Amazon river dolphin

(Inia geoffrensis)

Common or local names: boto, bufeo, pink river dolphin.

Introduction

IUCN Red List Conservation status: Endangered

The Amazon river dolphin is a unique freshwater species found only in the rivers of South America.  Its striking pink colour and flexible neck, which allows it to twist and turn through the flooded forests during high water seasons, make it a most unusual and unique dolphin species, and one that plays an important role in cultural beliefs and folklore throughout its range. Amazonian folklore includes the mystical ability of the Amazon river dolphin to morph into a handsome man who seduces young village women before leaving them to return to the river (the underwater world). Another legend says that if you swim alone in the Amazon River, you could be carried away by an Amazon river dolphin to “Encante,” a magical underwater city.  More recently, the species is now the focus of ecotourism in some parts of its habitat, and is under a range of threats in many parts of its range. 

Distribution and habitat

Amazon river dolphins are found in the Amazon, Orinoco, and Tocantins-Araguaia River Basins. Their presence has been confirmed in Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela1, and more recently there is photographic evidence of their presence in Guyana. The species is found in freshwater environments ranging from deltas to inland river systems and streams and even flooded forests in the heart of the South American continent. There are two recognised subspecies, the Bolivian bufeo (I. g. boliviensis), and the Common boto (I. g. geoffrensis)2.

Distribution map created by WWF Brazil. Note that the legend refers to the different distribution of the putative species of Amazon river dolphins, not yet recognised by the Society of Marine Mammalogy’s Committee on Taxonomy.

Abundance

Although a number of studies have been conducted to estimate Amazon river dolphin abundance in all the countries where the species occur 39, covering the full range of the species is incredibly challenging, and there is no estimate of population size for the species as a whole. Thus, researchers focus on strategic areas to study trends in abundance and to monitor the conservation status of the species. Where studies have been able to look at population trends, comparing, for example, estimates from a 15-year period in the Amazon River in Colombia (1993 to 2007), or the those from the Mamirauá Reserve in the Brazilian Amazon (23 year period of 1994 to 2017), there is strong evidence that populations are declining 9, 10. In the Mamirauá study area the population is thought to be halving each decade, with the decline attributed to the illegal fishery for the Piricatinga, a valuable species of catfish 10 .

Threats

One of the greatest threats to the continued survival of the Amazon river dolphin is entanglement in gillnets as bycatch. Additional threats are posed by intensive fishing for a species of catfish known as Piracatinga (Calophysus macropterus)11. Fishers use dolphin meat as bait and also employ other illegal fishing methods12. Additional threats include mercury contamination from gold mining 13, disturbance from irresponsible tourism practices 14, 15, increasing river traffic, water-related infrastructure such as dams and large navigation projects1, 6, 7, sand and gravel mining, river pollution and unusual drought exacerbated by climate change, causing low water levels.  Of these threats, water infrastructure, gold mining and gillnets pose the most danger. Dams and navigation projects compartmentalise free-flowing rivers, reducing the dolphins’ range and connectivity.  Dams affect river habitats by reducing water availability and quality, and blocking sediments, nutrients, dolphins and migratory fish (river dolphins’ main prey) from their spawning or feeding grounds. As the price of gold has soared, so too has gold mining and the related pollution it causes in the Amazon. In fact, the small-scale gold mining sector is the main source of mercury emissions in Latin America. Indigenous and traditional communities in the region, as well as river dolphins, accumulate mercury in their tissues over time through their consumption of mercury-contaminated fish18. Gold mining along rivers also destroys the gravel beds (where the gold is found) that migratory fish depend on for habitat and spawning. River dolphins are bio-indicators and a sentinel species of water health4, 19. Changes in their well-being should be interpreted as an (early) warning red flag indicating ecosystem degradation and human health risk.

Biology and ecology

Amazon river dolphins look very different from marine dolphins. Born grey, adult Amazon river dolphins turn pink or pinkish-grey as they mature20. Mature males are pinker than females, which could be due to frequent aggressive interactions with other males. As wounds heal, they are covered with pink scar tissue that eventually replaces their grey skin21.

Amazon river dolphin. Illustration by Uko Gorter.

Unlike other dolphins, the Amazon river dolphin has a flexible neck which allows it to move its head 180 degrees21, 22. This makes it possible to manoeuvre through underwater branches and roots in search of their prey. Almost blind and with a highly pronounced melon (the term for the dolphin’s forehead where echolocation clicks are amplified), the Amazon river dolphin uses its bisonar to displace and localize its prey in the turbid and muddy water23.

Amazon river dolphins have characteristically low, ridge-like dorsal fins on their backs shaped like a keel; long, slender snouts; plump bodies; ‘chubby’ cheeks with sensory whiskers; and round, bulbous foreheads. They can reach 2.8 m in length and 180 kg, making them the largest of the six river dolphin species. Their pectoral fins are large and have a wide angle of rotation. Like other dolphin species, Amazon river dolphins rely heavily on echolocation to navigate and find food in the dark and sometimes turbid waters where they live. However, their eyesight is thought to be more developed than other river dolphin species.

Reproduction and growth

Females reach sexual maturity between 6 and 10 years of age and a body length of 175-190 cm 20, 23, 24. Males reach sexual maturity at about nine years old. Females give birth for the first time at an average age of 9.7 years. Gestation lasts about 11 months, and females give birth to a single calf, which is roughly 75 cm long and weighs 7 kg25. Births can occur throughout the year, but peak during high water levels when the flooded forest provides a quiet and safe place between inundated roots and branches3. The average interval between births is 4.6 years26. Males have been observed apparently trying to ‘woo’ females with clever ‘tricks’ such as picking up floating plants or pieces of wood with their beaks, twirling in circles and then smacking the object on the water’s surface. They can live for up to 40 years23, 26.

The Amazon River Dolphin, Inia geoffrensis, Tapajos River, Santarem region, Pará State, Brazilian Amazon. © WWF-Brazil / Adriano Gambarini

Diet

Amazon river dolphins are the only dolphin species that have the equivalent of molar teeth, much like humans do.  All other dolphin species have only pointed conical teeth27. Molars allow them to crunch up particularly hard skinned fish. Their diet is quite broad, including over 40 species of fish, turtles, shrimp and crabs24.

Behaviour

The Amazon river dolphin, Inia geoffrensis, Tapajos River, Santarem region, Pará State, Brazilian Amazon. © WWF-Brazil / Adriano Gambarini

They are natural gymnasts, often pivoting, swimming backwards and upside down. Active day and night, they appear inquisitive and playful to human observers. They have been known to approach canoes, grabbing a paddle in their beaks and making off with their prize. They can also throw sticks and weeds, pull grass underwater and ‘play’ with turtles, snakes and fish. While observations of larger groups feeding together in one area have been recorded, these dolphins are usually found alone, in mother-calf pairs or in small pods of 3 or 4 individuals 3.

Also in this section

Ganges river dolphin

The Ganges river dolphin is known as the “Tiger of the Ganges” for the role it plays as a top predator, and an ecosystem indicator species; much like a tiger does in a forest.

Indus river dolphin

Indus river dolphins are also known as the blind dolphins because over millions of years they have lost the use of their eyes.

Irrawaddy dolphin

The Irrawaddy dolphin is found in marine, brackish, and freshwater environments throughout southeast Asia. The name is derived from the first specimens that were described from the Ayeyarwady river in Myanmar.

Tucuxi

Tucuxis overlap in range with Amazon river dolphins inhabiting the central region of the Amazon river basin.

Yangtze finless porpoise

The Yangtze finless porpoise is the only freshwater porpoise in the world. It can only be found in the Yangtze River, the longest river in Asia.

References

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  2. da Silva, V., Trujillo, F., Martin, A., Zerbini, A.N., Crespo, E., Aliaga-Rossel, E. & Reeves, R. 2018. Inia geoffrensis. The IUCN Red List of Threatened Species 2018: e.T10831A50358152. https://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T10831A50358152.en.
  3. Martin, A.R., V. Da Silva & D.L. Salmon. 2004. Riverine habitat preferences of botos (Iniageoffrensis) and tucuxis (Sotalia fluviatilis) in the central Amazon. Marine Mammal Science 20(2): 189-200
  4. Gómez-Salazar, C., Trujillo, F., and Whitehead, H. 2012b. Ecological factors influencing group sizes of river dolphins (Inia geoffrensis and Sotalia fluviatilis). Marine Mammal Science 28(2): E124-E142.
  5. Gómez-Salazar, C., Trujillo, F., Portocarrero-Aya, M., and Whitehead, H. 2012a. Population density estimates, and conservation of river dolphins (Inia and Sotalia) in the Amazon and Orinoco river basins. Marine Mammal Science 28: 124-153.
  6. Mosquera-Guerra F, Trujillo F, Meluk HM, Valência AV, Valderrama DHR, Navarro FAV, et al. (2015) Tamanho populacional, densidade e distribuição de Inia geoffrensis e Sotalia fluviatilis na bacia média do rio Caquetá. Momentos da Ciência 12 (2).
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  8. Pavanato, H.J., Melo-Santos, G., Lima, D.S., Portocarrero-Aya, M., Paschoalini, M., Mosquera, F., Trujillo, F., Meneses, R., Marmontel, M., and Maretti, C. 2016. Risks of dam construction for South American river dolphins: a case study of the Tapajós River. Endangered Species Research 31: 47-60.
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  10. Da Silva, V., C.E. Freitas, R.L. Dias & A.R. Martin. 2018. Both cetaceans in the Brazilian Amazon show sustained, profound population declines over two decades. PloS One 13(5): e0191304
  11. Trujillo et al. 2020. The piracatinga (Calophysus macropterus) fishery and its impact on river dolphin conservation: an update. International Whaling Commission Scientific Committee Report SC/68B/SM/01.
  12. Brum, S.M., V. Da Silva, F. Rossoni & L. Castello. 2015. Use of dolphins and caimans as bait for Calophysus macropterus (Lichtenstein 1819) (Siluriformes: Pimelodidae) in the Amazon. Applied Ichthyology 31: 675-680.
  13. Mosquera-Guerra, F., Trujillo, F., Parks, D., Oliveira da Costa, M., Van Damme, P., Echeverría, A., Franco, N., Carvajal-Castro, J., Mantilla-Meluk, H., Marmontel, M & D. Armenteras. 2019. Mercury in populations of river dolphins of the Amazon and Orinoco basins. EcoHealth 16, 743-758. https://doi.org/10.1007/s10393-019-01451-1
  14. Alves, L. C. P. de Sá; Andriolo, A.; Orams, M. B. Feeding Amazônia boto (Inia geoffrensis) as a tourism attraction. A path toward tragedy? VI International Congress on Coastal and Marine Tourism, 2009b. p. 225-235
  15. de Sá Alves, L. C. P., Andriolo, A., Orams, M. B., & de Freitas Azevedo, A. (2013). Resource defence and dominance hierarchy in the boto (Inia geoffrensis) during a provisioning program. Acta Ethologica, 16(1), 9-19.
  16. Pavanato, H.J., G. Melo-Santos, D.S. Lima, M. Portocarrero-Aya, M. Paschoalini, F., Mosquera, F. Trujillo, R. Meneses, M. Marmontel & C. Maretti. 2016. Risks of dam construction for South American river dolphins: a case study of the Tapajós River. Endangered Species Research 31: 47-60
  17. Paschoalini, M., R. Marques Almeida, F. Trujillo, G. Melo-Santos, M. Marmontel, H.J., Pavanato, F. Mosquera, N. Ristau & A. Novaes Zerbini. 2020. On the brink of isolation: Population estimates of the Araguaian river dolphin in a human-impacted region in Brazil. PLOS One, Biodiversity conservation issue 15(4): e0231224. https://doi.org/10.1371/journal.pone.0231224
  18. Venturieri, R., Oliveira-da Costa, M., Gama, C., & Jaster, C. B. (2017). Mercury contamination within protected areas in the brazilian Northern amazon-amapá state. American Journal of Environmental Science, 13, 11-21
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  25. Da Silva, V. M., Carter, A. M., Ambrosio, C. E., Carvalho, A. F., Bonatelli, M., Lima, M. C., & Miglino, M. A. (2007). Placentation in dolphins from the Amazon River Basin: the Boto, Inia geoffrensis, and the Tucuxi, Sotalia fluviatilis. Reproductive Biology and Endocrinology, 5(1), 1-6.
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  27. Loch, C., Swain, M. V., van Vuuren, L. J., Kieser, J. A., & Fordyce, R. E. (2013). Mechanical properties of dental tissues in dolphins (Cetacea: Delphinoidea and Inioidea). Archives of oral biology, 58(7), 773-779.