au. Also, Pack & Herman (1995) demonstrated that bottlenosed dolphins are able to extract shape information from their echolocative sense, suggesting that they are able to form an "echoic image" of their targets.

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 olfactory nerves as well as the olfactory lobe in the brain are missing. 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. Anecodotally, 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 blow hole (they lack vocal cords). However the exact physical mechanism for the production of vocalizations is controversial, the two primary theories being laryngeal , and nasal (monkey lips/dorsal bursae complex) origin. 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 (1984) 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.

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 since the late 70s through the present, and include the specific areas of: acoustic mimicry, behavioral mimicry (inter- and intra-specific), comprehension of novel sequences in an [animal language|artificial language]] (including non finite state grammars as well as novel anomalous sequences), memory, monitoring of self behaviors (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 behaviors 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 then many human beings could do. Some researchers include Louis Herman, Mark Xitco, John Gory, Stan Kuczaj, Lori Marino, Diana Reiss, Adam Pack, and many others.

Tool use and culture

In 1997, tool use was described in Bottlenose Dolphins in Shark Bay, Western Australia. A dolphin will stick a marine sponge on its beak, presumably to protect it when searching for food in the sandy sea bottom. (Smolker, et al., 1997) 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. An elaborate study in 2005 showed that mothers most likely teach the behaviour to their daughters. (Krutzen, et al., 2005). 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 behaviors are transmitted through cultural means. Rendell & Whitehead (2001) have proposed a structure for the study of culture in cetaceans, although this view has been controversial (e.g. see Premack & Hauser).

Sexuality and reproduction

The male has two slits at the bottom side of the body: one hiding the penis and further behind one for the anus. The female has one genital slit, housing the vagina and the anus.

Courtship behaviour of the male includes clinging along to that female, posing for the female, stroking, rubbing, nuzzling, mouthing, jaw clapping, and yelping. Copulation is preceded by lengthy foreplay; then the two animals arrange belly to belly, the penis extends out of its slit and is inserted into the vagina. The act lasts only 10-30 seconds, but is repeated numerous times, with several minutes break in between.

The gestation period is 12 months. The young are born in shallow water, sometimes assisted by a "midwife" (which may be male). A single calf is born, about 1 meter (3 feet) long at birth.

To speed up the nursing process, the mother can eject milk from her mammary glands. There are two slits, one on either side of the genital slit, each housing one nipple. The calf is nursed for 12 to 18 months.

The young live closely with their mother for up to 6 years; the males are not involved in the raising of their offspring. The females become sexually mature at age 5-12, the males a bit later, at age 10-12.

Janet Mann, a professor of biology and psychology at Georgetown University, argues that the common same-sex behaviour among male dolphin calves is about bond formation, and benefits the species evolutionarily. She cites studies showing that dolphins later in life as adults are bisexual, and the male bonds forged from homosexuality work for protection as well as locating females to reproduce with.

Male Bottlenose Dolphins have been observed working in pairs to follow and/or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. The same pairs have also been observed engaging in intense sexual play with each other.

Natural predators

Large shark species such as tiger sharks, dusky sharks, and bull sharks prey on Bottlenose Dolphins. However, the Dolphins are far from helpless against the predators as they have been known to fight back through charges. Orcas may also prey on them, 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 (and reported in Rice (1998), the standard work on the taxonomy of cetaceans [1]) 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
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.

Unfortunately 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. See [1].

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 (LeDuc, et al., 1999). The taxonomic situation of these animals is likely to remain in flux for some time to come.

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. (Curran, et al., 1996)

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.

Bottlenose Dolphins and humans

Some people kill Bottlenose Dolphins for food 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 don't 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.

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.

A unique collaboration has developed in the town of Laguna in south Brazil: a pod of Bottlenose Dolphins drive fish towards fishermen who stand at the beach in shallow waters. Then one dolphin rolls 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.

Bottlenose Dolphins in fiction

The popular television show Flipper, created by Ivan Tors, portrayed a botttlenose dolphin in a friendly relationship with two boys, Sandy and Bud; a kind of sea going 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 (actor), as well as a series running from 1995-2000 starring Jessica Alba.

In the 1988 anime series Gunbuster, dolphins are seen as part of the crew of the Earth space battleship Excelion.

In the television series Star Trek: The Next Generation, a crew of twelve Bottlenose Dolphins (Tursiops truncatus and Tursiops truncatus gilli) serve as elite specialists on the USS Enterprise-D and other Galaxy class starship ships who research guidance and navigation issues. They are supervised by two Takaya's Whale, a fictional subspecies (Orcinus orca takayai) of the orca or killer whale (named after the character Noriko Takaya from the anime series Gunbuster, of which Next Generation technical consultant Rick Sternbach is a fan). Though this cetacean team is described in Star Trek: The Next Generation Technical Manual and referred to peripherally in two episodes (" Relics" and " The Perfect Mate"), it is never actually shown on screen.

Ensign Darwin was a 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.

In The Life Aquatic with Steve Zissou, marine researcher Zissou (played by Bill Murray) has trained reconaissance dolphins which apparently are temperamental and rarely follow their instructions. In one scene, the dolphins' misbehavior elicits the following quote from Zissou: "Son of a bitch, I'm sick of these dolphins."

In the book Startide Rising by author David Brin, the spaceship Streaker is manned by neo-dolphins ( dolphins genetically engineered to match human intelligence). One of the mates of the ship is named Akeakamai, in honor of the real-life dolphin from Louis Herman's animal language research.

In Anne McCaffrey's Dragonriders of Pern book series, genetically-altered dolphins capable of speech work closely with human "dolphineers" in a variety of useful capacities.

Factual descriptions of the Bottlenose Dolphin date back into antiquity - the writings of Aristotle, Oppian and Pliny the Elder all mention the species (Perrin et al, 2002).