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Information about DolphinsDolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of Dolphin in seventeen genera. They vary in size from 1.2 m (4 ft) and 40 kg (90 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and 10 tonnes (9.8 LT; 11 ST) (the Orca or Killer Whale). They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. The family Delphinidae is the largest in the Cetacea, and relatively recent: dolphins evolved about ten million years ago, during the Miocene. Dolphins are considered to be amongst the most intelligent of animals and their often friendly appearance and seemingly playful attitude have made them popular in human culture.
Origin of the nameThe name is originally from Ancient Greek δελφίς (delphís; "Dolphin"), which was related to the Greek δελφύς (delphys; "womb"). The animal's name can therefore be interpreted as meaning "a 'fish' with a womb". The name was transmitted via the Latin delphinus, Middle Latin dolfinus and the Old French daulphin, which reintroduced the ph into the word.
The word is used in a few different ways. It can mean:
Any member of the family Delphinidae (oceanic dolphins),
Any member of the families Delphinidae and Platanistoidea (oceanic and river dolphins),
Any member of the suborder Odontoceti (toothed whales; these include the above families and some others),
Used casually as a synonym for Bottlenose Dolphin, the most common and familiar species of dolphin.
In this article, the second definition is used. Porpoises (suborder Odontoceti, family Phocoenidae) are thus not dolphins in this sense. Orcas and some closely related species belong to the Delphinidae family and therefore qualify as dolphins, even though they are called whales in common language. A group of dolphins can be called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".
AnatomyDolphins have a streamlined fusiform body, adapted for fast swimming. The tail fin, called the fluke, is used for propulsion, while the pectoral fins together with the entire tail section provide directional control. The dorsal fin, in those species that have one, provides stability while swimming.
Though it varies per species, basic colouration patterns are shades of grey usually with a lighter underside. It is often combined with lines and patches of different hue and contrast.
The head contains the melon, a round organ used for echolocation. In many species, the jaws are elongated, forming a distinct beak; for some species like the Bottlenose, there is a curved mouth which looks like a fixed smile. Teeth can be very numerous (up to two hundred and fifty) in several species. Dolphins breathe through a blowhole located on top of their head, with the trachea being anterior to the brain. The Dolphin brain is large and highly complex and is different in structure from that of most land mammals.
Unlike most mammals, dolphins do not have hair, but they are born with a few hairs around the tip of their rostrum which they lose shortly after birth, in some cases even before they are born. The only exception to this is the Boto river Dolphin, which does have some small hairs on the rostrum.
Their reproductive organs are located on the underside of the body. Males have two slits, one concealing the penis and one further behind for the anus. The female has one genital slit, housing the vagina and the anus. A mammary slit is positioned on either side of the female's genital slit.
SensesMost dolphins have acute eyesight, both in and out of the water, and their perception of sound extends ten times or more above the upper limit of adult human hearing. Though they have a small ear opening on each side of their head, it is believed that hearing underwater is also if not exclusively done with the lower jaw which conducts the sound vibrations to the middle ear via a fat-filled cavity in the lower jaw bone. Hearing is also used for echolocation, which seems to be an ability all dolphins have. It is believed that their teeth are arranged in a way that works as an array or antenna to receive the incoming sound and make it easier for them to pinpoint the exact location of an object. The Dolphin's sense of touch is also well-developed, with free nerve endings being densely packed in the skin, especially around the snout, pectoral fins and genital area. However, dolphins lack an olfactory nerve and lobes and thus are believed to have no sense of smell, but they can taste and do show preferences for certain kinds of fish. Since dolphins spend most of their time below the surface normally, just tasting the water could act in a manner analogous to a sense of smell.
Though most dolphins do not have any hair, they do still have hair follicles and it is believed these might still perform some sensory function, though it is unclear what exactly this may be. The small hairs on the rostrum of the Boto river Dolphin are believed to function as a tactile sense however, possibly to compensate for the Boto's poor eyesight.
BehaviourDolphins are often regarded as one of Earth's most intelligent animals, though it is hard to say just how intelligent dolphins are, as comparisons of species' relative intelligence are complicated by differences in sensory apparatus, response modes, and nature of cognition. Furthermore, the difficulty and expense of doing experimental work with large aquatics means that some tests which could yield meaningful results still have not been carried out, or have been carried out with inadequate sample size and methodology. Compared to many other species however, Dolphin behaviour has been studied extensively by humans, both in captivity and in the wild. See the cetacean intelligence article for more details.
Social behaviourDolphins are social, living in pods (also called "schools") of up to a dozen individuals. In places with a high abundance of food, pods can join temporarily, forming an aggregation called a superpod; such groupings may exceed a thousand dolphins. The individuals communicate using a variety of clicks, whistles and other vocalizations. They also use ultrasonic sounds for echolocation. Membership in pods is not rigid; interchange is common. However, the cetaceans can establish strong bonds between each other. This leads to them staying with injured or ill individuals, even actively helping them to breathe by bringing them to the surface if needed. This altruistic behaviour does not appear to be limited to their own species however. A Dolphin in New Zealand that goes by the name of Moko has been observed to seemingly help guide a female Pygmy Sperm Whale together with her calf out of shallow water where they had stranded several times. They have also been known to seemingly protect swimmers from sharks by swimming circles around the swimmers or charging the sharks to make them go away.
Dolphins also show cultural behaviour, something long believed to be a quality unique to humans (or to humans and some other primate species). In May 2005 (12 years ago), a discovery was made in Australia which shows this cultural aspect of Dolphin behaviour: Some dolphins, such as the Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) teach their young to use tools. The dolphins break sponges off and cover their snouts with them thus protecting their snouts while foraging. This knowledge of how to use a tool is mostly transferred from mothers to daughters, unlike simian primates, where the knowledge is generally passed on to both sexes. The technology to use sponges as mouth protection is not genetically inherited but a taught behaviour. Another such behaviour was discovered amongst river dolphins in Brazil, where some male dolphins apparently use objects such as weeds and sticks as part of a sexual display.
Dolphins are known to engage in acts of aggression towards each other. The older a male Dolphin is, the more likely his body is covered with scars ranging in depth from teeth marks made by other dolphins. It is suggested that male dolphins engage in such acts of aggression for the same reasons as humans: disputes between companions or even competition for other females. Acts of aggression can become so intense that targeted dolphins are known to go into exile, leaving their communities as a result of losing a fight with other dolphins.
Male Bottlenose Dolphins have been known to engage in infanticide. Dolphins have also been known to kill porpoises for reasons which are not fully understood, as porpoises generally do not share the same fish diet as dolphins and are therefore not competitors for food supplies.
Reproduction and sexualityDolphin copulation happens belly to belly and though many species engage in lengthy foreplay, the actual act is usually only brief, but may be repeated several times within a short timespan. The gestation period varies per species; for the small Tucuxi Dolphin, this period is around 11 to 12 months, while for the Orca the gestation period is around 17 months. They usually become sexually active at a young age, even before reaching sexual maturity. The age at which sexual maturity is reached varies per species and gender.
Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in acts of a homosexual nature. Various Dolphin species have been known to engage in sexual behaviour with other Dolphin species, this also having resulted in various hybrid Dolphin species as mentioned earlier. Sexual encounters may be violent, with male dolphins sometimes showing aggressive behaviour towards both females and other male dolphins. Occasionally, dolphins will also show sexual behaviour towards other animals, including humans.
FeedingVarious methods of feeding exist, not just between species but also within a species. Various methods may be employed, some techniques being used by only a single Dolphin population. Fish and squid are the main source of food for most Dolphin species, but the False Killer Whale and the Killer Whale also feed on other marine mammals.
One feeding method employed by many species is herding, where a pod will control a school of fish while individual members take turns plowing through the school, feeding. The tightly packed school of fish is commonly known as a bait ball. Coralling is a method where fish are chased to shallow water where they are more easily captured. In South Carolina, the Atlantic Bottlenose Dolphin takes this one step further with what has become known as strand feeding, where the fish are driven onto mud banks and retrieved from there. In some places, Orcas will also come up to the beach to capture sea lions. Some species also whack fish with their fluke, stunning them and sometimes sending fish clear out of the water.
Reports of cooperative human-dolphin fisheries date back to the ancient Roman author and natural philosopher Pliny the Elder. A modern human-dolphin fishery still takes place in Laguna, Santa Catarina, Brazil. Here, dolphins drive fish towards fishermen waiting along the shore and give them a signal when they can cast their nets. The dolphins then feed off the fish that manage to escape the nets.
VocalizationsDolphins are capable of making a broad range of sounds using nasal airsacs located just below the blowhole. Roughly three categories of sounds can be identified however; frequency modulated sounds which are usually just called whistles; burst-pulsed sounds and clicks. Whistles are used by dolphins to communicate, though the nature and extent of their ability to communicate in this way is not known. Research has shown however that at least some Dolphin species are capable of sending identity information to each other using a signature whistle; a whistle that refers specifically to the identity of a certain dolphin. The burst-pulsed sounds are also used for communication, but again the nature and extent of communication possible this way is not known. The clicks are directional and used by dolphins for echolocation and are often in a short series called a click train, the rate increasing when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by animals in the sea.
Pacific White-Sided Dolphins breaching
Jumping and playing
Dolphins occasionally leap above the water surface, sometimes performing acrobatic figures (e.g. the Spinner Dolphin). Scientists are not always quite certain about the purpose of this behaviour and the reason for it may vary; it could be to locate schools of fish by looking at above-water signs like feeding birds, they could be communicating to other dolphins to join a hunt, attempting to dislodge parasites, or simply doing it for fun.
Play is a fairly important part of dolphins' lives, and they can be observed playing with seaweed or play-fighting with other dolphins. At times they also harass other local creatures, like seabirds and turtles. Dolphins also seem to enjoy riding waves and frequently 'surf' coastal swells and the bow waves of boats. Occasionally, they're also willing to playfully interact with human swimmers.
SleepingBecause dolphins need to come up to the surface to breathe and have to be alert for possible predators, they do not sleep in the same way land mammals do. Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining some amount of consciousness required to breathe and keeping one eye open to keep a watch out for possible threats. The earlier stages of sleep can be observed in both hemispheres of the brain, however.
However, in captivity, dolphins have been observed to seemingly enter a fully asleep state where both eyes are closed and the animal does not respond to mild external stimuli, respiration being automatic with a tail kick reflex keeping the blowhole above the water. If not needed to keep the blowhole above the water, the tail kick reflex may subside. Dolphins kept unconscious using anesthetics initially show a similar tail kick reflex. Though a similar state has been observed with wild Sperm Whales, it is not known if this state is ever reached in the wild amongst any Dolphin species.
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