River dolphins are members of the infraorder Cetacea, which are descendants of land-dwelling mammals of the order Artiodactyla (even-toed ungulates). Phylogeny of cetaceans based on cytochrome b gene sequences, showing the distant relationship between Platanista and other river dolphins.
As of 2015, there were only four river dolphins in captivity. River dolphins are rarely kept in captivity breeding success has been poor and the animals often die within a few months of capture. River dolphins produce a variety of vocalizations, usually in the form of clicks and whistles. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them.
Male river dolphins typically mate with multiple females every year, but females only mate every two to three years. This makes them extremely vulnerable to habitat destruction. River dolphins are not very widely distributed they are all restricted to certain rivers or deltas. These species are well-adapted to living in warm, shallow waters, and, unlike other cetaceans, have little to no blubber. They have well-developed hearing that is adapted for both air and water they do not really rely on vision since the water they swim in is usually very muddy. River dolphins use their conical-shaped teeth and long beaks to capture fast-moving prey in murky water. They have streamlined bodies and two limbs that are modified into flippers. Several species exhibit sexual dimorphism, in that the males are larger than the females. They range in size from the 5-foot (1.5 m) long South Asian river dolphin to the 8-foot (2.4 m) and 220-pound (100 kg) Amazon river dolphin. River dolphins are relatively small compared to other dolphins, having evolved to survive in warm, shallow water and strong river currents. 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. There are five extant species of river dolphins. They comprise the families Platanistidae (the South Asian dolphins), the recently extinct Lipotidae (Yangtze river dolphin), Iniidae (the Amazonian dolphins) and Pontoporiidae. Extant river dolphins are placed in two superfamilies, Platanistoidea and Inioidea. They are an informal grouping of dolphins, which itself is a paraphyletic group within the infraorder Cetacea. River dolphins are a polyphyletic group of fully aquatic mammals that reside exclusively in freshwater or brackish water.
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