Bottlenose Dolphin
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| Bottlenose Dolphin Conservation status: Data deficient | ||||||||||||||
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| Tursiops truncatus Montagu, 1821 |
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.
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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. This 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. 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 appear has been restricted to the North Atlantic Ocean, where researchers (see 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
Bottlenose_Dolphin_KSC04pd0178.jpg
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. (On average, they breathe more often however, 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.
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. It is not uncommon for a diver to be investigated by a group of them. Occasionally, dolphins have rescued an injured diver by raising them to the surface, a behaviour they also show towards injured members of their own species. 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. (See [6].)
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 wacking" 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 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 turning down the volume 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 turned down.)
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.
By contrast, their sense of smell is very poor.
Bottlenose Dolphins communicate with body movements and with sounds they produce using six air sacs near their blow hole (they lack vocal cords). Each animal has a characteristic signature sound with which it identifies itself to others. Other communication uses about 30 distinguishable sounds, but a "dolphin language" has not been found. See also the article on the dolphin brain for some general information about the intelligence of dolphins.
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. The behavior 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 behavior to their daughters. See [7].
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 behavior of the male includes clinging along to that female, posing for the female, stroking, rubbing, nuzzling, mouthing, jaw clapping, and yelping. Coitus 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.
Natural predators
Large shark species such as tiger sharks, dusky sharks, and bull sharks prey on Bottlenose Dolphins. 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; 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.
Unfortunately much of the old scientific data in the field combines 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] (http://www.iucnredlist.org/search/details.php?species=22563).
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 ([3]). 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 under threat 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.
Bottlenose Dolphins and humans
Humans 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 performing.
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 attaching mines to enemy ships, locating sea mines, or fighting off enemy divers. The U.S. operations are 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 behavior; the collaboration has been going on at least since 1847.
Bottlenose Dolphins in fiction
The eponymous hero of the television series Flipper was a Bottlenose Dolphin living in the waters off the Florida Keys.
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. 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"), we never actually see them on screen.
Ensign Darwin was a 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 found to be a lot more intelligent than presumed and escape the Earth before it is exploded.
Notable books include the Uplift Universe series, by David Brin, which deals with alien civilizations and a spaceship from earth, crewed by geneticly modified dolphins.
References
- Rice, Dale W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. 231 pp.
- Hale, P.T., Barreto, A.S. and Ross, G.J.B. (2000). Comparative morphology and distribution of the aduncus and truncatus forms of bottlenose dolphin Tursiops in the Indian and Western Pacific Oceans. Aquatic Mammals 26(2): 101-110. Discussing distinguishing features between Bottlenose Dolphin species
- LeDuc R.G., Perrin W.F. and Dizon A.E. (1999). Phylogenetic relationships among the delphinids cetaceans based on full cyctochrome b sequences. Marine Mammal Science Vol 15, pages 619-648.
- Hersh, Sandra L. and Deborah A. Duffield. Distinction Between Northwest Atlantic Offshore and Coastal Bottlenose Dolphins Based on Hemoglobin Profile and Morphometry. In The Bottlenose Dolphin, edited by Stephen Leatherwood and Randall R. Reeves, pp. 129-139. San Diego Academic Press, 1990
- Curran, S., Wilson, B. and Thompson, P. 1996. Recommendations for the sustainable management of the bottlenose dolphin population in the Moray Firth. Scottish Natural Heritage Review. No. 56.
- Ainsley Thomson. Dolphins saved us from shark, lifeguards say (http://www.nzherald.co.nz/storydisplay.cfm?storyID=3613343&thesection=news&thesubsection=general), New Zealand Herald, 25 November 2004.
- Andreas von Bubnoff. Dolphin mothers pass tool use to daughters (http://www.nature.com/news/2005/050606/full/050606-2.html), Nature News Service, 6 June 2005
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