River dolphins are a

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...

group of fully

aquatic mammal Aquatic and semiaquatic mammals are a diverse group of mammals that dwell partly or entirely in bodies of water. They include the various marine mammals who dwell in oceans, as well as various freshwater species, such as the European otter. The ...

s that reside exclusively in freshwater or

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...

. They are an informal grouping of

dolphin A dolphin is an aquatic mammal within the infraorder Cetacea. Dolphin species belong to the families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the ...

s, which itself is a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group within the infraorder

Cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...

. Extant river dolphins are placed in two superfamilies, Platanistoidea and Inioidea. They comprise the families Platanistidae (the South Asian dolphins), the recently extinct

Lipotidae Lipotidae is a family of river dolphins containing the possibly extinct baiji of China and the fossil genus ''Parapontoporia'' from the Late Miocene and Pliocene of the Pacific coast of North America. The genus '' Prolipotes'', which is based on ...

(Yangtze river dolphin),

Iniidae Iniidae is a family of river dolphins containing one living genus, ''Inia'', and four extinct genera. The extant genus inhabits the river basins of South America, but the family formerly had a wider presence across the Atlantic Ocean. Iniidae a ...

(the Amazonian dolphins) and Pontoporiidae. There are five extant species of river dolphins. River dolphins, alongside other cetaceans, belong to the clade

Artiodactyla The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

, with

even-toed ungulate The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

s, and their closest living relatives the

hippopotamuses The hippopotamus ( ; : hippopotamuses or hippopotami; ''Hippopotamus amphibius''), also called the hippo, common hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extan ...

, from which they diverged about 40 million years ago. Specific types of Dolphins can be pink. River dolphins are relatively small compared to other dolphins, having evolved to survive in warm, shallow water and strong river currents. They range in size from the long

South Asian river dolphin South Asian river dolphins are toothed whales in the genus ''Platanista'', which inhabit fresh water habitats in the northern Indian subcontinent. They were historically considered to be one species (''P. gangetica'') with the Ganges river dol ...

to the and

Amazon river dolphin The Amazon river dolphin (''Inia geoffrensis''), also known as the boto, bufeo or pink river dolphin, is a species of toothed whale classified in the family Iniidae. Three subspecies are currently recognized: ''I. g. geoffrensis'' (Amazon river ...

. Several species exhibit

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

, in that the males are larger than the females. They have streamlined bodies and two limbs that are modified into flippers. River dolphins use their conical-shaped teeth and long beaks to capture fast-moving prey in murky water. 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. Instead, they tend to rely on echolocation when hunting and navigating. These species are well-adapted to living in warm, shallow waters, and, unlike other cetaceans, have little to no blubber. River dolphins are not very widely distributed; they are all restricted to certain rivers or deltas. This makes them extremely vulnerable to

habitat destruction Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...

. 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. River dolphins are rarely kept in captivity; breeding success has been poor and the animals often die within a few months of capture. , there were only four river dolphins in captivity.

Taxonomy and evolution

Classification

Four

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

of river dolphins (Iniidae, Pontoporiidae, Lipotidae and Platanistidae) are currently recognized, comprising three superfamilies (Inioidea, Lipotoidea and Platanistoidea). Platanistidae, containing the two subspecies of

, is the only living family in the superfamily 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 The baiji (; IPA: ; ''Lipotes vexillifer'', ''Lipotes'' meaning "left behind" and ''vexillifer'' "flag bearer") is a possibly extinct species of freshwater dolphin native to the Yangtze river system in China. It is thought to be the first dolph ...

'' (commonly known as the baiji, or Chinese river dolphin) and declared the species

functionally extinct Functional extinction is the extinction of a species or other taxon such that: #It disappears from the fossil record, or historic reports of its existence cease; #The reduced population no longer plays a significant role in ecosystem function; or ...

. With their disappearance, one of the recently accepted superfamilies, Lipotoidea, has become extinct. The current classification of river dolphins is as follows: Arktocara NT small

* Superfamily Platanistoidea ** Family Platanistidae *** Genus '' Platanista'' ****

, ''Platanista gangetica'', with two subspecies *****

Ganges river dolphin The Ganges river dolphin (''Platanista gangetica'') is a species of toothed whale classified in the family Platanistidae. It lives in the Ganges and related rivers of South Asia, namely in the countries of India, Nepal, and Bangladesh. It is rel ...

(''susu''), ''P. g. gangetica'' *****

Indus river dolphin The Indus river dolphin (''Platanista minor''), also known as the ''bhulan'' in Urdu and Sindhi, is a species of toothed whale in the family Platanistidae. It is endemic to the Indus River Basin of Pakistan and northwestern India. This dolphin ...

(''bhulan''), ''P. g. minor'' ** Family †

Allodelphinidae Allodelphinidae is a family of primitive platanistoid river dolphins found in marine deposits in the eastern North Pacific region, Alaska, and Japan. Description Kimura and Barnes (2016, pp. 3–4) diagnose the family as follows: System ...

(

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

) ** Family † Squalodelphinidae (

to Miocene) ** Family †

Squalodontidae Squalodontidae or the shark-toothed dolphins is an extinct family of large toothed whales who had long narrow jaws. Squalodontids are known from all continents except Antarctica, from the Oligocene to the Neogene, but they had a maximal diversi ...

(Oligocene to Miocene) ** Family † Waipatiidae (Oligocene to Miocene) * Superfamily Inioidea ** Family

*** Genus ''

Inia ''Inia'' is a genus of river dolphins from South America containing one to four species. Taxonomy The genus was described by Alcide d'Orbigny in 1834 when ''Delphinus geoffrensis'', described by Henri Marie Ducrotay de Blainville in 1817, was ...

****

(''boto''), ''Inia geoffrensis'' ***** ''Inia geoffrensis geoffrensis'' ***** ''Inia geoffrensis humbotiana'' **** Araguaian river dolphin, ''Inia araguaiaensis'' ****

Bolivian river dolphin The Bolivian river dolphin (''Inia boliviensis'') is a species of the genus ''Inia''. Taxonomy Bolivian river dolphins were discovered by the Western world in 1832 by French researcher Alcide d'Orbigny. The Bolivian river dolphin was briefly th ...

, ''Inia boliviensis'' *** Genus †''

Meherrinia ''Meherrinia'' is an extinct genus of inioid river dolphin from the Meherrin River, North Carolina, in the United States. First described in 2012, the dolphin is, in most respects, intermediate in form between the living Amazon river dolphin and ...

'' (late Miocene) *** Genus †''

Isthminia ''Isthminia'' is an extinct genus of river dolphin-like cetacean from the Late Miocene epoch ( Hemphillian in the NALMA classification) that lived in Panama. The type species is ''I. panamensis''.Pontoporiidae *** Genus †''

Auroracetus ''Auroracetus'', from ''Aurora'' (the location the holotype was discovered, Aurora N.C.) and the Latin ''cetus'' (whale), is a genus of Pontoporiidae, the river dolphins, and contains one extinct species. The species ''Auroracetus bakerae'' was p ...

'' **** †''Auroracetus bakerae'' *** Genus '' Pontoporia'' Qiqi, a Chinese River Dolphin (Baiji) 26

**** La Plata dolphin (''Franciscana''), ''Pontoporia blainvillei'' * Superfamily †

Lipotoidea Lipotidae is a family of river dolphins containing the possibly extinct baiji of China and the fossil genus ''Parapontoporia'' from the Late Miocene and Pliocene of the Pacific coast of North America. The genus '' Prolipotes'', which is based ...

** Family †

*** Genus † '' Lipotes'' **** † '' Baiji'' (or Chinese river dolphin), ''Lipotes vexillifer'' In 2012 the

Society for Marine Mammalogy The Society for Marine Mammalogy was founded in 1981 and is the largest international association of marine mammal scientists in the world. Mission The mission of the Society for Marine Mammalogy (SMM) is to promote the global advancement of mari ...

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 The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...

, continue to consider the Bolivian population to be a subspecies of ''Inia geoffrensis''.Reeves, R.R., Jefferson, T.A., Karczmarski, L., Laidre, K., O’Corry-Crowe, G., Rojas-Bracho, L., Secchi, E.R., Slooten, E., Smith, B.D., Wang, J.Y. & Zhou, K. (2011). ''Inia geoffrensis''. The IUCN Red List of Threatened Species In October 2014, the Society for Marine Mammalogy took ''Inia boliviensis'' and ''Inia araguaiaensis'' off their list of

species and subspecies and currently does not recognize these species-level separations.

Evolution

River dolphins are members of the infraorder ''

'', which are descendants of land-dwelling mammals of the order

(even-toed

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, cam ...

s). They are related to the ''

Indohyus ''Indohyus'' is an extinct genus of digitigrade artiodactyls known from Eocene fossils in Asia. This small chevrotain-like animal found in the Himalayas is one of the earliest known non-cetacean ancestors of whales. Discovery The fossils were di ...

'', an extinct chevrotain-like ungulate, from which they split approximately 48 million years ago. The primitive cetaceans, or

archaeocetes Archaeoceti ("ancient whales"), or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene (). Representing the earliest cetacean radiation, they include the initial ...

, first took to the sea approximately 49 million years ago and became fully aquatic by 5–10 million years later. It is unknown when river dolphins first ventured back into fresh water. River dolphins are thought to have relictual distributions, that is, 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 Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

; thus, a grouping of all river dolphins is

. Amazon river dolphins are actually more closely related to

oceanic dolphins Oceanic dolphins or Delphinidae are a widely distributed family of dolphins that live in the sea. Close to forty extant species are recognised. They include several big species whose common names contain "whale" rather than "dolphin", such as the ...

than to

s. '' Isthminia panamensis'' is an extinct genus and species of river dolphin, living 5.8 to 6.1 million years ago. Its fossils were discovered near Piña, Panama. 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. 'True' river dolphins are descendants of ancient evolutionary lineages that evolved in freshwater environments. Some species of cetacean live in rivers and lakes, but are more closely related to

or porpoises and entered fresh water more recently. Such species are considered facultative freshwater cetaceans as they can use both marine and freshwater environments. These include species such as the

Irrawaddy dolphin The Irrawaddy dolphin (''Orcaella brevirostris'') is a euryhaline species of oceanic dolphin found in scattered subpopulations near sea coasts and in estuaries and rivers in parts of the Bay of Bengal and Southeast Asia. It closely resembles the ...

, ''Orcaella brevirostris'', found in the

Mekong The Mekong or Mekong River is a trans-boundary river in East Asia and Southeast Asia. It is the world's List of rivers by length, twelfth longest river and List of longest rivers of Asia, the third longest in Asia. Its estimated length is , ...

Mahakam The Mahakam River ( Indonesian: ''Sungai Mahakam'') is third longest and volume discharge river in Borneo after Kapuas River and Barito River, it is located in Kalimantan, Indonesia. It flows from the district of Long Apari in the highlands ...

, the

Irrawaddy River The Irrawaddy River ( Ayeyarwady River; , , from Indic ''revatī'', meaning "abounding in riches") is a river that flows from north to south through Myanmar (Burma). It is the country's largest river and most important commercial waterway. Origi ...

s, as well as the

Yangtze The Yangtze or Yangzi ( or ; ) is the longest river in Asia, the third-longest in the world, and the longest in the world to flow entirely within one country. It rises at Jari Hill in the Tanggula Mountains (Tibetan Plateau) and flows ...

finless porpoise ''Neophocaena'' is a genus of porpoise native to the Indian and Pacific oceans, as well as the freshwater habitats of the Yangtze River basin in China China, officially the People's Republic of China (PRC), is a country in East Asia. ...

''Neophocaena phocaenoides asiaeorientalis''. Some oceanic cetacean populations are known to live semi-permanently in river and estuarine systems, such as the

Indo-Pacific bottlenose dolphin The Indo-Pacific bottlenose dolphin (''Tursiops aduncus'') is a species of bottlenose dolphin. This dolphin grows to long, and weighs up to . It lives in the waters around India, northern Australia, South China, the Red Sea, and the eastern ...

group resident in the Swan River of

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

which travel as far inland as Belmont. The

tucuxi The tucuxi (''Sotalia fluviatilis''), alternatively known in Peru ''bufeo gris'' or ''bufeo negro'', is a species of freshwater dolphin found in the rivers of the Amazon basin. The word ''tucuxi'' is derived from the Tupi language word ''tuchu ...

(''Sotalia fluviatilis'') in the

Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of t ...

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. The tucuxi was until recently considered conspecific with the

Guiana dolphin The Guiana dolphin (''Sotalia guianensis''), also known as the estuarine dolphin or costero, is a dolphin found in the coastal waters to the north and east of South America, and east of Central America. It is a member of the oceanic dolphin fami ...

(''Sotalia guianensis''), which inhabits marine waters. It may also be true for the Irrawaddy dolphin and the finless porpoise that, although the species may be found in both freshwater and marine environments, individual animals found in rivers may not be able to survive in the ocean, and vice versa. The tucuxi is currently classified as an oceanic dolphin (Delphinidae). The

Franciscana The La Plata dolphin, franciscana or toninha (''Pontoporia blainvillei'') is a species of dolphin found in coastal Atlantic waters of southeastern South America. It is a member of the river dolphin group and the only one that lives in the ocean ...

(''Pontoporia blainvillei'') has shown a converse evolutionary pattern, and has an ancient evolutionary lineage in freshwater, but inhabits estuarine and coastal waters.

Biology

Anatomy

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. River dolphin skulls have small eye orbits, a long snout and eyes placed on the sides of the head. River dolphins are rather small, ranging in size from the long

to the and

. They all have female-biased

, with the females being larger than the males. River dolphins are polygynous, meaning 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 have conical teeth, used to catch swift prey such as small river fish. They also have very long snouts, with some measuring , four times longer than most of their oceanic counterparts. They have a two-chambered stomach that is similar in structure to that of terrestrial carnivores. They have

fundic The gastric glands are glands in the lining of the stomach that play an essential role in the process of digestion. All of the glands have mucus-secreting foveolar cells. Mucus lines the entire stomach, and protects the stomach lining from the e ...

and pyloric chambers. Breathing involves expelling stale air from their blowhole, followed by inhaling fresh air into their lungs. They do not have the iconic spout, as this only forms when the warm air exhaled from the lungs meets cold external air, which does not occur in their tropical habitats. River dolphins have a relatively thin layer of blubber. Blubber can help with buoyancy, protection from predators (they would have a hard time getting through a thick layer of fat), energy for leaner times, and insulation from harsh climates. The habitats of river dolphins lack these needs.

Locomotion

River dolphins have two flippers and a tail fin. These flippers contain four digits. Although river dolphins do not possess fully developed hind limbs, some possess discrete rudimentary appendages, which may contain feet and digits. River dolphins are slow swimmers in comparison to oceanic dolphins, which can travel at speeds up to ; the

can only travel at about . Unlike other cetaceans, their neck vertebrae are not fused together, meaning they have greater flexibility than other non-terrestrial

s, at the expense of speed. This means they can turn their head without actually moving their entire body. When swimming, river dolphins rely on their tail fins to propel themselves through the water. Flipper movement is continuous. 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. All species have a

dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through conv ...

Senses

The ears of river dolphins have specific adaptations to their aquatic environment. In humans, the

middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...

works as an impedance equalizer between the outside air's low impedance and the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory org ...

r fluid's high impedance. In river dolphins, and other cetaceans, there is no great difference between the outer and inner environments. Instead of sound passing through the outer ear to the middle ear, river dolphins receive sound through the throat, from which it passes through a low-impedance fat-filled cavity to the inner ear. The ear is acoustically isolated from the skull by air-filled sinus pockets, which allows for greater directional hearing underwater. Dolphins send out high frequency clicks from an organ known as a

melon A melon is any of various plants of the family Cucurbitaceae with sweet, edible, and fleshy fruit. The word "melon" can refer to either the plant or specifically to the fruit. Botanically, a melon is a kind of berry, specifically a "pepo". The ...

. This melon consists of fat, and the skull of any such creature containing a melon will have a large depression. This allows river dolphins to produce

biosonar Echolocation, also called bio sonar, is a biological sonar used by several animal species. Echolocating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these ...

for orientation. They are so dependent on echolocation that they can survive even if they are blind. Beyond locating an object, echolocation also provides the animal with an idea on the object's shape and size, though how exactly this works is not yet understood. The small hairs on the rostrum of the Amazon river dolphin are believed to function as a

tactile sense Haptic perception ( gr, haptόs, script=latn, italics=yes "palpable", ''haptikόs'' "suitable for touch") means literally the ability "to grasp something". Perception in this case is achieved through the active exploration of surfaces and objects b ...

, possibly to compensate for their poor eyesight. River dolphins have very small eyes for their size, and do not have a very good sense of sight. In addition, the eyes are placed on the sides of the head, so the vision consists of two fields, rather than a binocular view like humans have. When river dolphins surface, their lens and cornea correct the nearsightedness that results from the refraction of light. They have both rod and

cone A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines con ...

cells, meaning they can see in both dim and bright light. Most river dolphins have slightly flattened eyeballs, enlarged pupils (which shrink as they surface to prevent damage), slightly flattened corneas and a

tapetum lucidum The ''tapetum lucidum'' ( ; ; ) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light a ...

; these adaptations allow for large amounts of light to pass through the eye and, therefore, a very clear image of the surrounding area. They also have glands on their eyelids and an outer corneal layer that act as protection for the cornea.

Olfactory lobes The olfactory bulb (Latin: ''bulbus olfactorius'') is a neural structure of the vertebrate forebrain involved in olfaction, the sense of smell. It sends olfactory information to be further processed in the amygdala, the orbitofrontal cortex (OF ...

are absent in river dolphins, suggesting that they have no sense of smell. River dolphins are not thought to have a sense of taste, as their taste buds are atrophied or missing altogether. However, some dolphins have preferences between different kinds of fish, indicating some sort of attachment to taste.

Interactions with humans

Threats

Development

Development and agriculture have had devastating impacts on the habitats on river dolphins. The total population of Araguaian river dolphins is estimated to be between 600 and 1,500 individuals, and genetic diversity is limited. The ecology of their habitat has been adversely affected by agricultural, ranching and industrial activities, as well as by the use of dams for hydroelectric power. The inhabited section of the Araguaia River probably extends over about out of a total length of . The

Tocantins river The Tocantins River ( pt, Rio Tocantins, link=no , , Parkatêjê: ''Pyti'' ɨˈti is a river in Brazil, the central fluvial artery of the country. In the Tupi language, its name means " toucan's beak" (''Tukã'' for "toucan" and ''Ti'' for "beak ...

habitat is fragmented by six hydroelectric dams, so the population there is at particular risk. Its probable eventual

status is vulnerable or worse. Both subspecies of

s have been very adversely affected by human use of the river systems in the

subcontinent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

. Irrigation has lowered water levels throughout both subspecies' ranges. Poisoning of the water supply from industrial and agricultural chemicals may have also contributed to population decline. 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. Currently, three subpopulations of Indus river dolphins are considered capable of long-term survival if protected. As China developed economically, pressure on the baiji river dolphin grew significantly.

Industrial Industrial may refer to: Industry * Industrial archaeology, the study of the history of the industry * Industrial engineering, engineering dealing with the optimization of complex industrial processes or systems * Industrial city, a city dominate ...

and residential waste flowed into the Yangtze. The riverbed was dredged and reinforced with concrete in many locations. Ship traffic multiplied, boats grew in size, and fishermen employed wider and more lethal nets. Noise pollution caused the nearly blind animal to collide with propellers. Stocks of the dolphin's prey declined drastically in the late 20th century, with some fish populations declining to one thousandth of their pre-industrial levels. In the 1950s, the population was estimated at 6,000 animals, but declined rapidly over the subsequent five decades. Only a few hundred were left by 1970. Then the number dropped down to 400 by the 1980s and then to 13 in 1997 when a full-fledged search was conducted. 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.

Competition

The region of the Amazon in Brazil has an extension of containing diverse fundamental ecosystems. One of these ecosystems is a

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

, or a

várzea forest A várzea forest is a seasonal floodplain forest inundated by whitewater rivers that occurs in the Amazon biome. Until the late 1970s, the definition was less clear and várzea was often used for all periodically flooded Amazonian forests. Althoug ...

, and is home to a large number of fish species which are an essential resource for human consumption. The várzea is also a major source of income through excessive local commercialized fishing. Várzea consist of muddy river waters containing a vast number and diversity of nutrient-rich species. The abundance of distinct fish species lures the Amazon River dolphin into the várzea areas of high water occurrences during the seasonal flooding. 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. Human fishing activities directly compete with the dolphins for the same fish species, the tambaqui (''Colossoma macropomum'') and the pirapitinga (''Piaractus brachypomus''), resulting in deliberate or unintentional catches of the Amazon river dolphin. The local fishermen overfish, and when the Amazon river dolphins remove the commercialized fish from the nets and lines, it damages the equipment and the capture and causes a negative reaction from the local fishermen. The

Brazilian Institute of Environment and Renewable Natural Resources Brazilian Institute of the Environment and Renewable Natural Resources ( pt, Instituto Brasileiro do Meio Ambiente e dos Recursos Naturais Renováveis, IBAMA) is the Brazilian Ministry of the Environment's administrative arm. IBAMA supports anti- ...

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.

Bycatch

During the process of catching the commercialized fish, the Amazon river dolphins get caught in the nets and exhaust themselves until they die, or the local fishermen deliberately kill the dolphins that become entangled in their nets. The carcasses are discarded, consumed, or used as bait to attract a scavenger catfish, the

piracatinga ''Calophysus macropterus'' (also known as the Piracatinga, Vulture Catfish, or Zamurito) is a species of catfish (order Siluriformes) of the monotypic genus ''Calophysus'' of the family Pimelodidae. This fish has also been placed in its own fam ...

(''Calophysus macropterus''). The use of the Amazon river dolphin carcass as bait for the piracatinga dates back from 2000. 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. For example, of the 15 dolphin carcasses found in the Japurá River in 2010–2011 surveys, 73% of the dolphins were killed for bait, disposed of, or abandoned in entangled gillnets. 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. Scavenger species feed upon them and the complexity of the river currents makes it nearly impossible to locate all the carcasses. More importantly, the local fishermen do not report these deaths out of fear that legal action will be taken against them, as the Amazon river dolphin and other cetaceans are protected under the Brazilian federal law, prohibiting any takes, harassments, and kills of the species.

In captivity

A baiji conservation

dolphinarium A dolphinarium is an aquarium for dolphins. The dolphins are usually kept in a pool, though occasionally they may be kept in pens in the open sea, either for research or public performances. Some dolphinariums consist of one pool where dolphins pe ...

was established at the Institute of Hydrobiology (IHB) in Wuhan in 1992. 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. The aim is to also generate income from tourism which can be put towards the baiji plight. The pools are not very large, only kidney shaped tanks with dimensions of arc width and depth, diameter, deep and diameter, deep, and are not capable of holding many baijis at one time.

Douglas Adams Douglas Noel Adams (11 March 1952 – 11 May 2001) was an English author and screenwriter, best known for ''The Hitchhiker's Guide to the Galaxy''. Originally a 1978 The Hitchhiker's Guide to the Galaxy (radio series), BBC radio comedy, ''The H ...

and

Mark Carwardine Mark Carwardine ( IPA: /kɑːwɑːdiːn/; born 9 March 1959) is a British zoologist who achieved widespread recognition with his 20-year conservation project – '' Last Chance to See'' – which involved round-the-world expeditions with Dougla ...

documented their encounters with the endangered animals on their conservation travels for the BBC programme ''

Last Chance to See ''Last Chance to See'' is a 1989 BBC radio documentary series and its accompanying book, written and presented by Douglas Adams and Mark Carwardine. In the series, Adams and Carwardine travel to various locations in the hope of encountering s ...

''. The book by the same name, published in 1990, included pictures of a captive specimen, a male named Qi Qi (淇淇) that lived in the Wuhan Institute of Hydrobiology

from 1980 to July 14, 2002. Discovered by a fisherman in

Dongting Lake Dongting Lake () is a large, shallow lake in northeastern Hunan Province, China. It is a flood basin of the Yangtze River, so its volume depends on the season. The provinces of Hubei and Hunan are named after their location relative to the la ...

, he became the sole resident of the Baiji Dolphinarium (白鱀豚水族馆) beside East Lake. 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

s since 1990, was flooded. The Amazon river dolphin has historically been kept in dolphinariums. Today, only three exist in captivity: one in

Acuario de Valencia Valencia's Aquarium (officially Acuario de Valencia, Fundación Seijas) is a recreational park located in Valencia, Venezuela. It is the largest aquarium in Venezuela, and it also has a small zoo with species from Venezuela. It is operated by th ...

in Venezuela, one in Zoologico de Guistochoca in Peru, and one in

Duisburg Zoo The Duisburg Zoo, founded on 12 May 1934, is one of the largest zoological gardens in Germany. It is especially well known for its dolphinarium and, since 1994, for breeding koalas. Far less well known are the breeding successes in other areas, f ...

in Germany. Several hundred were captured between the 1950s and 1970s, and were distributed in dolphinariums throughout the US, Europe, and Japan. Around 100 went to US dolphinariums, and of that, only 20 survived; the last (named Chuckles) died in

Pittsburgh Zoo __NOTOC__ The Pittsburgh Zoo & Aquarium is one of only six major zoo and aquarium combinations in the United States. Located in Pittsburgh, Pennsylvania's Highland Park, the zoo sits on of park land where it exhibits more than 4,000 animals repr ...

in 2002.

In mythology

Asia

In Hindu mythology, the

is associated with Ganga, the deity of the

Ganges The Ganges ( ) (in India: Ganga ( ); in Bangladesh: Padma ( )). "The Ganges Basin, known in India as the Ganga and in Bangladesh as the Padma, is an international river to which India, Bangladesh, Nepal and China are the riparian states." is ...

river. The dolphin is said to be one of the creatures which heralded the goddess' descent from the heavens, and Ganga's mount, the

Makara ''Makara'' ( sa, मकर, translit=Makara) is a legendary sea-creature in Hindu mythology. In Hindu astrology, Makara is equivalent to the Zodiac sign Capricorn. Makara appears as the vahana (vehicle) of the river goddess Ganga, Narmada, a ...

, is sometimes depicted as a dolphin. In Chinese mythology, the baiji has many origin stories. For example, near the mouth of the

, the baiji was a princess that had lost her parents and had lived with her stepfather, whom she had longed to get away from. 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. The enraged stepfather tried to take her, but she plunged herself into the river, was transformed into a dolphin before she drowned, and swam away from her abusive stepfather, who also fell in and was transformed into a porpoise. In another story, the baiji was the daughter of a general deported from the city of

Wuhan Wuhan (, ; ; ) is the capital of Hubei, Hubei Province in the China, People's Republic of China. It is the largest city in Hubei and the most populous city in Central China, with a population of over eleven million, the List of cities in China ...

during a war who ran away while her father was in duty. Later, the general met a woman who told him how her father was a general. When he realized that she was his daughter, he threw himself into the river out of shame, and his daughter ran after him and also fell into the river. Before they were drowned, the daughter was transformed into a dolphin and the general into a porpoise.

South America

s, known by the

native Native may refer to: People * Jus soli, citizenship by right of birth * Indigenous peoples, peoples with a set of specific rights based on their historical ties to a particular territory ** Native Americans (disambiguation) In arts and entert ...

s as the

boto Boto is a Portuguese name given to several types of dolphins and river dolphins native to the Amazon and the Orinoco River tributaries. A few botos exist exclusively in fresh water, and these are often considered primitive dolphins. Classificatio ...

, encantados or toninas, are very prevalent in the mythology of the native South Americans. 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 A utopia ( ) typically describes an imaginary community or society that possesses highly desirable or nearly perfect qualities for its members. It was coined by Sir Thomas More for his 1516 book ''Utopia (book), Utopia'', describing a fictional ...

full of wealth and without pain or death, the encantados crave the pleasures and hardships of human societies. Transformation into human form is said to be rare, and usually occurs at night. The encantado will often be seen running from a festa, despite protests from the others for it to stay, and can be seen by pursuers as it hurries to the river and reverts to dolphin form. When it is under human form, it wears a hat to hide its blowhole, which does not disappear with the

shapeshift In mythology, folklore and speculative fiction, shape-shifting is the ability to physically transform oneself through an inherently superhuman ability, divine intervention, demonic manipulation, sorcery, spells or having inherited the ...

. Besides the ability to shapeshift into human form, encantados frequently wield other magical abilities, such as controlling storms, hypnotizing humans into doing their will, transforming humans into encantados, and inflicting

illness A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...

insanity Insanity, madness, lunacy, and craziness are behaviors performed by certain abnormal mental or behavioral patterns. Insanity can be manifest as violations of societal norms, including a person or persons becoming a danger to themselves or t ...

, and even

death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

Shamans Shamanism is a religious practice that involves a practitioner (shaman) interacting with what they believe to be a spirit world through altered states of consciousness, such as trance. The goal of this is usually to direct spirits or spiritu ...

often intervene in these situations.

Kidnapping In criminal law, kidnapping is the unlawful confinement of a person against their will, often including transportation/asportation. The asportation and abduction element is typically but not necessarily conducted by means of force or fear: the p ...

is also a common theme in such

folklore Folklore is shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. This includes oral traditions such as tales, legends, proverbs and jokes. They include material culture, ranging ...

. Encantados are said to be fond of abducting humans with whom they fall in love, children born of their illicit love affairs, or just about anyone near the river who can keep them company, and taking them back to the Encante. The fear of this is so great among people who live near the Amazon River that both children and adults are terrified of going near the water between dusk and dawn, or entering water alone. Some who supposedly have encountered encantados while out in their

canoe A canoe is a lightweight narrow water vessel, typically pointed at both ends and open on top, propelled by one or more seated or kneeling paddlers facing the direction of travel and using a single-bladed paddle. In British English, the term ...

s have been said to have gone insane, but the creatures seem to have done little more than follow their boats and nudge them from time to time.