Amazon river dolphin

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Amazon river dolphin
Amazonas Flussdelfin Apure Orinoco Duisburg 01.jpg
Amazon river dolphin size.svg
Size compared to an average human
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Eutheria
Order: Artiodactyla
Infraorder: Cetacea
Parvorder: Odontoceti
Superfamily: Inioidea
Family: Iniidae
Genus: Inia
Species: I. geoffrensis
Binomial name
Inia geoffrensis
(Blainville, 1817)
Cetacea range map Amazon River Dolphin.PNG
Amazon river dolphin range

The Amazon river dolphin (Inia geoffrensis), also known as the Boto or pink river dolphin, is a species of toothed whale classified in the family Iniidae. Three subspecies are currently recognised: I. g. geoffrensis (Araguaian river dolphin), I. g. boliviensis (Bolivian river dolphin) and I. g. humboldtiana (Orinoco river dolphin), which are distributed in the Amazon basin, the upper Madeira River in Bolivia, and the Orinoco basin, respectively.

The Amazon river dolphin is the largest species of river dolphin, with adult males reaching 185 kilograms (408 lb) in weight, and 2.5 metres (8.2 ft) in length . Adults acquire a pink color, more prominent in males, giving it its nickname "pink river dolphin". Sexual dimorphism is very evident, with males measuring 16% and weighing 55% more than females. Like other toothed whales, they have a melon, and organ that is used for biosonar. The dorsal fin short in height, but nonetheless long, and their pectoral fins are also large. This feature, along with its medium size and the unfused cervical vertebrae, gives it great maneuverability to navigate the flooded forests and capture their prey.

They has one of the widest diets among toothed whales; they feed mainly on fish, as many as 53 different species, such as croakers, catfish, tetras and piranhas, while also consuming other animals such as river turtles and freshwater crabs.[2]

In 2008, it was ranked by the International Union for Conservation of Nature (IUCN) as a Data Deficient , due to the uncertainty regarding their population trends and the impact of threats. There has been significant hunting, but in recent decades, habitat loss and bycatch in fishing gear have become major threats. For its striking pink tinge, it is the only species of river dolphin kept in captivity, mainly in the United States, Venezuela and Europe ; however, it is difficult to train and a high mortality in captive species.


An Amazon river dolphin as depicted in Brehms Tierleben, 1860's

The species Inia geoffrensis was described by Henri Marie Ducrotay de Blainville in 1817. The Amazon river dolphin belonged to the superfamily Platanistoidea (river dolphins), which used to constitute all river dolphins, which made them paraphyletic.[3] Today, however, the Amazon river dolphin has been reclassified into the superfamily Inioidea.[4] There is no consensus on when and how they penetrated the Amazon basin; they may have done during the Miocene from the Pacific Ocean, before the formation of the Andes, or from the Atlantic Ocean.[5][6]

Three subspecies are recognized:[7] I. g. geoffrensis (Araguaian river dolphin), I. g. boliviensis (Bolivian river dolphin) and I. g. humboldtiana (Orinoco river dolphin).[4] However, based on skull morphology in 1994, it was proposed that I. g. boliviensis was a different species.[8] In 2002, following the analysis of mitochondrial DNA specimens from the Orinoco basin, the Putumayo River (tributary of the Amazon) and the Tijamuchy and Ipurupuru rivers , in the Bolivian Amazon, a study concluded that genus Inia was divided into at least two evolutionary lineages: one restricted to the river basins of Bolivia and the other widely distributed in the Orinoco and Amazon; until 2009, the matter remained unresolved.[9][10]


The Bolivian river dolphin is a subspecies of Inia geoffrensis

Inia geoffrensis geoffrensis:[4] it inhabits most of the Amazon River, including rivers Tocantins, Araguaia , low Xingu and Tapajos, the Madeira to the rapids of Porto Velho, and rivers Purus, Yurua, Ica, Caqueta, Branco, and the Black River through the channel of Casiquiare to San Fernando de Atabapo in the Orinoco river, including its tributary: the Guaviare.
Inia humboldtiana geoffrensis:[4] they are located in the Orinoco River basin, including the Apure and Meta rivers . This subspecies is restricted, at least during the dry season, to the waterfalls of Black River rapids in the Orinoco between Samariapo and Puerto Ayacucho, and the Casiquiare canal.
Inia geoffrensis boliviensis:[4] it has populations in the upper reaches of the Madeira River, upstream of the rapids of Teotonio, in Bolivia. It is confined to the Mamore River and its main tributary, the Iténez.[11]

Biology and ecology[edit]


Male Amazon river dolphins are either solid pink or mottled grey/pink

The Amazon river dolphin is the largest river dolphin. Adult males reach a maximum length and weight of 2.55 metres (8.4 ft) (average 2.32 metres (7.6 ft)) and 185 kilograms (408 lb) (average 154 kilograms (340 lb)), while females reach a length and weigh of 2.15 metres (7.1 ft) (mean 2 metres (6.6 ft)) and 150 kilograms (330 lb) (average 100 kilograms (220 lb)). It has very evident sexual dimorphism, with males measuring and weighing between 16% and 55% more than females, making it unique among cetaceans in which females are generally larger than males.[12]

The texture of the body is robust and strong but flexible enough; unlike oceanic dolphins; their cervical vertebrae are not fused, allowing the head to turn 90°.[13] The flukes are broad and triangular; the dorsal fin, which is keel-shaped, is short in height, but is very long and extends from the middle of the body to the caudal region. The pectoral fins are large and paddle-shaped. The length of these fins allow them to perform circular movement, giving exceptional maneuverability to swim through the flooded forest, but this decreases speed.[14]

The body colour varies with age. Newborns and young have a dark gray tint; in adolescence, it is transformed into light gray and adults turn pink as a result of repeated abrasion of the skin surface. Males tend to be more pink than females due to more frequent trauma from intraspecific aggression (between individuals of the same species). The colour of adults varies between solid and mottled pink. In some adults, the dorsal surface is darker; it is believed that the difference in color depends on the temperature, water transparency, and geographical location. There is one albino on record, kept in an aquarium in Germany.

Amazon river dolphins have a heterodont dentition

The skull of the species is slightly asymmetrical compared to the other toothed whales. It has a prominent nose, with 25 to 28 pairs of teeth long and slender to each side of both jaws. They have a heterodont dentition, meaning the teeth differ in shape and length; anterior teeth are conical and later have ridges on the inside of the crown. The eyes are small, but seem to have good eyesight in and out of water. They have a melon on the head; the shape can be modified by muscular control when used for biosonar.[14]

It has a long, thin snout with 25–28 pairs of teeth in the hemimaxilias. The front teeth are sharp, while the back teeth are flatter and cupped. The two tooth types serve different functions: grab and crush prey. Breathing takes place every 30 to 110 seconds. Gestation lasts 315 days, after which a baby is born two years, staying beside the mother.[14]


Apure the dolphin lived for forty years at the Duisburg Zoo

Life expectancy of the Amazon river dolphin in the wild is unknown, but in captivity, the longevity of healthy individuals has been recorded to be between ten and thirty years. However, the average longevity in captive animals is only 33 months. It is estimated that an individual named Apure at Zoo Duisburg, Germany, managed to live more than forty years, being captured from the wild at age nine.[15]


The Amazon river dolphin tends to be solitary, but is sometimes seen in groups. Pods usually contain up to four individuals. Usually, social bonds occur between mother and child, but may consist of heterogeneous groups or bachelor groups. The largest congregations are seen in areas with abundant food, and in the mouths of rivers. There is significant segregation during the rainy season, in which males are placed in the channels rivers, while females and their offspring are located in flooded areas; in the dry season, there is no such separation.[13][16]

Captive studies have shown that the Amazon river dolphin is less shy than the bottlenose dolphin, but also less sociable. It is very curious and remarkable lack of fear of foreign objects. However, in captivity may not show the same behavior in their natural environment. Wild dolphins exhibit a variety of behaviours; they have been reported to: hold the oars of the fishermen, rub against the boat, pluck underwater plants, and playing with sticks, logs, clay, turtles, snakes, and fish.[4]

They are slow swimmers; they commonly travel at speeds of 1.5 to 3.2 kilometres per hour (0.93 to 1.99 mph), but have been recorded to swim at speeds up to 14 to 22 kilometres per hour (8.7 to 13.7 mph). When it surfaces, the tip of the snout, melon and dorsal fin appear simultaneously on the surface, rarely showing the tail before diving. They can also shake fins, pull the tail fin and the head above the water, but the latter do so to observe the environment; they can jump out of the water, though rarely, with some jumping as high as a meter (3.14 ft). They are harder to train than most other dolphins.[4]


Females reach maturity between six or seven years. Breeding is seasonal, coinciding with the dry season when the water level is low. Gestation lasts for eleven months with delivering occurring during the flood season. Pups weigh 80 kilograms (180 lb) at birth and lactation lasts up to a year at intervals of two to three years between pregnancies.[4]

Adult males have been observed carrying objects in their mouths, objects such as branches or other floating vegetation, or balls of hardened clay. The males appear to carry these objects as a sociosexual display which is part of their mating system. The behaviour is "triggered by an unusually large number of adult males and/or adult females in a group, or perhaps it attracts such into the group. A plausible explanation of the results is that object carrying is aimed at females and is stimulated by the number of females in the group, while aggression is aimed at other adult males and is stimulated by object carrying in the group."[17] Before determining that the species had an evident sexual dimorphism, it was postulated that the river dolphins were monogamous. Later, it was shown that males were larger than females and are documented wielding an aggressive sexual behavior in the wild and in captivity. Males have a significant degree of damage in the dorsal, caudal, and pectoral fin as well as the blowhole due to bites and abrasions, in addition to the numerous secondary teeth-raking scars. This suggests a fierce competition for access to females. This suggests a polygynous mating system, but polyandry and promiscuity cannot be excluded.[18]

In captivity, courtship and mating foreplay have been documented. Males take the initiative by nibbling the fins of the female, but react aggressively if female is not receptive. A high frequency of copulations in a couple was observed; they used three different positions: contacting the womb at right angles, parallel lying head to head, or head to tail.[6]

Breeding is seasonal and births occur between May and June. The period of birthing coincides with the flood season, and this may provide an advantage because the females and their offspring remain in flooded areas longer than males. As the water level begins to decrease, the density of dams in flooded areas increases due to loss of space, providing the energy required for infants to meet the high demands required for growth. Gestation is estimated to be around eleven months and captive births take 4 to 5 hours. At birth, they are 80 centimetres (31 in) long and in captivity have registered a growth of 0.21 metres (0.69 ft) per year. Lactation takes about a year. The interval between births is estimated between 15 and 36 months, and the river dolphins are thought to be dependent within two to three years.[6]

The relatively long duration of breastfeeding and parenting suggests a strong mother-child bond. Most couples observed in their natural environment consist of a female and her calf. This suggests that long periods of parental care contribute to learning and development of young.[6]


Amazon river dolphin feeding

The diet of the Amazon river dolphin is the most diverse among toothed whales. It consists of at least 53 different species of fish grouped in 19 families. The prey size is between 5 and 80 centimetres (2.0 and 31.5 in), with an average of 20 centimetres (7.9 in). The most frequently consumed fish belong to the families Sciaenidae (croakers), Cichlidae, and Characidae (tetras and piranhas); their dentition allows them to access shells of river turtles and freshwater crabs.[2][4] Their diet is more diverse during the wet season, when fish are spread in flooded areas outside riverbeds, and become more difficult to catch, and becomes more selective during the dry season when prey density is greater.[14]

Usually, they are active during the day and night and are feeding themselves throughout; however, they are predominantly crepuscular; they consume about 5.5% of their body weight per day. They sometimes takes advantage of the disturbances made by boats to catch their prey disoriented. Sometimes, they are associated with tucuxis and giant otters (Pteronura brasiliensis) to hunt in a coordinated manner, by gathering and attacking fish stocks at the same time. Apparently, there is little competition for food between these species, as each prefers different prey. It has also been observed that captive dolphins share food.[2][4]


The species, like other dolphins, use whistling tones to communicate. The issuance of these sounds is related to the time they return to the surface, before diving, suggesting they are related to food. The acoustic analysis revealed that the vocalisations are different in structure to the typical whistles of other species of dolphins.[19]

Distribution and population[edit]

Main branch of the Amazon River near Fonte Boa , Brazil, with multiple floodplains, lakes and smaller channels. The Amazon river dolphin is observed here throughout the year

The Amazon river dolphin is the most widespread river dolphin. It is present in six countries in South America: Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela, in an area covering about 7,000,000 square kilometres (2,700,000 sq mi). Its boundaries are set by waterfalls, such as the Xingu and Tapajos rivers in Brazil, as well as very shallow water. A series of rapids and waterfalls in the Madeira River have isolated the population, recognised as the subspecies I. g. boliviensis, south of the Amazon basin in Bolivia.[4]

They are also distributed in the basin of the Orinoco River, except the Caroni River and the upper Caura River in Venezuela. The only connection between the Orinoco and the Amazon is through the Casiquiare canal. The distribution of dolphins in the rivers and surrounding areas depend on the time of year; in the dry season it is located in the river beds, but in the rainy season, when the rivers overflow, disperse to the flooded areas, both the forests and the plains.[4]

Studies to estimate the population are difficult to analyze due to the difference in the methodology used. In a study conducted in the stretch of the Amazon called Solimões River, with a length of 1,200 kilometres (750 mi) between the cities of Manaus and Tabatinga, a total of 332 individuals was sighted ± 55 per inspection and density was estimated to be 0.08- 0.33 animals per square kilometer in the main channels, and 0.49 to 0.93 animals per square kilometer in the branches. In another study, on a stretch of 120 kilometres (75 mi) at the confluence of Colombia, Brazil and Peru, 345 individuals with a density of 4.8 in the tributaries around the islands 2.7 and 2.0 were observed along the banks. Additionally, another study was conducted in the Amazon at the height of the mouth of the Caqueta River for six days. As a result of the studies conducted, it was found that the density is higher in the riverbanks, 3.7 per km, decreasing towards the center of the river. In studies conducted during the rainy season, the density observed in floodplain was 18 animals per square kilometer, while on the banks of rivers and lakes ranged from 1.8 to 5.8 individuals per square kilometer. These observations suggest that the Amazon river dolphin is found in higher density than any other cetacean.[4]


The Amazon river dolphin is located in almost all habitats, including river basins, major courses of rivers, canals, river tributaries, lakes and at the end of the rapids and waterfalls. Cyclical changes in the water level of rivers takes place during the rainy season and dry season throughout the year. During the dry season, they occupy the main river channels. During the rainy season, the dolphins can move easily to smaller tributaries to the forest and floodplains.[6]

Males and females appear to have selective habitat preferences; the males return to the main river channels when water levels are still high while the females and their offspring remain in the flooded areas almost year-round. This is probably because it decreases the risk of aggression by males toward the young and predation by other species.[6]


In the Pacaya Samiria National Reserve , Peru, photoidentification is used to recognize individuals based on pigmentation patterns, scars and abnormalities in its beak. Seventy two copies were recognized, of which 25 were again observed between 1991 and 2000. The intervals between sightings ranged from one day to 7 and a half years. The maximum range of motion was 220 kilometres (140 mi), with an average of 60.8 kilometres (37.8 mi). The longest distance in one day was 120 kilometres (75 mi), with an average of 14.5 kilometres (9.0 mi).[20] In a previous study conducted at the center of the Amazon River, a dolphin was observed that moved only a few dozen kilometers from the dry season and wet season. However, three of the reviewed 160 animals were observed over 100 kilometres (62 mi) from where they were first registered.[11]

Interactions with humans[edit]

In captivity[edit]

A trained Amazon river dolphin at the Acuario de Valencia

The Amazon river dolphin has historically been kept in dolphinariums. Today, only three exist in captivity: one in Acuario de Valencia in Spain, one in Zoologico de Guistochoca in Peru, and one in Duisburg Zoo 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 died in Pittsburgh Zoo in 2002.[13]


The region of the Amazon in Brazil has an extension of 5,000,000 km2 (1,900,000 sq mi) containing diverse fundamental ecosystems.[21][22] One of these ecosystems is a floodplain, or a várzea forest, and is home to a large number of fish species which are an essential resource for human consumption.[23] The várzea is also a major source of income through excessive local commercialized fishing.[21][24][25] Várzea consist of muddy river waters containing a vast number and diversity of nutrient rich species.[17] The abundance of distinct fish species lures the Amazon River dolphin into the várzea areas of high water occurrences during the seasonal flooding.[26]

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.[27][28][29][21][30][31][32][33] The local fishermen overfish and when the Amazon River dolphins remove the commercialized fish from the nets and lines, it causes damages to the equipment and the capture, as well as a negative reaction from the local fishermen.[29] [31][32] The negative reactions of the local fishermen is also attributed to the Brazilian Institute of Environment and Renewable Natural Resources prohibiting from killing the Amazon river dolphin, yet not compensating the fishermen for the damage done to their equipment and capture.[33]

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.[23] The carcasses are discarded, consumed, or used as bait to attract a scavenger catfish, the piracatinga (Calophysus macropterus).[23][34] The use of the Amazon river dolphin carcass as bait for the piracatinga dates back from 2000.[34] 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.[33]

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.[23] The data do 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.[23][28][31] Scavenger species feed upon the carcasses and the complexity of the river currents make it nearly impossible to locate all the carcasses.[23] More importantly, the local fishermen do not report these deaths out of fear that a legal course of action will be taken against them,[23] as the Amazon river dolphin and other cetaceans are protected under the Brazilian federal law prohibiting any takes, harassments, and kills of the species.[35]


The Amazon river dolphin is listed on appendix II[36] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). It is listed on Appendix II as it has an unfavourable conservation status or would benefit significantly from international co-operation organized by tailored agreements. In September 2012, Bolivian President Evo Morales enacted a law to protect the dolphin and declared it a national treasure.[13][37]

In 2008, the species was listed on the red list of endangered species, but in 2011, the IUCN stated it as Data Deficient (DD).[1] The species was previously listed as "vulnerable" but their conservation status changed due to the limited amount of current available information on threats, ecology, and population trends. In areas where these dolphins have been studied, they appear well extended and relatively abundant. However, these areas represent only a small proportion of the total distribution of the species and are often sites where they are protected. However, the information of these areas may not be representative and may not be valid in the long term.[1]

However, pollution and gradual destruction of the Amazon rainforest and the vulnerability of the species have been taken for their protection in all countries it inhabits. Their biggest threats are deforestation and other human activities that contribute to disrupt and alter their environment.[4] A source of concern is the difficulty of keeping them alive in captivity, due to intraspecific aggression and low longevity. Captive breeding is not a conservation option for this species.[38]

In 2008, the International Whaling Commission (IWC) expressed concern for the capture botos for use as bait in the Central Amazon, which is an emerging problem that has spread on a large scale. The species is listed in Appendix II of the Convention on International Trade in Endangered Species Fauna and Flora (CITES), and Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals.[39]

According to the assessment by the Scientific Committee of the International Whaling Commission in 2000, the population of botos appears great and there is little or no evidence of population decline in numbers and range. However, increased human intervention on their habitat is expected to, in the future, be the most likely cause of the decline of its range and population. A series of recommendations were issued to ensure proper follow-up to the species, among which is the implementation and publication of studies on the structure of populations, making a record of the distribution of the species, information about potential threats as the magnitude of fishing operations and location of pipelines, more detailed records on risks, distribution, amount of each of the populations.[40]

In mythology[edit]

Main article: Encantado

In traditional Amazon River folklore, at night, an Amazon river dolphin becomes a handsome young man who seduces girls, impregnates them, and then returns to the river in the morning to become a dolphin again. This dolphin shapeshifter is called an encantado. The myth has been suggested to have arisen partly because dolphin genitalia bear a resemblance to those of humans. Others believe the myth served (and still serves) as a way of hiding the incestuous relations which are quite common in some small, isolated communities along the river.[41] In the area, tales relate it is bad luck to kill a dolphin. Legend also states that if a person makes eye contact with an Amazon river dolphin, he or she will have lifelong nightmares. Local legends also state the dolphin is the guardian of the Amazonian manatee, and that, should one wish to find a manatee, one must first make peace with the dolphin.

Associated with these legends is the use of various fetishes, such as dried eyeballs and genitalia.[41] These may or may not be accompanied by the intervention of a shaman. A recent study has shown, despite the claim of the seller and the belief of the buyers, none of these fetishes is derived from the boto. They are derived from Sotalia guianensis, are most likely harvested along the coast and the Amazon River delta, and then are traded up the Amazon River. In inland cities far from the coast, many, if not most, of the fetishes are derived from domestic animals such as sheep and pigs.[42]

See also[edit]


  1. ^ a b c 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. (2013). "Inia geoffrensis". IUCN Red List of Threatened Species. Version 2015.3. International Union for Conservation of Nature. Retrieved 21 October 2015.  Database entry includes a lengthy justification of why this species is data-deficient.
  2. ^ a b c American Cetacean Society Fact Sheet. "Boto (Amazon river dolphin)". American Cetacean Society. Retrieved December 6, 2011. 
  3. ^ The Paleobiology database. "Superfamily Platanistoidea". fossilworks. Retrieved 21 November 2015. 
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  9. ^ Banguera-Hinestroza, E; Cardenas, H; Ruiz-García, M; Marmontel, M; Gaitan, E; Vazquez Garcia-Vallejo, F (2002). "Molecular identification of evolutionarily significant units in the Amazon River dolphin Inia sp. (Cetacea: Iniidae)" 93 (5). pp. 312–322. 
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