<P> River dolphins are not very widely distributed; they are all restricted to certain rivers or deltas . This makes them extremely vulnerable to habitat destruction . River dolphins feed primarily on fish . Male river dolphins typically mate with multiple females every year, but females only mate every two to three years . Calves are typically born in the spring and summer months and females bear all the responsibility for raising them . River dolphins produce a variety of vocalizations, usually in the form of clicks and whistles . </P> <P> River dolphins are rarely kept in captivity; breeding success has been poor and the animals often die within a few months of capture . As of 2015, there were only three river dolphins in captivity . </P> <P> Four families of river dolphins (Iniidae, Pontoporiidae, Lipotidae and Platanistidae) are currently recognized, comprising three superfamilies (Inioidea, Lipotoidea and Platanistoidea). Platanistidae, containing the two subspecies of South Asian river dolphin, is the only accepted family of Platanistoidea . Previously, many taxonomists had assigned all river dolphins to a single family, Platanistidae, and treated the Ganges and Indus River dolphins as separate species . 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 . With their disappearance, one of the recently accepted superfamilies, Lipotoidea, has become extinct . </P> <P> The current classification of river dolphins is as follows: </P>

How many species of river dolphin are there
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