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 endemic to South America and is classified in the family Iniidae. Three subspecies are currently recognized: I. g. geoffrensis (Amazon river dolphin), I. g. boliviensis (Bolivian river dolphin) and I. g. humboldtiana (Orinoco river dolphin). The position of the Araguaian river dolphin (I. araguaiaensis) within the clade is still unclear. The three subspecies 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 many adult males reaching in weight, and 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% longer and weighing 55% more than females. Like other toothed whales, they have a melon, an organ that is used for bio sonar. The dorsal fin, although short in height, is regarded as long, and the pectoral fins are also large. The fin size, unfused vertebrae, and its relative size allow for improved maneuverability when navigating flooded forests and capturing prey.

They have one of the widest-ranging diets among toothed whales, and feed on up to 53 different species of fish, such as croakers, catfish, tetras and piranhas. They also consume other animals such as river turtles, aquatic frogs, and freshwater crabs. Today, however, the Amazon river dolphin has been reclassified into the superfamily Inioidea.

There is ongoing debate about the classification of species and subspecies. The IUCN and Committee on Taxonomy of the Society for Marine Mammalogy recognize two subspecies: I. g. geoffrensis (Amazon river dolphin) and I. g. boliviensis (Bolivian river dolphin). Molecular analysis suggests that the Orinoco dolphins are derived from the Amazon population and are not genetically distinct. Comparative morphological research also indicates that I. g. humboldtiana is not distinguishable from I. g. geoffrensis. However, Cañizales (2020) found that I. g. humboldtiana skulls were morphologically distinct and recommended that it be elevated to species status.

In 2012, it was proposed that I. g. boliviensis was a different species based on skull morphology. A recent study, with more comprehensive sampling of the Madeira system, including above and below the Teotonio Rapids (which were thought to obstruct gene flow), found that the Inia above the rapids did not possess unique mtDNA. As such the species level distinction once held is not supported by current results. Therefore, the Bolivian river dolphin is currently recognized as a subspecies. In addition, a 2014 study identifies a third species in the Araguaia-Tocantins basin,

Subspecies

thumb|The [[Bolivian river dolphin is a subspecies of Inia geoffrensis]]

Inia geoffrensis geoffrensis 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 Rio Negro through the channel of Casiquiare to San Fernando de Atabapo in the Orinoco river, including its tributary: the Guaviare.

Inia geoffrensis boliviensis

Inia geoffrensis humboldtiana

The texture of the body is robust and strong but flexible. Unlike in oceanic dolphins, the cervical vertebrae are not fused, allowing the head to turn 90 degrees. but decreasing its speed.

thumb|Amazon river dolphins have a [[heterodont dentition]]

The skull of the species is slightly asymmetrical compared to the other toothed whales. It has a long, thin snout, with 25 to 28 pairs of long and slender teeth to each side of both jaws. Dentition is heterodont, meaning that the teeth differ in shape and length, with differing functions for both grabbing and crushing prey. Anterior teeth are conical and later have ridges on the inside of the crown. Despite small eyes, the species seems to have good eyesight in and out of the water. It has a melon on the head, the shape of which can be modified by muscular control when used for biosonar. Breathing takes place every 30 to 110 seconds.

Until his death in December 2020, an individual named "Baby" lived up to (at least) 46 years at the Duisburg Zoo in Germany. At the time of his death, he had lived at Duisburg Zoo for 45 years and 9 months.

Biology and ecology

The Amazon river dolphin are commonly seen singly or in pairs, but may also occur in pods that rarely contain more than eight individuals. Pods as large as 37 individuals have been seen in the Amazon, but the average is three. In the Orinoco, the largest observed groups number 30, but the average is just above five. Typically, social bonds occur between mother and child, but may also been seen in heterogeneous groups or bachelor groups. The largest congregations are seen in areas with abundant food, and at the mouths of rivers. There is significant segregation during the rainy season, with males occupying the river channels, while females and their offspring are located in flooded areas. However, in the dry season, there is no such separation. Due to the high level of prey fish, larger group-sizes are seen in large sections that are directly influenced by whitewater (such as main rivers and lakes, especially during low water season) than in smaller sections influenced by blackwater (such as channels and smaller tributaries).

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 often have a significant degree of damage in the dorsal, caudal, and pectoral fins, as well as the blowhole, due to bites and abrasions. They also commonly have numerous secondary teeth-raking scars. This suggests fierce competition for access to females, with a polygynous mating system, though polyandry and promiscuity cannot be excluded.

In captivity, courtship and mating foreplay have been documented. The male takes the initiative by nibbling the fins of the female, but reacts aggressively if the 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, lying head to head, or head to tail. The dolphin's dentition allows it to access shells of river turtles (such as Podocnemis sextuberculata) and freshwater crabs (such as Poppiana argentiniana). By sending out clicks of lower amplitude only nearby objects will cast back detectable echoes, and hence fewer echoes need to be sorted out, but the cost is a reduced biosonar range. Toothed whales generally do not produce a new echolocation click until all relevant echoes from the previous click have been received, so if detectable echoes are only reflected back from nearby objects, the echoes will quickly return, and the Amazon river dolphin is then able to click at a high rate.

Male Amazon river dolphins have been observed to exhibit an unusual aerial urination behavior, predominantly in social contexts involving other males. During these events, a urinating male positions itself upside down, ejecting a urine stream into the air, while a receiving male actively seeks the stream with its rostrum, possibly detecting chemical signals through specialized bristles. Researchers speculate that this behavior likely serves a social or communicative function beyond waste elimination, potentially facilitating chemical communication in a species with limited olfactory capabilities.

Distribution and population

thumb|The main branch of the Amazon River near [[Fonte Boa, Amazonas|Fonte Boa, Brazil, with multiple floodplains, lakes and smaller channels. The Amazon river dolphin is observed here throughout the year]]

Amazon river dolphins are the most widespread river dolphins. They are present in six countries in South America: Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela, in an area covering about . The boundaries are set by waterfalls, such as the Xingu and Tapajós rivers in Brazil, as well as very shallow water. A series of rapids and waterfalls in the Madeira River have isolated one population, recognized as the subspecies I. g. boliviensis, in the southern part of the Amazon basin in Bolivia. 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 from where they were first registered.

Interactions with humans

In captivity

thumb|A trained Amazon river dolphin at the [[Acuario de Valencia]]

Historically, this species had been kept across select zoos and dolphinariums. Several hundred individuals were captured between the 1950s and the mid-1970s, and were distributed throughout the United States, Europe, and Japan.

In the USA alone, over 70 were directly imported, and of those individuals, only 20 would survive. On February 20 2002, "Chuckles", a 34-year-old male, died at the Pittsburgh Zoo & Aquarium as the last of his species in the United States.

Threats

The region of the Amazon in Brazil has an extension of containing diverse fundamental ecosystems. 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. The várzea is also a major source of income through excessive local commercialized fishing. Várzea consists of muddy river waters containing a vast number and diversity of nutrient rich species.

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 are removed from the commercial catch in the nets and lines, it causes damages to the equipment and the capture, as well as generating ill will from the local fishermen. The use of the Amazon river dolphin carcass as bait for the piracatinga dates back to 2000.

The Amazon river suffers from pollution by mercury from industrial and mining sources. High mercury concentrations were found in muscle tissue from a male Amazon river dolphin in the Tapajos River (Brazil), and in the body and milk of female dolphins.

In September 2023, 154 Amazon river dolphins died in Brazil's Lake Tefé following record-high water temperatures of and reduced water levels during a drought. While there are ongoing studies by the Mamirauá Institute for Sustainable Development to determine the cause of the deaths, the leading hypothesis is that the elevated temperature caused algae in the lake to release a toxin that attacks the central nervous system.

Conservation

In 2008, the International Whaling Commission (IWC) expressed concern for captured 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, because it has an unfavorable conservation status or would benefit significantly from international co-operation organized by tailored agreements.

According to a previous 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.

In September 2012, Bolivian President Evo Morales enacted a law to protect the dolphin and declared it a national treasure.

The Global Declaration for River Dolphins seeks to reverse the decline of river dolphin populations throughout the world. As of early 2024, 11 of the 14 countries that have river dolphins have signed the declaration. Local legends also state that the dolphin is the guardian of the Amazonian manatee, and that if one should 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.

External links

Omacha Foundation—A non-government and non-profit organization created to study, research and protect river dolphins and other fauna and aquatic ecosystems in Colombia. Winner of the 2007 Whitley Awards (UK).
River Dolphin Research Program —Research project devoted to the study of the ecology and conservation of river dolphins in the Amazon basin, based in the Federal University of Western Pará. The scope of this research project focuses on ecological studies, as well as the impact that human activities have on their survival.
Convention on Migratory Species page on the Amazon Dolphin