Araguaian River Dolphin

The Araguaian river dolphin, or Araguaian boto, (Inia araguaiaensis), is a new species of river dolphin population whose identification as a species distinct from the Amazon river dolphin was announced in 2014. It is native to the Araguaia–Tocantins basin of Brazil. The Araguaian dolphins are closely related to two other species, the Bolivian river dolphin and Amazon river dolphin; the Araguaian dolphins were only described as a separate species in 2014,. The Araguaian river dolphin or Araguaian boto (Inia araguaiaensis) is a South American river dolphin population native to the Araguaia–Tocantins basin of Brazil Contents 1 Discovery and species recognition. In river dolphin The Araguaian boto ( I. Araguaiaensis ), which is physically similar to the Amazon river dolphin, was classified as a separate species in 2014 on the basis of its distinct DNA. This species inhabits the Araguaia-Tocantins river system in Brazil.

One Mammal that is shrouded in mystery are the Araguaian river dolphins of Brazil. The dolphins was considered to be quite solitary, with little social structure that would require communication. But a biologist at the University of Vermont, Laura May Collado, and her colleagues have discovered that the dolphin can indeed make hundreds of different sounds to communicate, finding that could help uncover how communication revolved in marine mammals. The biologists published their findings in the journal Peerj.


Collado said that they discovered that the dolphin interacts socially and are making more sounds than previously thought. They have quite a diverse vocal repertoire.

River Dolphin In Captivity

The biologists used underwater cameras and microphones to record sounds and interactions between the dolphins at the market and took some genetic samples. They were able to identify 237 different types of sounds the dolphins make, but even with 20 hours of recordings, the researchers don't believe they captured the entire acoustic repertoire of the animals. The most common sounds were short, two-part calls that baby dolphins made when they were approaching their mothers.


May Collado explained that it is exciting that the marine dolphins like the bottlenose use signature whistles for contact, and here we have a different sound used by river dolphins for the same purpose. Also, these animals made longer calls and whistles, but these were much rarer, and the reasons for them are not yet clear to the biologists. There is some indication, however, that whistles serve the opposite purpose than in bottlenose dolphins, with the botos using them to maintain distance rather than for group cohesion.

The characteristics of the acoustic of the calls are also exciting. These calls fall somewhere between the low-frequency calls used by baleen whales to communicate over long distances, and the high-frequency ones used by marine dolphins for short distances. May Collado speculation is that the river environment may have shaped those characteristics.

The next step for May Collado and her colleagues is to study whether the same diversity of communication is seen in other populations of Araguaian river dolphins that are less accustomed to humans. They will also compare them to their relatives elsewhere in South America.

The Araguaian dolphins are closely related to two other species, the Bolivian river dolphin and the Amazon river dolphin. The sole description of the Araguaian dolphins is that of a separate species in 2014, and that classification is still under debate. But this appears to be a large number of variations in the repertoire of sounds each species makes.

May Collado concluded by saying that they cannot say what the evolutionary story is yet until they get to know what sounds the other river dolphins produce in the Amazon area, and how that relates to what they found. These are a new set of questions to explore.

Two Araguaian botos swimming together. Photo credit: Gabriel Melo Alves dos Santos

Seven years ago, Gabriel Melo Alves dos Santos happened upon a remarkable sight: a rare and highly reclusive Amazonian river dolphin, the Araguaian river dolphin, gathering in numbers at a marketplace along the Tocantins River in the Amazonian town of Mocajuba to feast on market scraps, fearlessly interacting with the local humans.

The dolphin, colloquially called theboto, is difficult to study, threatened by steadily increasing human activityand remains largely a mystery to science.

Melo-Santos seized the opportunity to study this extraordinary mammal in a relatively controlled setting, a rare condition in field biology. Long interested in understanding how the boto uses echolocation to navigate its murky environment and communicate with kin, Melo-Santos and his colleagues recorded the dolphin’s surprisingly complex repertoire of sounds. Their results do much to untangle the complex evolution of vocalization among the toothed whales. Perhaps more critical to the boto’s current situation, though, they show that the boto may be more social than previously suspected.

Conservation: Choose the Right Strategy

An animal’s social habits inform the strategies we can adopt to conserve them. Relocating a few isolated members of a social species to a new habitat, for instance, risks mistakenly hastening that species’ decline.

Through 20 hours of hydrophone recordings at the market, Melo-Santos group discovered that the Araguaian boto is much more talkative than previously thought. Not only did the researchers observe the dolphins “chatting” with each other, but they identified a special type of signal that appeared to be reserved for communication between mothers and calves.


If corroborated by future studies, this finding implies that the boto is more of a social animal than had been assumed. Melo-Santos, however, cautions drawing too fast a conclusion, given that the market, in which the study was conducted, presents a somewhat unusual setting for the dolphins. The botos found in Mocajuba are what scientists call “habituated”, in that they are accustomed to human interaction and may exhibit behaviours distinct from those of unhabituated botos.

Dr. Melo-Santos next steps will be to build upon his current research by finding and recording such unhabituated botos and using their vocalisations to refine his understanding of those made in Mocajuba’s market.

Araguaian River Dolphin

Threats Outpace Knowledge

The more we know about a threatened animal, the better able we are to provide effective conservation. In this context, the Araguaian boto’s greatest threat may be our lack of knowledge about it. In 2008, the International Union for the Conservation of Nature (IUCN) changed the status of the entire family of botos, to which the Araguaian boto belongs, from “vulnerable” to “data deficient,” in the face of the increasingly outdated information regarding the dolphins’ numbers and known threats.

In his paper first describing the Araguaian boto, Dr. Tomas Hrbek of Brazil’s Federal University of Amazonas, estimated the total population at 1,033 individuals. A later estimate by Melo-Santos arrived at nearly the same number.

This small population faces a number of threats within the confines of its increasingly busy rivers. Six dams currently constrict the botos’ range. Agricultural drainage, often laden with toxins, constantly leaks into their waters. Boat traffic brings noise that potentially confuses the dolphins’ echolocation. Frustratingly for human fishermen, the botos’ have learned to apply their rare ability to swim backwards to more effectively remove fish from nets.

The single greatest threat to all Brazilian river dolphins is likely fishing. In this excellent deep dive on the topic, Sophie Yeo describes the terrifying effect that the world market for piracatinga, a type of Amazonian catfish, is having on river dolphin populations, whose flesh fishermen use as bait for the catfish.

Dredging the River

Further threatening the dolphin’s future, the Brazilian government plans to make some sections within the dolphins’ territory more navigable through dredging, which will remove the dolphins’ access to prey and cover.

The Tocantins River is considered an important waterway, along which to ferry iron ore and grains from Midwestern Brazil to the northern state of Pará. In 2016, the engineering firm DTA Engenharia won a concession to dredge a roughly 43km long stretch of the river in Pará State, known as the Pedral do Lourenço. Shallow and rocky, the Pedral do Lourenço forms a natural barrier to large boat traffic on the river. Following the recent completion of a government-mandated environmental review, the Brazilian Institute of Environment and Renewable Natural Resources (Ibama, for its name in Portuguese) approved DTA Engenharia’s plan (linked article in Portuguese). The firm intends to begin work in the coming austral summer.

“The fate of this region is very worrying,” says Dr. Melo-Santos. “Some of the scenic beauty might be lost. The area they want to dredge is extraordinarily beautiful, with gorgeous rock formations, lots of birds [and] dolphins.”

Inia Araguaiaensis

Unfortunately, the results of the official environmental impact study commissioned by Ibama have not been made public and at the time of this publication, neither DTA Engenharia nor Ibama have responded to requests for comment. In Brazil’s current political climate, however, environmental concerns seem unlikely to sway any plans for industrial progress.

Effects of Loss

What happens to the boto’s ecosystem, should the dolphin disappear? Melo-Santos points out that the Araguaian boto is an apex predator, meaning that it occupies the top of its food chain. The loss of such top predators typically signals detrimental changes to their habitats. Free from the fear of predators, herbivores can wreck havoc on an ecosystem, reproducing without check and devouring more plants than ever before. This phenomenon, called a trophic cascade, has happened many times in the past, with the loss of wolves in Yellowstone National Park providing one good example.

Baby River Dolphin

Evolutionary History

The value of the recordings of Melo-Santos and his colleagues extend beyond the basic biology and conservation of the Araguaian boto. The boto’s vocal repertoire provides an important piece to the puzzle of how vocal communication evolved in toothed whales.

Dr.Laura May-Collado, study’s principal investigator, comments that river dolphinsare the “only living representatives” of a lineage that branched from whalesmillions of years ago. To this day, scientists remain uncertain as to manyaspects of the vocal evolution of these related groups of animals. To whatextent, for instance, have their social preferences affected theirvocalisations? Have the calls of the river dolphins evolved to avoid otherechoes found in a river environment? By comparing the calls made by riverdolphins with those of marine dolphins, killer whales and their kin, more ofthe evolutionary development of this branch of the animal kingdom may come tolight.

Assuming, of course, that the Araguaian boto remains alive long enough to be studied.

River Dolphin Habitat

Corrections: An earlier version of this post referred to Dr. Melo-Santos as Dr. Alves. Melo-Santos is used professionally.