Bottlenose Dolphins

Tursiops

Summary 3

Bottlenose dolphins, the genus Tursiops, are the most common and well-known members of the family Delphinidae, the family of oceanic dolphin. Recent molecular studies show the genus contains two species, the common bottlenose dolphin (Tursiops truncatus) and the Indo-Pacific bottlenose dolphin (Tursiops aduncus), instead of one. Research in 2011 revealed a third species, the Burrunan dolphin (Tursiops australis). Bottlenose dolphins inhabit warm and temperate seas worldwide.

Description 4

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 countershading makes them hard to see, both from above and below, when swimming. Adults range from 2 to 4 metres (6.6 to 13.1 ft), and 150 to 650 kilograms (330 to 1,430 lb). Males are, on average, slightly longer and considerably heavier than females. In most parts of the world, the adult's are about 2.5 m (8.2 ft), and 200 to 300 kilograms (440 to 660 lb). Their size varies considerably with habitat. Except in the eastern Pacific, dolphins in warmer, shallower waters tend to be smaller than those in cooler, pelagic waters.

Bottlenose dolphins can live for more than 40 years. However, one study of a population off Sarasota, Florida, indicated an average lifespan of 20 years or less.

Ecology 4

A dolphin's diet consists mainly of small fish, crustaceans, and squid. Although this varies by location, many populations share an appetite for fish from the mullet, the tuna and mackerel, and the drum and croaker families. Its cone-like teeth serve to grasp, but do not chew food. When they encounter a shoal of fish, they work as a team to herd them towards the shore to maximize the harvest. They also hunt alone, often targeting bottom-dwelling species. The bottlenose dolphin sometimes hits a fish with its fluke, sometimes knocking it out of the water, using a strategy called "fish whacking". "Strand feeding", is an inherited feeding technique used by bottlenose dolphins near and around coastal regions of Georgia and South Carolina. When a pod finds a school of fish, they will circle the school and trap the fish in a mini whirlpool. Then, the dolphins will charge at the school and push their bodies up onto a mud-flat, forcing the fish on the mud-flat, as well. The dolphins then crawl around on their sides, consuming the fish they washed up on shore.

One type of feeding behavior seen in bottlenose dolphins is mud ring feeding.

Bottlenose dolphins conflict with small-scale coastal commercial fisheries in some Mediterranean areas. Common bottlenose dolphins are probably attracted to fishing nets because they offer a concentrated food source.

Dolphins can exhibit altruistic behaviour toward other sea creatures. On Mahia Beach, New Zealand, on March 10, 2008, two pygmy sperm whales, a female and calf, stranded on the beach. Rescuers, including Department of Conservation officer Malcolm Smith, attempted to refloat them four times. Shortly, a playful bottlenose dolphin known to local residents as Moko arrived and, after apparently vocalizing at the whales, led them 200 m (660 ft) along a sandbar to the open sea, saving them from imminent euthanasia.

The bottlenose dolphin can behave aggressively. Males fight for rank and access to females. During mating season, males compete vigorously with each other through displays of toughness and size, with a series of acts, such as head-butting. They display aggression towards sharks and smaller dolphin species. At least one population, off Scotland, has practiced infanticide, and also has attacked and killed harbour porpoises. University of Aberdeen researchers say the dolphins do not eat their victims, but are simply competing for food. However, Dr. Read of Duke University, a porpoise expert researching similar cases of porpoise killings that had occurred in Virginia in 1996 and 1997, holds a different view. He states dolphins and porpoises feed on different types of fish, thus food competition is an unlikely cause of the killings. Similar behaviour has been observed in Ireland. In the first half of July, 2014, four attacks with three Porpoise fatalities were observed and caught on video by the Cardigan Bay Marine Wildlife Centre in the Cardigan Bay.

The bottlenose dolphin sometimes forms mixed species groups with other species from the dolphin family, particularly larger species, such as the short-finned pilot whale, the false killer whale and Risso's dolphin. They also interact with smaller species, such as the Atlantic spotted dolphin and the rough-toothed dolphin. While interactions with smaller species are sometimes affiliative, they can also be hostile.

Some large shark species, such as the tiger shark, the dusky shark, the great white shark and the bull shark, prey on the bottlenose dolphin, especially calves. The bottlenose dolphin is capable of defending itself by charging the predator; dolphin 'mobbing' behavior of sharks can occasionally prove fatal for the shark. Targeting a single adult dolphin can be dangerous for a shark of similar size. Killer whale populations in New Zealand and Peru have been observed preying on bottlenose dolphins, but this seems rare, and other orcas may swim 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 mobbing techniques to batter the predator to death or force it to flee.

Life history 4

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. Dolphins are voluntary breathers, who must deliberately surface and open their blowholes to get air. They can store almost twice as much oxygen in proportion to their body weight as a human can: the dolphin can store 36 milliliters (ml) of oxygen per kg of body weight, compared with 20 ml per kg for humans. This is an adaptation to diving. The bottlenose dolphin typically rises to the surface to breathe through its blowhole two to three times per minute, although it can remain submerged for up to 20 minutes.

Dolphins can breathe while "half-asleep". During the sleeping cycle, one brain hemisphere remains active, while the other hemisphere shuts down. The active hemisphere handles surfacing and breathing behavior. The daily sleeping cycle lasts for about 8 hours, in increments of minutes to hours. During the sleeping cycle, they remain near the surface, swimming slowly or "logging", and occasionally closing one eye.

Both sexes have genital slits on the underside of their bodies. The male can retract and conceal his penis through his slit. The female's slit houses her vagina and anus. Females have two mammary slits, each housing one nipple, one on each side of the genital slit. The ability to stow their reproductive organs (especially in males) allows for maximum hydrodynamics. The breeding season produces significant physiological changes in males. At that time, the testes enlarge, enabling them to hold more sperm. Large amounts of sperm allow a male to wash away the previous suitor's sperm, while leaving some of his own for fertilization. Also, sperm concentration markedly increases. Having less sperm for out-of-season social mating means it wastes less. This suggests sperm production is energetically expensive. Males have large testes in relation to their body size.

During the breeding season, males compete for access to females. Such competition can take the form of fighting other males or of herding females to prevent access by other males. In Shark Bay, male bottlenose dolphins have been observed working in pairs or larger groups to follow and/or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. These coalitions, also known as male reproductive alliances, will fight with other coalitions for control of females.

Mating occurs belly to belly. Dolphins have been observed engaging in intercourse when the females are not in their estrous cycles and cannot produce young, suggesting they may mate for pleasure. The gestation period averages 12 months. Births can occur at any time of year, although peaks occur in warmer months. The young are born in shallow water, sometimes assisted by a (possibly male) "midwife", and usually only a single calf is born. Twins are possible, but rare. Newborn bottlenose dolphins are 0.8 to 1.4 m (2.6 to 4.6 ft) long and weigh 9 to 30 kg (20 to 66 lb), with Indo-Pacific bottlenose dolphin infants being generally smaller than common bottlenose dolphin infants. To accelerate nursing, the mother can eject milk from her mammary glands. The calf suckles for 18 months to up to 8 years, and continues to closely associate with its mother for several years after weaning. Females sexually mature at ages 5–13, males at ages 9–14. Females reproduce every two to six years. Georgetown University professor Janet Mann argues the strong personal behavior among male calves is about bond formation and benefits the species in an evolutionary context. She cites studies showing these dolphins as adults are inseparable, and that early bonds aid protection, as well as in locating females.

Adult males live mostly alone or in groups of two to three, and join pods for short periods of time. Adult females and young dolphins normally live in groups of up to 15 animals. However, they live in fission-fusion societies of varying group size, within which individuals change associations, often on a daily or hourly basis. Group compositions are usually determined by sex, age, reproductive condition, familial relations and affiliation histories. In a dolphin community near Sarasota, Florida, the most common group types are adults females with their recent offspring, older subadults of both sexes and adult males either alone or in bonded pairs. Smaller groups can join to form larger groups of 100 or more, and occasionally exceed 1,000. The social strategies of marine mammals such as bottlenose dolphins "provide interesting parallels" with the social strategies of elephants and chimpanzees.

Bottlenose dolphins studied by Bottlenose Dolphin Research Institute researchers off the island of Sardinia show random social behavior while feeding, and their social behavior does not depend on feeding activity. 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.

Sources and Credits

  1. (c) Alexandre Roux, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/30142279@N07/15168565369/
  2. (c) cskk, some rights reserved (CC BY-NC-ND), http://www.flickr.com/photos/58792328@N00/2490937815
  3. (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Tursiops
  4. (c) Wikipedia, some rights reserved (CC BY-SA), http://en.wikipedia.org/wiki/Bottlenose_dolphin

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