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.
- Superfamily Platanistoidea
- Family Platanistidae
- Family †Allodelphinidae (Miocene)
- Family †Squalodelphinidae (Oligocene to Miocene)
- Family †Squalodontidae (Oligocene to Miocene)
- Family †Waipatiidae (Oligocene to Miocene)
- Superfamily Inioidea
- Superfamily †Lipotoidea
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.
Differences between marine and river dolphins
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.
Extinction of the baiji
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.
- The Hungry Tide, by Amitav Ghosh, centres around a character studying Irrawaddy dolphins in Bangladesh.
Facultative freshwater cetaceans and non-river dolphins in riverine environments
River dolphin is considered a taxonomic description – suggesting an evolutionary relationship among the group. 'True' river dolphins are an ancient evolutionary linage evolved in freshwater environments.
Some species of cetacean live in rivers and lakes, but are more closely related to oceanic dolphins or porpoises, and more recently entered freshwater. Such species are considered facultative freshwater cetaceans as they can use both marine and freshwater environments. These include species such as the Irrawaddy dolphin Orcaella brevirostris, found in the Mekong, Mahakam, and Ayeyarwady Rivers, and the Yangtze finless porpoise Neophocaena asiaeorientalis asiaeorientalis.
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 freshwater. The Tucuxi was until recently considered conspecific with the Costero Sotalia guianensis, which inhabits marine waters. It may also be true for Irrawaddy dolphin and Finless Porpoise that the species might be found in both freshwater and marine environments, but the individual animals found in rivers may not be able to survive in the ocean, and vice versa.
The Franciscana Pontoporia blainvillei, has shown a converse evolutionary pattern, and is descended from the 'true' river dolphins, but inhabits estuarine and coastal waters.
- Rice, D. W. (1998). Marine mammals of the world: systematics and distribution. Society of Marine Mammalogy Special Publication Number 4. p. 231.
- Cassens, I., S. Vicario, V. G. Waddell, H. Balchowsky, D. Van Belle, W. Ding, C. Fan, R. S. L. Mohan, P. C. Simoes-Lopes, R. Bastida, A. Meyer, M. J. Stanhope, and M. C. Milinkovitch (2000). "Independent adaptation to riverine habitats allowed survival of ancient cetacean lineages". Proceedings of the National Academy of Sciences of the United States of America 97 (21): 11343–11347. doi:10.1073/pnas.97.21.11343. PMC 17202. PMID 11027333.
- Hamilton, H., S. Caballero, A. G. Collins, and R. L. Brownell Jr. (2001). "Evolution of river dolphins". Proceedings of the Royal Society B 268 (1466): 549–556. doi:10.1098/rspb.2000.1385. PMC 1088639. PMID 11296868.
- Turvey, S. T., R. L. Pitman, B. L. Taylor, J. Barlow, T. Akamatsu, L. A. Barrett, X. Zhao, R. R. Reeves, B. S. Stewart, K. Wang, Z. Wei, X. Zhang, L. T. Pusser, M. Richlen, J. R. Brandon and D. Wang (2007). "First human-caused extinction of a cetacean species?". Journal of the Royal Society, Biology Letters 3 (5): 537–540. doi:10.1098/rsbl.2007.0292. PMC 2391192. PMID 17686754.
- All Headline News, Dec. 2006.
- Lee, Y., et al. (2012). "First record of a platanistoid cetacean from the middle Miocene of South Korea". Journal of Vertebrate Paleontology 32 (1): 231–234. doi:10.1080/02724634.2012.626005.
- "" Committee on Taxonomy. 2012. List of marine mammal species and subspecies. Society for Marine Mammalogy, www.marinemammalscience.org, consulted on May 6, 2012.
- R.R. Reeves, T.A. Jefferson, L. Karczmarski, K. Laidre, G. O'Corry-Crowe, L. Rojas-Bracho, E.R. Secchi, E. Slooten, B.D. Smith, J.Y. Wang, & K. Zhou (2011). "Inia geoffrensis". IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. International Union for Conservation of Nature and Natural Resources.
- HarperCollins; New Ed edition (3 May 2005), ISBN 978-0-00-714178-4
- Reeves, Randall R. et al. (2002). National Audubon Society guide to marine mammals of the world. Alfred A. Knopf. 527 pp.
South Asian River Dolphin: South Asian river dolphin