How many river dolphins exist?


By Earl Filskov

We know so little about the planet Earth and its inhabitants, both past and present. National Geographic estimates that we know about less than 15 percent of the species currently alive. At the current rate of extinction, we may not know about many animals in our lifetime or even our grandchildren’s.

The new discovery of any species is exciting, take for example the rarely-seen river dolphin. Most people know about dolphins in the ocean, of course, that can be seen often on television shows, in the movies and at zoos. These are all salt-water species of dolphins.

The discovery a new river dolphin species in Brazil has reignited the conversation on what we are missing right in front of us. The last discovery of a river dolphin was in 1918, when researchers identified Lipotes vexillifer, the Yangtze River dolphin, or baiji, in China. After its last sighting in 2001, the International Union for Conservation of Nature declared the Yangtze river dolphin extinct in 2006 making it the first river dolphin whose demise was caused by human.

Inia araguaiaensis became the first new river dolphin discovered in almost 100 years. Located in the Araguaia River in Brazil on the outside of the Amazon basin; the Araguaia is not a tributary to the Amazon River. This makes the discovery even more exciting and scientifically puzzling.

Depending on how you choose to define the river dolphin there are between three and seven known species. The World Wildlife Foundation lists seven, but they include dolphins that travel between salt water and fresh water. Most define a true river dolphin as one that lives only in a fresh water environment.

The list of dolphins that live in fresh water includes:

· Inia araguaiaensis (Araguaian river dolphin)

· Inia geoffrensis (Amazon river dolphin)

· Inia boliviensis (Bolivian river dolphin)

· Pontoporia blainvillei (La Plata dolphin)

· Platanista gangetica (South Asian river dolphin)

· Lipotes vexillifer (baiji or Yangtze river dolphin) – Declared extinct in 2006.

First published in Plos One by Dr. Thomas Hrbek and his colleagues from the Federal University of Amazonas; they wrote that the Araguaian is a distinct species from the Amazon and Bolivian river dolphins. DNA samples were collected from 45 Amazon, 45 Bolivian and 32 Araguaian dolphins and they determined that the Araguaian split away from their cousins during the Pleistocene period around 2.1 million years ago.

“It is very similar to the other ones,” Hrbek wrote. “It was something that was very unexpected, it is an area where people see them all the time, they are a large mammal, the thing is nobody really looked. It is very exciting.”

Araguaian river dolphins are large mammals. Ranging from 5 feet to 8.5 feet in length with unusually small eyes, their vision is poor due to living in the muddy waters of the rivers, rendering eyesight mostly useless. Adults weigh an average of 350 pounds. They appear gray to pink in color. Possessing extra long snouts for betting hunting, they have fewer teeth than their cousins, but have a broader forehead suggesting a large brain. Disconnected neck vertebrae allows for moving their heads from side to side.

Natural barriers such as waterfalls and rapids, and manmade barriers such as dams separated the river cousins over the years. These separations, along with human populations competing with the dolphins for food, are among the reasons river dolphins are extremely rare. Due to their low numbers and bleak outlook, the IUCN has placed river dolphins on its Red List of Threatened Species.

“Its future is pretty bleak,” Hrbek told the New Scientist. “The Araguaia-Tocantins basin suffers huge human disturbance and there are probably less than 1,000 inia araguaiaensis in existence.”

Watson, Tracy. National Geographic News. August 25, 2011. 86 Percent of Earth’s Species Still Unknown? Web Accessed on March 19, 2015
Hrbeck, Thomas. Plos One. January 22, 2014. DOI: 10.1371/journal.pone.0083623. A New Species of River Dolphin from Brazil or: How Little Do We Know Our Biodiversity. Web Accessed March 19, 2015.
Barnett, Adrian. New Scientist. January 22, 2014. New species of river dolphin born of Amazon rapids. Web Accessed March 20, 2015.