The Bottlenose Dolphin (Tursiops truncatus) The Bottlenose dolphin 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. 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 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 Senses and communication 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). 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 Tool use and culture Sexuality and reproduction Courtship behavior 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 behavior 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 Taxonomy 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 Pacific Bottlenose Dolphin (T. gillii or T. truncatus gillii), living in the Pacific; has a black line from the eye to the forehead 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 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 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. An interesting 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. Similar cooperative fisheries also exist in Africa, and have been reported through recorded history. |
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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.
