Etymology
The name is originally from
Greek δελφίς (
delphís), "dolphin",
[1] which was related to the Greek
δελφύς(
delphus), "womb".
[2] The animal's name can therefore be interpreted as meaning "a 'fish' with a womb".
[3]The name was transmitted via the
Latin delphinus[4] (the
romanization of the later Greek δελφῖνος –
delphinos[5]), which in
Medieval Latin became
dolfinus and in
Old French daulphin, which reintroduced the
ph into the word. The term
mereswine (that is, "sea pig") has also historically been used.
[6]
The term 'dolphin' can be used to refer to, under the suborder
odontoceti, all the species in the family
delphinidae (marine dolphins including orcas and pilot whales) and the river dolphin superfamily
Platanistoidea, which includes the families
Iniidae (Amazon river dolphin),
Lipotidae (Yangtze river dolphin) and
Platanistidae (Ganges river dolphin and Indus river dolphin).
[7][8] This term has often been misused in the US, mainly in the fishing industry, where all small
cetaceans (dolphins and
porpoises) are considered porpoises, while the fish
dorado is called dolphin fish.
[9] In common usage the term 'whale' is used only for the larger species under cetaceans,
[10] while the smaller ones with a beaked or longer nose are considered 'dolphins'.
[11] The name 'dolphin' is used casually as a synonym for
bottlenose dolphin, the most common and familiar species of dolphin.
[12] Orcas and some closely related species belong to the delphinidae family and therefore qualify as dolphins, though they are called
killer whales in common usage. Though the terms 'dolphin' and 'porpoise' are used interchangeably, porpoises are not considered dolphins and have different physical features such as a shorter beak and spade-shaped teeth; they even differ in their behavior. Porpoises belong to the family
Phocoenidae and share a common ancestry with the delphinidae under the suborder odontocetes.
[12]
A group of dolphins is called a "school" or a "pod". Male dolphins are called "bulls", females "cows" and young dolphins are called "calves
Evolution and anatomy
The anatomy of a dolphin, showing its skeleton, major organs, tail, and body shape
Evolution
Along with whales and porpoises, dolphins are descendants of terrestrial mammals, most likely of the
Artiodactyl order. The ancestors of the modern-day dolphins entered the water roughly 55 million years ago, in the
Eocene epoch.
[20]
Hind limb buds are apparent on an embryo of a spotted dolphin in the fifth week of development as small bumps (hind limb buds) near the base of the tail. The pin is approximately 2.5 cm (1.0 in) long.
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
finson each side of its genital slit, which scientists believe to be an unusually pronounced development of these vestigial hind limbs.
[21]
Anatomy
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 varying by species, basic coloration patterns are shades of grey, usually with a lighter underside and 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.
[22]
Unlike most mammals, dolphins do not have hair, except for a few hairs around the tip of their
rostrum (beak) which they lose shortly before or after birth.
[23] The only exception to this is the
Boto river dolphin, which has persistent small hairs on the rostrum.
[24]
Dolphins' 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.
[25] The female has one genital slit, housing the
vagina and the
anus. Two
mammary slits are positioned on either side of the female's genital slit.
[26][27][28]
Dolphins can tolerate and recover from extreme injuries such as
shark bites although the exact methods used to achieve this are not known. The
healing process is rapid and even very deep wounds do not cause dolphins to
hemorrhage to death. Furthermore, even gaping wounds restore in such a way that the animal's body shape is restored, and
infection of such large wounds seems rare.
[29]
A study at the U.S. National Marine Mammal Foundation revealed that dolphins, like humans, 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.
[30]
Senses
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.
[31] Though they have a small ear opening on each side of their head, it is believed 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. Dolphin teeth are believed to function as antennae to receive incoming sound and to pinpoint the exact location of an object.
[32] Beyond locating an object, echolocation also provides the animal with an idea on the object's shape and size, though how exactly this works is not yet understood.
[33] The
Indus Dolphin is effectively blind. This may be because not much light penetrates the waters of the
Indus river (due to suspended sediments), making the need for vision unnecessary.
[34]
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.
[35] 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.
[36] 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.
[37]
Behavior
A pod of Indo-Pacific bottlenose dolphins in the
Red Sea.
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.
Social behavior
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. Individual dolphins communicate using a variety of clicks, whistle-like sounds and other vocalizations. Membership in pods is not rigid; interchange is common. Dolphins can, however, establish strong social bonds; they will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed.
[38] This
altruismdoes not appear to be limited to their own species. 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.
[39] They have also been seen protecting swimmers from sharks by swimming circles around the swimmers
[40][41] 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 dolphins (
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
simianprimates, where knowledge is generally passed on to both sexes. Using sponges as mouth protection is a learned behavior.
[42] Another learned behavior was discovered among river dolphins in Brazil, where some male dolphins use weeds and sticks as part of a sexual display.
[43]
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 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 after 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.
Feeding
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 orca also feed on other marine mammals. Orcas on occasion also hunt whale species larger than themselves.
[50]
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.
[50] Coralling is a method where dolphins chase fish into shallow water to catch them more easily.
[50]Orcas and bottlenose dolphins have also been known to drive their prey onto a beach to feed on it, a behaviour known as beach or strand feeding.
[51][52] Some species also whack fish with their flukes, stunning them and sometimes knocking them out of the water.
[50]
Vocalizations
Spectrogram of dolphin vocalizations. Whistles, whines, and clicks are visible as upside down V's, horizontal striations, and vertical lines, respectively.
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 whistle-like sounds produced by vibrating connective tissue, similar to the way human vocal cords function,
[56] and through burst-pulsed sounds, though the nature and extent of that ability is not known. 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.
[57]
Bottlenose dolphins have been found to have signature whistles, a whistle that is unique to a specific individual. These whistles are used in their communication to identify an individual. It can be seen as the dolphin equivalent of a
name.
[58] The signature whistle of male bottlenose dolphins tends to be similar to that of their mother, while the signature whistle of female bottlenose dolphins tends to be more unique.
[59]Bottlenose dolphins have a strong memory when it comes to these signature whistles, being able to relate to a signature whistle of an individual they have not encountered for over twenty years.
[60] Research done on signature whistle usage by other dolphin species is relatively limited. The research on other species done so far has yielded varied outcomes and inconclusive results.
[61][62][63][64]
Jumping and playing
Pacific white-sided dolphins porpoising
Dolphins frequently leap above the water surface, this being done for various reasons. When travelling, jumping can save the dolphin energy as there is less friction while in the air.
[65]This type of travel is known as porpoising.
[65] Other reasons include orientation, social displays, fighting,
non-verbal communication, entertainment and attempting to dislodge
parasites.
[66][67]
Dolphins show various types of playful behavior, often including objects, self-made
bubble rings, other dolphins or other animals.
[8][68][69] When playing with objects or small animals, common behavior includes carrying the object or animal along using various parts of the body, passing it along to other members of the group or taking it from another member, or throwing it out of the water.
[68]Dolphins have also been observed harassing animals in other ways, for example by dragging birds underwater without showing any intent to eat them.
[68] Playful behaviour that involves an other animal species with active participation of the other animal can also be observed however. Playful human interaction with dolphins being the most obvious example, however playful interactions have been observed in the wild with a number of other species as well, such as
Humpback Whales and
dogs.
[70][71]
Sleeping
Sleeping dolphin in captivity: a tail kick reflex keeps the dolphin's blowhole above the water
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.
[72][73][74] In captivity, dolphins seemingly enter a fully asleep state where both eyes are closed and there is no response to mild external stimuli. In this case, respiration is automatic; a tail kick
reflex keeps the blowhole above the water if necessary.
Anesthetized dolphins initially show a tail kick reflex.
[75] Though a similar state has been observed with wild
sperm whales, it is not known if dolphins in the wild reach this state.
[76] The
Indus river dolphinhas a sleep method that is different from that of 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.
[77]
Threats
Natural threats
Except for humans (discussed below), dolphins have few natural enemies. Some species or specific populations have none, making them
apex predators. For most of the smaller species of dolphins, only a few of the larger sharks, such as the
bull shark,
dusky shark,
tiger shark and
great white shark, are a potential risk, especially for calves.
[78] Some of the larger dolphin species, especially
orcas (killer whales), may also prey on smaller dolphins, but this seems rare.
[79][80] Dolphins also suffer from a wide variety of
diseases and
parasites.
[81][82]
Human threats
Pesticides, heavy metals, plastics, and other industrial and agricultural pollutants that do not disintegrate rapidly in the environment concentrate in predators such as dolphins.
[84]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.
[85] 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.
[86][87] In some parts of the world, such as
Taiji in Japan and the
Faroe Islands, dolphins are traditionally considered food and are killed in
harpoon or
drive hunts.
[88]Dolphin meat is high in mercury and may thus pose a health danger to humans when consumed.
[89]
Dolphin safe labels attempt to reassure consumers that fish and other marine products have been caught in a dolphin-friendly way. The earliest campaigns with "Dolphin safe" labels were initiated in the 1980s as a result of cooperation between marine activists and the major tuna companies, and involved decreasing incidental dolphin kills by up to 50% by changing the type of nets used to catch tuna. The dolphins are netted only while fishermen are in pursuit of smaller tuna. Albacore are not netted this way, making albacore the only truly dolphin-safe tuna.
[citation needed]
Conservation
Several scientists who have researched dolphin behaviour have proposed that dolphins' unusually high intelligence in comparison to other animals means that dolphins should be seen as
non-human personswho should have their own specific rights and that it is
morally unacceptable to keep them captive for entertainment purposes or to kill them either intentionally for consumption or unintentionally as by-catch.
[92] [93] Four countries –
Chile,
Costa Rica,
Hungary, and
India – have declared dolphins to be "non-human persons" and have banned the capture and import of live dolphins for entertainment.
[94][95]
Relationships with humans
Mythology
Dolphins have long played a role in human culture. Dolphins are common in
Greek mythology, and many coins from ancient Greece have been found which feature a man, a boy or a deity riding on the back of a dolphin.
[96] The
Ancient Greekswelcomed dolphins; spotting dolphins riding in a ship’s wake was considered a good omen.
[97] In both ancient and later art,
Cupid is often shown
riding a dolphin.
