River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water.

River dolphins, alongside other cetaceans, belong to the clade Artiodactyla, with even-toed ungulates, and their closest living relatives the hippopotamuses, from which they diverged about 40 million years ago.

River dolphins use their conical-shaped teeth and long beaks to capture fast-moving prey in murky water.

[2] Four families of river dolphins (Iniidae, Pontoporiidae, Lipotidae and Platanistidae) are currently recognized, comprising three superfamilies (Inioidea, Lipotoidea and Platanistoidea).

A December 2006 survey found no members of Lipotes vexillifer (commonly known as the baiji, or Chinese river dolphin) and declared the species functionally extinct.

[7][9] River dolphins are members of the infraorder Cetacea, which are descendants of land-dwelling mammals of the order Artiodactyla (even-toed ungulates).

[12][13] Many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution; thus, a grouping of all river dolphins is polyphyletic.

[15][16] River dolphin has been considered a taxonomic description, suggesting an evolutionary relationship among the group, although it is now known that they form two distinct clades.

[12] Some species of cetacean live in rivers and lakes, but are more closely related to oceanic dolphins or porpoises and entered fresh water more recently.

[18] The tucuxi (Sotalia fluviatilis) in the Amazon River is another species descended from oceanic dolphins; however, it does not perfectly fit the label of 'facultative' either, as it occurs only in fresh water.

[21] River dolphins have a torpedo shaped body with a flexible neck, limbs modified into flippers, non-existent external ear flaps, a tail fin, and a small bulbous head.

[26] Unlike other cetaceans, their neck vertebrae are not fused together, meaning they have greater flexibility than other non-terrestrial aquatic mammals, at the expense of speed.

River dolphins swim by moving their tail fins and lower bodies up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering.

The small hairs on the rostrum of the Amazon river dolphin are believed to function as a tactile sense, possibly to compensate for their poor eyesight.

Perhaps the most significant issue is the building of more than 50 dams along many rivers, causing the segregation of populations and a narrowed gene pool in which the dolphins can breed.

On December 13, 2006, the baiji (Lipotes vexillifer) was declared "functionally extinct", after a 45-day search by leading experts in the field failed to find a single specimen.

[42][43] One of these ecosystems is a floodplain, or a várzea forest, and is home to a large number of fish species which are an essential resource for human consumption.

[47] The abundance of distinct fish species lures the Amazon River dolphin into the várzea areas of high water occurrences during the seasonal flooding.

[48] In addition to attracting predators such as the Amazon river dolphin, these high-water occurrences are an ideal location to draw in the local fisheries.

[51] [53][54] The Brazilian Institute of Environment and Renewable Natural Resources prohibit fishermen from killing the Amazon river dolphin, yet they are not compensated for the damage to their equipment and the loss of their catch.

[56] The increasing consumption demand by the local inhabitants and Colombia for the piracatinga has created a market for distribution of the Amazon river dolphin carcasses to be used as bait throughout these regions.

[44] The data does not fully represent the actual overall number of deaths of the Amazon river dolphins, whether accidental or intentional, because a variety of factors make it extremely complicated to record and medically examine all the carcasses.

[44] More importantly, the local fishermen do not report these deaths out of fear that legal action will be taken against them,[44] as the Amazon river dolphin and other cetaceans are protected under the Brazilian federal law, prohibiting any takes, harassments, and kills of the species.

This was planned as a backup to any other conservation efforts by producing an area completely protected from any threats, and where the baiji could be easily observed.

The site includes an indoor and outdoor holding pool, a water filtration system, food storage and preparation facilities, research labs and a small museum.

Douglas Adams and Mark Carwardine documented their encounters with the endangered animals on their conservation travels for the BBC programme Last Chance to See.

A sexually mature female was captured in late 1995, but died after half a year in 1996 when the Shishou Tian-e-Zhou Baiji Semi-natural Reserve (石首半自然白鱀豚保护区), which had contained only finless porpoises since 1990, was flooded.

The stepfather wanted to trade her since she would be sold for a great sum of money, but as they were crossing the river to get to the trader, a storm rolled in.

They are often characterized in mythology with superior musical ability, seductiveness and love of sex that often results in illegitimate children, and attraction to parties.

Despite the fact that the Encante are said to come from a utopia full of wealth and without pain or death, the encantados crave the pleasures and hardships of human societies.