海豚的尺寸大小，小至1.2公尺長40公斤重的毛利海豚(Maui's Dolphin)，大到9.5公尺長10公噸重的殺人鯨(Killer Whale or the Orca)。
名字的起源Origin of the name
à任何海豚科(family Delphinidae)及淡水豚總科(family Platanistoidea)的成員：海洋中及河流中的海豚
至於殺人鯨(Orcas or Killer Whale)和一些相近的種類，即使一般口語上稱為「鯨」，但他們確實屬於海豚科，因此被界定為海豚。
一般瓶鼻海豚的混種海豚都居住在加州海域，其它混種海豚分布在世界各地，有些是人工飼養，而有些則是野生混種海豚，像是瓶鼻和大西洋斑點圓海豚混種(Bottlenose-Atlantic Spotted hybrid)。
最著名的混種海豚是瓶鼻海豚和偽虎鯨雜交(False Killer Whale-Bottlenose Dolphin hybrid)生育的後代，稱為Wolphin。Wolphin是可以繁殖後代的混種海豚。目前有2頭Wolphin生活在夏威夷的Sea Life Park，第一隻在1985年出生，他的父母是雄性偽虎鯨和雌性瓶鼻海豚。除了人工飼養以外，也曾在野外發現Wolphin的蹤跡。
演化和解剖Evolution and anatomy
唯一有毛髮的海豚是Boto river dolphin，出生後少量的毛髮會持續存在吻突處。
雖然海豚沒有毛髮，他們卻有毛囊可以執行某些感覺功能。長在Boto river dolphin吻突處的微少毛髮，它的功能被認為是為了彌補不良視力的觸覺能力。
社會行為 Social behavior
繁殖與性行為Reproduction and sexuality
一種常見的方法是herding，一群海豚迫使大魚群成一個數量較小的餌球，海豚群的成員則輪流如犁田般捕食形成球狀，驚慌失措的魚群。Corralling是一種海豚把追趕到淺水以更容易抓到他們的捕食方法。在南卡羅萊納州，大西洋瓶鼻海豚更進一步的with strand feeding,，將獵物驅趕至泥堆中，使捕食更容易。在某些地方，虎鯨甚至會到海灘捕食海獅。
海豚能夠使用位於吸呼氣孔下方的鼻腔氣囊發出各式各樣的聲音，粗略可概界定出3種聲音：調頻哨聲(frequency modulated whistle)、突發如脈衝的聲音(burst-pulsed sounds)和咔達聲(clicks)。海豚的溝通透過哨聲和突發脈衝聲，雖然他們這種能力的本質和可發揮到何種程度並不被我們所了解。
咔達聲是用來定向和回聲定位，通常他們會發出一小段連串的咔達聲稱為「a click train」。
太平洋斑海豚擊浪Pacific White-Sided Dolphins breaching
一般而言，海豚睡覺時，一次只有1個腦半球處於慢波睡眠，因此可以維持清醒的意識呼吸空氣並觀察可能出現的掠食者或威脅。較早的睡眠階段可以同時發生在2個腦半球。人工飼養的海豚似乎進入全然沉睡的狀態，他們的雙眼緊閉並且對輕微的外部刺激沒有反應，他們的呼吸是自動的，如果必要的話，他們的尾巴會反射性的打水(a tail kick reflex)使呼吸孔保持在水面之上。
除了人類(於下文討論)，海豚少有天敵。有些種類的海豚或一些特別的族群是沒有天敵的，這使他們成為頂級掠食者，而對大多體型較小的海豚而言，特別是這些海豚的幼豚，潛在的危險只有體型較大的種類，像是公牛鯊(bull shark)、灰色白眼鮫(dusky shark)、虎鯊(tiger shark)和大白鯊(great white shark)。
各種補魚方法都不經意的殺害了許多海豚，最有名的如圍網捕捉金槍魚(seine fishing for tuna)和使用流刺網(drift and gill nets)。流刺網和防止掠食者的魚網(antipredator nets)意外的捕獲在保護海洋漁場是常見的，但卻對當地的海豚族群造成風險。
有些地區如日本的太地町(Taiji)和丹麥的法羅群島(Faroe Islands)在傳統上認為海豚是一種食物，且以魚叉及驅趕漁法(drive hunts)殘害海豚。
海豚安全標籤(Dolphin safe labels)試圖安撫消費者魚魚和其它海中產品的捕殺方式是友善無痛苦的。
Bottlenose Dolphin breaching in the bow wave of a boat.
Dolphins 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 Cetacean order, and relatively recent: dolphins evolved about ten million years ago, during the Miocene. Dolphins are among the most intelligent animals and their often friendly appearance and seemingly playful attitude have made them popular in human culture.
Origin of the name
The 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.
This article uses the second definition and does not describe Porpoises (suborder Odontoceti, family Phocoenidae). 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 is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves".
In 1933, three strange dolphins beached off the Irish coast; they appeared to be hybrids between Risso's and Bottlenose Dolphins. This mating was later repeated in captivity producing a hybrid calf. In captivity, a Bottlenose Dolphin and a Rough-toothed Dolphin produced hybrid offspring. A Common-Bottlenose hybrid lives at SeaWorld California. Other dolphin hybrids live in captivity around the world or have been reported in the wild, such as a Bottlenose-Atlantic Spotted hybrid. The best known hybrid is the Wolphin, a False Killer Whale-Bottlenose Dolphin hybrid. The Wolphin is a fertile hybrid. Two Wolphins currently live at the Sea Life Park in Hawaii; the first was born in 1985 from a male False Killer Whale and a female Bottlenose. Wolphins have also been observed in the wild.
Evolution and anatomy
Dolphins, along with whales and porpoises, are descendants of terrestrial mammals, most likely of the Artiodactyl order. The ancestors of the modern day dolphins entered the water roughly fifty million years ago, in the Eocene epoch.
Modern dolphin skeletons have two small, rod-shaped pelvic bones thought to be vestigial hind limbs. In October 2006 an unusual Bottlenose Dolphin was captured in Japan; it had small fins on each side of its genital slit which scientists believe to be a more pronounced development of these vestigial hind limbs.
Dolphins 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 coloration patterns are shades of grey usually with a lighter underside, often with lines and patches of different hue and contrast.
The head contains the melon, a round organ used for echolocation. In many species, elongated jaws form a distinct beak; species such as the Bottlenose have a curved mouth which looks like a fixed smile. Some species have up to 250 teeth. Dolphins breathe through a blowhole on top of their head. The trachea is 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, except for a few hairs around the tip of their rostrum which they lose shortly before or after birth. The only exception to this is the Boto river dolphin, which has persistent small hairs on the rostrum.
Dolphin’s 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.
A recent study at the US National Marine Mammal Foundation revealed that dolphins are the only animals other than humans that develop a natural form of Type 2 Diabetes, which may lead to a better understanding of the disease and new treatments for both humans and dolphins.
Most dolphins have acute eyesight, both in and out of the water, and they can hear frequencies 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 sound to the middle ear via a fat-filled cavity in the lower jaw bone. Hearing is also used for echolocation, which all dolphins have. It is believed that dolphin teeth function as an antenna to receive incoming sound and to pinpoint the exact location of an object. The dolphin's sense of touch is also well-developed, with free nerve endings 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. They do have a sense of taste and show preferences for certain kinds of fish. Since dolphins spend most of their time below the surface, tasting the water could function like smelling, in that substances in the water can signal the presence of objects that are not in the dolphin’s mouth.
Though most dolphins do not have hair, they do have hair follicles that may perform some sensory function. The small hairs on the rostrum of the Boto river dolphin are believed to function as a tactile sense possibly to compensate for the Boto's poor eyesight.
Dolphins are often regarded as one of Earth's most intelligent animals, though it is hard to say just how intelligent. Comparing species' relative intelligence is complicated by differences in sensory apparatus, response modes, and nature of cognition. Furthermore, the difficulty and expense of experimental work with large aquatic animals has so far prevented some tests and limited sample size and rigor in others. Compared to many other species however, dolphin behavior has been studied extensively, both in captivity and in the wild. See cetacean intelligence for more details.
Dolphins are social, living in pods of up to a dozen individuals. In places with a high abundance of food, pods can merge temporarily, forming a superpod; such groupings may exceed 1,000 dolphins. Individuals communicate using a variety of clicks, whistles and other vocalizations. They make ultrasonic sounds for echolocation.
Membership in pods is not rigid; interchange is common. However, dolphins can establish strong social bonds. Dolphins will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed.
This altruism does not appear to be limited to their own species however. The dolphin Moko in New Zealand has been observed guiding a female Pygmy Sperm Whale together with her calf out of shallow water where they had stranded several times. They have also been seen protecting swimmers from sharks by swimming circles around the swimmers or charging the sharks to make them go away.
Dolphins also display culture, something long believed to be unique to humans (and possibly other primate species). In May 2005, a discovery in Australia found Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) teaching their young to use tools. They cover their snouts with sponges to protect them while foraging. This knowledge is mostly transferred by mothers to daughters, unlike simian primates, where knowledge is generally passed on to both sexes. Using sponges as mouth protection is a learned behavior. Another learned behavior was discovered among river dolphins in Brazil, where some male dolphins use weeds and sticks as part of a sexual display.
Dolphins engage in acts of aggression towards each other. The older a male dolphin is, the more likely his body is to be covered with bite scars. Male dolphins engage in such acts of aggression apparently for the same reasons as humans: disputes between companions and competition for females. Acts of aggression can become so intense that targeted dolphins sometimes go into exile as a result of losing a fight.
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 diet as dolphins and are therefore not competitors for food supplies.
Reproduction and sexuality
Dolphin copulation happens belly to belly and though many species engage in lengthy foreplay, the actual act is usually 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 of sexual maturity varies by species and gender.
Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in homosexual behavior. Various species sometimes engage in sexual behavior including copulation with other dolphin species. Sexual encounters may be violent, with male dolphins sometimes showing aggressive behavior towards both females and other males. Occasionally, dolphins behave sexually towards other animals, including humans.
Various methods of feeding exist among and within species, some apparently exclusive to a single population. Fish and squid are the main food, but the False Killer Whale and the Killer Whale also feed on other marine mammals.
One common feeding method is herding, where a pod squeezes a school of fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish. Coralling is a method where dolphins chase fish into shallow water to more easily catch them.
In South Carolina, the Atlantic Bottlenose Dolphin takes this further with strand feeding, driving prey onto mud banks for easy access. In some places, Orcas come to the beach to capture sea lions. Some species also whack fish with their fluke, stunning them and sometimes knocking them out of the water.
Reports of cooperative human-dolphin fishing date back to the ancient Roman author and natural philosopher Pliny the Elder. A modern human-dolphin partnership currently operates in Laguna, Santa Catarina, Brazil. Here, dolphins drive fish towards fishermen waiting along the shore and signal the men to cast their nets. The dolphins’ reward is the fish that escape the nets.
Dolphins 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: frequency modulated whistles, burst-pulsed sounds and clicks. Dolphins communicate with their whistles and burst-pulsed sounds, though the nature and extent of that ability is not known. At least some dolphin species can identify themselves using a signature whistle. The clicks are directional and are for echolocation, often occurring in a short series called a click train. The click rate increases when approaching an object of interest. Dolphin echolocation clicks are amongst the loudest sounds made by marine animals.
Pacific White-Sided Dolphins breaching
Dolphins occasionally leap above the water surface, sometimes performing acrobatic figures (e.g. the Spinner Dolphin). Scientists are not certain about the purpose(s) of the acrobatics. Possibilities include locating schools of fish by looking at above-water signs like feeding birds, communicating with other dolphins, dislodging parasites or simple amusement.
Play is an important part of dolphin culture. Dolphins play with seaweed and play-fight with other dolphins. At times they harass other local creatures, like seabirds and turtles. Dolphins enjoy riding waves and frequently surf coastal swells and the bow waves of boats, at times “leaping” between the dual bow waves of a moving catamaran. Occasionally, they playfully interact with swimmers.
Generally, dolphins sleep with only one brain hemisphere in slow-wave sleep at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats. Earlier sleep stages can occur simultaneously in both hemispheres. In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external stimuli. Respiration is automatic; a tail kick reflex keeps the blowhole above the water if necessary. Anesthetized dolphins initially show a tail kick reflex. Though a similar state has been observed with wild Sperm Whales, it is not known if dolphins in the wild reach this state. The Indus river dolphin has a different sleep method from other dolphin species. Living in water with strong currents and potentially dangerous floating debris, it must swim continuously to avoid injury. As a result, this species sleeps in very short bursts which last between 4 and 60 seconds.
Except for humans (discussed below), dolphins have few natural enemies. Some species or specific populations have none, making them apex predators. For most smaller species, only a few larger species of shark such as the bull shark, dusky shark, tiger shark and great white shark are a potential risk, especially for calves. Some of the larger dolphin species such as Orcas may also prey on some of the smaller species, but this seems rare. Dolphins also suffer from a wide variety of diseases and parasites.
Some dolphin species face an uncertain future, especially some river dolphin species such as the Amazon River Dolphin, and the Ganges and Yangtze River Dolphin, which are critically or seriously endangered. A 2006 survey found no individuals of the Yangtze River Dolphin, which now appears to be functionally extinct.
Pesticides, heavy metals, plastics, and other industrial and agricultural pollutants that do not disintegrate rapidly in the environment concentrate in predators including dolphin from their prey. Injuries or deaths due to collisions with boats, especially their propellers, are also common.
Various fishing methods, most notably purse seine fishing for tuna and the use of drift and gill nets, unintentionally kill many dolphins. Accidental by-catch in gill nets and incidental captures in antipredator nets that protect marine fish farms are common and pose a risk for mainly local dolphin populations. In some parts of the world such as Taiji in Japan and the Faroe Islands, dolphins are traditionally considered as food, and killed in harpoon or drive hunts. Dolphin meat is high in mercury and may thus pose a health danger to humans when consumed.
Dolphin safe labels attempt to reassure consumers that fish and other marine products have been caught in a dolphin-friendly way.
Loud underwater noises, for example resulting from naval sonar use, live firing exercises or certain offshore construction projects such as wind farms may be harmful to dolphins, increasing stress, damaging hearing and causing decompression sickness by forcing them to surface too quickly to escape the noise.