Dolphins are marine mammals closely related to whales and porpoises. There are almost forty species of dolphin in 17 genera. Dolphins are among the most intelligent animals, and their often friendly appearance, an artifact of the "smile" of their mouthline, and seemingly playful attitude have made them very popular in human culture.
Dolphins vary in size from 1.2 m and 40 kg (Maui's dolphin), up to 9.5 m and 10 tonnes (the orca or killer whale).
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, much like a rudder on a boat. The dorsal fin, in those species that have one, provides stability while swimming. Though it varies by 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 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 (beak) 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.
Though the exact methods used to achieve this are not known, dolphins can tolerate and recover from extreme injuries, such as shark bites. 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.
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 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. 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. The dolphin's sense of touch is also well-developed, with free nerve endings densely packed in the skin, especially around the snout and pectoral fins. However, dolphins lack an olfactory nerve and lobes, and thus are believed to have no sense of smell.
The dolphin's sense of touch is also well-developed, with free nerve endings densely packed in the skin, especially around the snout and pectoral fins. However, dolphins lack an olfactory nerve and lobes, and thus are believed to have no sense of smell.
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.
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, whistle-like sounds and other vocalizations. Membership in pods is not rigid; interchange is common. However, dolphins can 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. 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 dolphins 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.
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.
The gestation period varies with 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. Typically dolphins give birth to a single calf, which is, unlike most other mammals, born tail first in most cases. 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.
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.
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 catch them more easily. 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 flukes, 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 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 whistle-like sounds produced by vibrating connective tissue, similar to the way human vocal cords function, 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.