Temporal range: Oligocene–Recent
|Baiji (Lipotes vexillifer)|
River dolphins are the four living species of dolphin that reside in freshwater rivers and estuaries. River dolphins inhabit areas of Asia and South America. They are classed in the Platanistoidea superfamily of cetaceans. Three species live in fresh water rivers. The fourth species, the La Plata dolphin, lives in saltwater estuaries and near-shore marine environments. However, it is scientifically classed in the river dolphin group rather than the oceanic dolphin family.
The largest river dolphins usually grow up to 2.4 meters (8 feet) long, but most of the animals are smaller. River dolphins may be white, pink, yellow, brown, gray, or black.
The four families of river dolphins are classified by Rice, 1998 as belonging to the superfamily Platanistoidea. Formerly, Platanistidae was listed as the only extant family of the Platanistoidea superfamily. The previously accepted classification treated all four families as belonging to this family and treated the Ganges and Indus River dolphins as separate species.
Five lineages of dolphin have evolved to live in big, muddy rivers. River dolphins are thought to have relictual distributions. Their ancestors originally occupied marine habitats, but were then displaced from these habitats by modern dolphin lineages. Many of the morphological similarities and adaptations to freshwater habitats arose due to convergent evolution, thus the superfamily Platanistoidea is paraphyletic.
In 2012 the Society for Marine Mammalogy began considering the Bolivian (Inia geoffrensis boliviensis) and Amazonian (Inia geoffrensis geoffrensis) subspecies as full species Inia boliviensis and Inia geoffrensis, respectively; however, much of the scientific community, including the IUCN, consider the boliviensis population to be a subspecies of Inia geoffrensis.
Both river dolphins and marine dolphins belong to a group of mammals called cetaceans, but they differ somewhat in appearance. For example, the snout of a river dolphin measures about 58 centimeters (2 ft) long, approximately four times as long as that of most marine dolphins. River dolphins have smaller eyes than marine dolphins, and their vision is poorly developed because they live in dark, muddy water. This environment also makes river dolphins less active than marine dolphins. River dolphins feed primarily on fish.
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. The last verified sighting was in September 2004. In August 2007, reports surfaced that a man saw and videotaped what appears to be a baiji in the Yangtze River. A team of scientists attempted to verify the sighting beginning in September 2007.
Overfishing, damming and subaquatic sonar pollution (which interfered with the dolphins' sonar-based method of locating food), is believed to have led to their extinction. Reuters news reported this as their first record of an aquatic mammalian extinction in 50 years.
Some oceanic dolphins live in fluvial environments, such as the Irrawaddy dolphin, but are not classified as river dolphins. The river dolphins should also not be confused with freshwater populations of porpoises, such as the finless porpoise.
South Asian River Dolphin: South Asian river dolphin
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