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 the boto, is difficult to study, threatened by steadily increasing human activity and 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 boto. Credit: Paulo Castro 
Araguaian boto. Credit: Paulo Castro
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.”
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.
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 dolphins are the “only living representatives” of a lineage that branched from whales millions of years ago. To this day, scientists remain uncertain as to many aspects of the vocal evolution of these related groups of animals. To what extent, for instance, have their social preferences affected their vocalisations? Have the calls of the river dolphins evolved to avoid other echoes found in a river environment? By comparing the calls made by river dolphins with those of marine dolphins, killer whales and their kin, more of the evolutionary development of this branch of the animal kingdom may come to light.
Assuming, of course, that the Araguaian boto remains alive long enough to be studied.
Corrections: An earlier version of this post referred to Dr. Melo-Santos as Dr. Alves. Melo-Santos is used professionally.