Dolphins are aquatic mammals which are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They vary in size from 1.2 meters (4 ft) and 40 kilograms (88 lb) (Maui's Dolphin), up to 9.5 m (30 ft) and ten tonnes (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.
Dolphins, along with whales and porpoises, are thought to be
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
Hind Limb Buds on Dolphins An embryo of a Spotted Dolphin in the fifth week of development. The hind limbs are present as small bumps (hind limb buds) near the base of the tail. The pin is approximately 1 inch (~2,5 cm) long.
Modern dolphin skeletons have two small, rod-shaped pelvic
bones thought to be vestigial hind legs. 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
Dolphins have a streamlined fusiform body, adapted for fast swimming. The basic coloration patterns are shades of grey with a light underside and a distinct dark cape on the back. 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. The dolphin brain is large and has a highly structured cortex, which often is referred to in discussions about their advanced intelligence.
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 after some time, 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.
Most dolphins have acute eyesight, both in and out of the water, and their sense of hearing is superior to that of humans. 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. 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. 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 tacticle sense however, 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 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 meaningfully be done still have not been carried out, or have been carried out with inadequate sample size and methodology. Dolphin behavior has been studied extensively by humans however, both in captivity and in the wild. See the cetacean intelligence article for more details.
Dolphins 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.
In May 2005, researchers in Australia discovered a cultural aspect of dolphin behavior: Some dolphins (Tursiops aduncus) teach their children 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.
They are also willing to occasionally approach humans and playfully interact with them in the water. Dolphins have also been known to protect swimmers from sharks by swimming circles around them.
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.
Dolphin 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.
Dolphins are one of the few animals other than humans 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 behavior with other dolphin species, this also having resulted in various hybrid dolphin species as mentioned earlier. Occasionally, dolphins will also show sexual behavior towards humans.
Individual species may employ a number of methods of hunting, but even within a species various feeding methods may be employed, some 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 bait ball. Corralling 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 seals. 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. A modern human-dolphin fishery still takes place in Laguna, Santa Catarina, Brazil.
Dolphins often 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 behavior 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 very important part of dolphins' lives, and they can often be observed playing with seaweed or play-fighting with other dolphins. They even harass other locals, like seabirds and turtles. Dolphins also seem to enjoy riding waves and frequently 'surf' coastal swells and the bow waves of boats.
In more recent times, the 1963 Flipper movie and the subsequent
popular Flipper television series, contributed to the popularity
of dolphins in Western society. The series, created by Ivan Tors,
portrayed a dolphin in a friendly relationship with two boys,
Sandy and Bud; a kind of seagoing Lassie. Flipper, a Bottlenose
Dolphin, understood English unusually well and was a marked
hero. A second Flipper movie was made in 1996, which was based
on the story of the original movie. A Bottlenose Dolphin also
played a prominent role in the 1990s science fiction television
series seaQuest DSV in which the animal, named Darwin, could
communicate with English speakers using a vocoder, a fictional
invention which translated the clicks and whistles to English
More well known from this time period is probably the movie Free Willy however, which made famous the Orca playing Willy, Keiko. The 1977 horror movie Orca paints a less friendly picture of the animal. Here, a male Orca takes revenge on fishermen after the killing of his mate. In the 1973 movie The Day of the Dolphin trained dolphins are kidnapped and made to perform a naval military assassination using explosives.
The renewed popularity of dolphins in the 1960s resulted in the appearance of many dolphinariums around the world, which have made dolphins accessible to the public. Though criticism and more strict animal welfare laws have forced many dolphinariums to close their doors, hundreds still exist around the world attracting large amount of visitors. In the United States, best known are the SeaWorld marine mammal parks, and their common Orca stage name Shamu, which they have trademarked, has become well known. Southwest Airlines, an American airline, has even painted three of their Boeing 737 aircraft in Shamu colors as an advertisement for the parks and have been flying with such a livery on various aircraft since 1988.
Occasionally, dolphins make an appearance in computer games. Best known is the Ecco the Dolphin game series. The games are named after their main character, Ecco, a young Bottlenose Dolphin. The Ecco the Dolphin games hinge on the idea that cetaceans are sapient beings and have their own underwater society.
A well known American National Football League (NFL) team is named the Miami Dolphins. Their logo depicts an aqua-colored Bottlenose Dolphin wearing an American football helmet and jumping in front of a coral-colored sunburst.
This Dolphin Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub