RIVER DOLPHINS AND FRESHWATER DOLPHINS
River dolphins are found in several rivers and estuaries in Asia as well as the Amazon basin in South America. They range in size from 1.5 tp 2.4 meters (five to eight feet) in length and weigh up to 200 kilograms (440 pounds). They are blueish grey in color and can survive in both freshwater and saltwater but prefers freshwater. They are shy, slow swimming. They have very long snouts lined with teeth that they seem willing to use in defense, unlike most dolphins. They swim in small groups with two or three individuals
Mark Jenkins wrote in National Geographic: River dolphins parted company with its oceanic ancestors about 15 million years ago, during the Miocene epoch. Sea levels were higher then, large parts of the mainland areas where river dolphins now reside may have been flooded by shallow, more or less brackish water. When this inland sea retreated, some scientists have hypothesized, river dolphins remained in various river basin, evolving into striking creatures that bear little resemblance to our beloved Flipper.[Source: Mark Jenkins, in National Geographic, June 2009]
“The Amazon dolphin, or boto, is the largest species of river dolphin. All river dolphins are superficially similar yet they don't belong to the same family. DNA studies by Healy Hamilton of the California Academy of Sciences in San Francisco and others have shown that river dolphins evolved from archaic marine cetaceans (the order that also includes whales) on at least three separate occasions — first in India, later in China and in South America — before modern marine dolphins themselves had emerged as a distinct group. In an example of what's known as convergent evolution, geographically isolated and genetically distinct species developed similar characteristics because they were adjusting to similar environments.
River dolphins feed on fish and invertebrates, making dives that rarely last more than a few minutes. Group size varies from single individuals to 10 or 12. In general, little is known about their ecology and social behavior.
Websites and Resources: Britain-based Whale and Dolphin Conservation Society uk.whales.org ; Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures
River Dolphin Species and Taxonomy
There are five generally recognized species of freshwater dolphin: 1) Amazon river dolphin (Inia geoffrensis); 2) Tucuxi (Sotalia fluviatilis), which also lives in the Amazon basin; 3) the Ganges river dolphin (Platanista gangetica), found in the Ganges River in India; 4) Indus River Dolphin (Platanista minor), found in the Indus River in Pakistan; and 5) the Irrawaddy Dolphin (Orcaella brevirostris).
Irrawaddy dolphins (Scientific name: Orcaella brevirostris) are not considered true river dolphin, but an oceanic dolphin because they can live in brackish water near coasts, river mouths and in estuaries. Even so they also live a considerable distance upriver in freshwater as they do in the Mekong and Irrawaddy Rivers.
River dolphins are are classed in the cetacean superfamily Platanistoidea. All species live in fresh water rivers and have adaptations for fish catching: 1) a long, forceps-like beak with numerous small teeth in both jaws, 2) broad flippers to allow tight turns, 3) small eyes, and 4) unfused neck vertebrae to allow the head to move in relation to the body (most whales and dolphins have fused neck vertebrae). A sixth species, the La Plata dolphin, lives in saltwater estuaries and the ocean in the Río de la Plata between Argentina and Uruguay but is scientifically classed in the river dolphin family rather than the oceanic dolphin family. [Source: Wikipedia]
The species in Platanistidae family are placed in four genera: 1) Iniidae (South American river dolphins), 2) Lipotidae (Chinese river dolphins), 3) Platanistidae (Indian river dolphins), and 4) Pontoporidae (La Plata river dolphin). [Source: Animal Diversity Web (ADW)]
The baiji (Yangtze river dolphin, Lipotes vexillifer) is considered by some to be an Irrawaddy dolphin. But it looks really different. It has a long slender snout and it is hard to believe it is an Irrawaddy dolphin. In any case it is now believed that the baiji is extinct. There is also the Yangtze Finless Porpoise (Neophocaena asiaeorientalis) found in the Yangtze River in China.
Dolphin head sound production All dolphins and whales have blowholes about the size of a quarter with inner valves and out outer valves to seal out water when the dolphin is under water. When they surface dolphins inhales four to ten liters of air in one second. They can blow bubbles from their blowholes. Bottlenose dolphins come to the surface an average of once every 28 seconds to breath. In aquariums they have been observed making rings and other shapes with bubbles and playing with them.
Dolphins and whales have hair and blubber — milky, white fat underneath a animal’s skin. Like whales, dolphins have a horizontal fluke rather than vertical tail fins like fish. Dolphins propel themselves forward by moving their fluke up and down. A subdermal sheath is attached to the dolphin’s muscles and skeleton. Dolphins are able to keep their skins clear of parasites that attach to whales through special features in their skins: nanometer-size ripples and ridges with a gel-like coating. Scientists are studying dolphin skin for clues on how to keep ships clear of barnacles and tubeworms
Dolphins have several stomachs, like cows, which may be remnants from its terrestrial ancestors that lives tens of millions of years ago. Dolphins collect salt in their kidneys like camels.
River Dolphin Characteristics
Mark Jenkins wrote in National Geographic: River dolphins “have fat, bulbous foreheads and skinny, elongated beaks suited to snatching fish from a tangle of branches or to rooting around in river mud for crustaceans. The jaw muscles can its elongated beak down on prey with crocodilain ferocity.
Unlike marine dolphins, they have unfused neck vertebrae that allow them to bend at up to a 90-degree angle — ideal for slithering through trees. They also have broad flippers, a reduced dorsal fin (a larger one would just get in the way in tight spots), and small eyes — echolocation helps them pinpoint prey in muddy water.
David Attenborough wrote: “The most primitive of tooth whales alive today are the dolphins that swim in some of the world’s great rivers, the Ganges and the Indus, the Yangtze and the Amazon...Their neck bones are still separate so they can move their heads from side to side and the bones in their fingers are still indistinguishable and separate withing the gristle of their flippers.
According to Animal Diversity Web: River dolphins and freshwater dolphins (platanistids) are generally small, ranging from 1 to 3 meters (3.3 to 10 feet) in length and from roughly 20 to 225 kilograms (44 to 500 pounds) in weight, depending on species. They have a long, slender beak, above which rises a sharply differentiated, bulging forehead. The eyes are small, and in some species, appear atrophied. [Source: Animal Diversity Web (ADW)]
Unlike most other cetaceans,river dolphins have a distinct neck. The flippers are broad and either rounded or sharply curved. The dorsal fin is low in all but the La Plata dolphin, in which it is moderately high. Colors tend to be muted grays, pinks, and browns, usually darker above and paler below.
Technically, these species are characterized by narrow facial depressions, with the lateral edges of the depression (formed by maxilla and frontal bones) not roofing over the temporal fossa and concealing the zygomatic arch. The zygomatic arch is strongly developed and arched. The rostrum is very long and slender; the mandibular symphysis is long, from 46 to 72 percent of the length of the ramus; and the teeth are numerous, ranging from 25/24 to 61/61. The teeth of most species are simple pegs, but in one species the posterior teeth are slightly tricusped, and in another they have a well defined cingulum. /=\
Dolphin Hearing, Noise Making and Echolocation
All toothed whales and dolphins produce sound to communicate, navigate, and locate prey and have a melon, an oval fat-filled organ in their foreheads. A melon focuses outgoing sounds. It evolved from a sac off the main nasal passage for moving air back and forth to create sound vibrations. The lower jaw helps catch returning vibrations. Dolphins whistle, grunt, squawk and make clicking noises. They make a variety of hissing noises like air escaping from a balloon by blowing air through nasal sacs inside their skull. Most sounds are made in the nasal passages not the larynx. Some believe the clicking noises are generated by forcing air from two sacks near the blowholes. Others believe they are formed by forcing nasal plugs against bony edges of the skull.
Dolphins “hear” using their jaws. They don’t have an outer ear. Instead, sound travels to the inner ear through a thin "window" in the lower jawbone. Clicks can be rattled off at a rate of up to 700 per second or generated as a drawn-out individual sound. The frequency of these clicks varies between 20 cycles per second and 170,000 cycles per second. The human ear can only detect sounds as high as 16,000 cycles per second.
Dolphins — like bats and many whales — use echolocation (emitting sound waves and sensing their reflections to determine the location of objects) to locate objects. They make whistles, and ultra-sounds with frequencies of 200,000 vibrations a second with their larynx and produce clicks by forcing air through special passages and sinuses in the heads. The clicks and ultra-sounds pass through the melon, where the clicks are focused, and strike objects outside the dolphin and return to the dolphin brain like radar, enabling a dolphin to determine the distance and certain things about the objects. Blindfolded dolphins can detect a object three inches in diameter from a distance of a 120 meters.
Dolphins rely on echolocation more than sight to sense objects. Not only can they deduce the presence of an object with echolocation they can deduce its size and what it is. They can also determine if a container is full or empty and distinguish between a rock and a piece of flesh. Studies have shown that dolphins are almost as good at picking out objects of different shapes and sizes when they blindfolded as when they are not blindfolded.
River Dolphin Sonar and Communication
River dolphins are nearly blind. They have very small eyes. Indus and Ganges dolphins even lack lenses and can do little more than distinguish light and dark and night and day. The water they swim in is often so muddy that even if the could see they could only see a few inches in front of them. River dolphins rely on echolocation more than vision to locate their prey. In the sea, dolphins use their echolocation primarily to locate and catch fish. River dolphins use it primarily to navigate through the murky water so they can sweep for fish. They swim on their sides and sweep their long bony snouts in wide arcs across the river bottom, emitting long trains of echolocation clicks that let them hunt fish in all but opaque waters."
David Attenborough wrote: “The Ganges dolphin lives in waters that are so cloudy and thick with suspended mud that no one swimming in it, animal or human, could see more than a few inches ahead. The dolphin certainly cannot. For it is blind. Its eyes are tiny and do not even have lenses within them. The animal finds it way around by moving its mobile neck from side to side and making a series of clicking noises. These sounds are very high-pitched and, like those produced by bats, have important components that are far beyond the range of the human ear. They are produced in passages within the skull and then focused, amplified and transmitted forward by a large lump of fat in the dolphin’s forehead, known as the melon. When the beam of these ultra-sounds strikes a hard object, they are reflected back as echoes and received by another lump of fat in the lower jaw, which extends upward towards the dolphin’s ear. So even though the animal is blind, it has a detailed mental picture of its surroundings.”
“The Ganges dolphin feeds in its own peculiar way, tipping on its side and ploughing its flipper through the mud, stirring it up in clouds. As it does so, it greatly increases the frequency of the clicks so that, converted down into our audibility, they sound like the chatter of a machine gun. The effect for the dolphin must be like shining a torch in the twilight and it enables the animal to locate and snap up crabs, shrimp and bottom-living catfish.”
River Dolphin Mating
Mark Jenkins wrote in National Geographic: Vera da Silva have found that female Amazon dolphins in particular stray far into the forest when the Amazon floods during the wet season — perhaps to take refuge from aggressive, bright pink males. The females are mostly gray; the males' pink color, Martin and da Silva believe, is scar tissue. [Source: Mark Jenkins, in National Geographic, June 2009]
Tony Martin of the University of Kent in England has studied Amazon dolphins since the mid 1990s rold National Geographic, "The males beat the hell out of each other. They are brutal. They can snap each other's jaws, tails, flippers, lacerate blowholes. The large males are literally covered with scar tissue." Only a small percentage of males turn bright pink, Martin says, and those are the ones females are most attracted to — at least during the mating season, when the water retreats back into the river channel and both sexes are thrown together.
Being pink is not the only strategy males have for impressing females. They also sometimes pick up weeds or a piece of wood with their beaks, twirl in a circle, and smash the object on the water. Locals long believed the dolphins were just playing, but Martin discovered that only males carried objects, and only in the presence of females. What's more, they were 40 times more likely to get into fights when engaged in such ostentatious behavior. No other mammals besides humans and chimps carry objects for display, says Martin. "It's like a guy showing off — the equivalent of having a Ferrari," he explains.
Endangered River Dolphins
Mark Jenkins wrote in National Geographic: “River dolphins have no predators, except for humans. In December 2006 the Yangtze river dolphin, called the baiji, succumbed to pollution, propeller blades, dams, and overfishing; it became the first cetacean to be declared "functionally" extinct — meaning the species cannot renew itself, even if a specimen or two still exist. "Losing the baiji is like taking a chain saw to the cetacean tree of life," says Hamilton. "We've lost 20 million years of independent evolution." The Ganges river dolphin is also in grave danger; only a few thousand remain, and they live in some of the most polluted rivers on Earth. [Source: Mark Jenkins, in National Geographic, June 2009]
“The Amazon species probably has the best prospects; though its numbers are uncertain, Martin thinks at least 100,000 are left. But the trend is worrisome. At the Mamirauá Reserve, Martin's study population has declined by half over the past seven years. Fishermen hunt dolphins for catfish bait, he says, and they also kill them accidentally in their gill nets.
Freshwater Dolphins in China
For millions of years river dolphins have inhabited the Yangtze. The first are believed to have migrated up the Yangtze 20 million years ago. There is also a species of river porpoise: the finless porpoise. In 1993 2,700 lived in the Yangtze. Less than 1,000 live there now according to an expedition in 2006. So they don't go the same way as the baiji and colony of 30 of them has been established in nature preserve, far from the polluted river. The hope is that they will reproduce.
Baijis (Scientific name: Lipotes vexillifer) are a freshwater dolphin species found in China. They are the rarest and most endangered of all whale, porpoise of dolphin species. They live on a 1,000-mile stretch of the Yangtze river between the mouth of the river and the Three Gorges.
Baiji have traditionally been found in China from the mouth of the Chang Jiang (Yangtze) to a point about 1,900 kilometers (1,200 miles) up the river, as well as in the middle and lower regions of the Quintangjiang River and in the Dongting and Poyang lakes. They prefer to stay near large eddies that form next to sandbars and have been observed in lakes, ponds, rivers, streams as well as estuaries and areas adjacent to rivers and other water bodies. [Source: Allison Poor and Sarah Grigg, Animal Diversity Web (ADW) /=]
Image Sources: Wikimedia Commons, NOAA
Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.
Last Updated June 2023