A Fresh Look at River Dolphins

By Vidal, Omar | Americas (English Edition), March-April 1994 | Go to article overview

A Fresh Look at River Dolphins


Vidal, Omar, Americas (English Edition)


In the Amazon scientists are discovering how these ancient animals are adapting to an ever-changing environment

In January 1986 I visited the Amazonian rain forest for the first time. Jorge Reynolds, a Colombian electronic engineer who for more than two decades has been researching cardiac stimulation in animals, invited me to take part in an expedition to obtain electrocardiograms of river dolphins. (Reynolds has taken EKGs of animals as diverse as mosquitoes, electric eels, sea turtles, iguanas, dolphins, and whales.) During this and subsequent research trips, I became aware of the great research potential of these dolphins.

Two dolphin species--the boto and the tucuxi--share habitat in the Amazon and Orinoco rivers, the largest river system on the planet. The boto, as it is known in Brazil (Inia geoffrensis), better known by Colombian and Peruvian fishermen and rural people as bufeo colorado, or pink dolphin, belongs to the superfamily Platanistoidea, or river dolphins. They include four other dolphin species--the baiji of the Yangtze River in China; the two susus of India, Bangladesh, Nepal, Bhutan, and Pakistan; and the franciscana, the only one of the group that lives in the sea, off the coasts of Brazil, Uruguay, and Argentina. The tucuxi (Sotalia fluviatilis) belongs to the superfamily Delphinoidea, which includes all marine dolphins.

Besides being the largest river dolphin, the boto covers the widest geographical area, and can be found in Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, and Venezuela. The tucuxi inhabits the Amazon and Orinoco rivers, as well as the sea, and can be found in the Caribbean off the coast of Panama (recently, groups of these dolphins were sighted off the Nicaraguan coast), and also along South America's north and northeastern shores, from Colombia to Brazil.

Dolphin fossils indicate that river dolphins are older and more primitive than marine dolphins. Ancestors of the river dolphins existed from the Late Oligocene and Early Miocene epochs, about 20 to 25 million years ago; while marine dolphins are known to exist from the Late Miocene epoch, about 10 to 15 million years ago. Platanistoid dolphin fossils were found in marine deposits dating back to the Oligocene, Miocene, and Pliocene epochs, which indicates that the freshwater habitat is secondary and that the ancestors of modern river dolphins moved from the sea to the rivers. It has been suggested that the ancestors of the boto entered the Amazon River either from the Atlantic Ocean between 2 and 5 million years ago, or from the Pacific Ocean about 15 million years ago. The ancestors of the tucuxi entered the Amazon River at a much later date. Consequently, the species differ despite sharing the same environment. Botos had more time to adapt to freshwater than tucuxis and, therefore, experienced more anatomical changes that allow them to live in and use this environment perhaps more efficiently.

In 1956, American biologists James Layne and Ross Allen participated in an expedition to Leticia, Colombia, whose main objective was to capture live dolphins to be exhibited in the United States. Layne and Allen's observations (published in 1958), while limited, were the first published description of Amazon dolphins' ecology and behavior--and the first to examine the natural history of any river dolphin. Thirty years passed before the dolphins of this region again became of interest to scientists. And, it wasn't until last year that a multidisciplinary, long-term study of the biology, ecology, and behavior of the boto and tucuxi was begun. This research includes scientists from Colombia, Mexico, Peru, and the United States, and is sponsored by the United Nations Environment Programme (UNEP), the Monterrey Institute of Technology and Higher Studies (ITESM)--Guaymas Campus (Mexico), and the Colombian government, through the National Institute for Renewable Natural Resources and the Environment (INDERENA). …

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