Life in the Amazon

A detailed look at South American river dolphins may help scientists understand why so many plants and animals call the Amazon rain forest home.

The Amazon rain forest is a place of almost magical natural bounty, boasting more plants and animals than perhaps any other terrestrial region on the planet. And though many researchers have spent their lives studying the Amazon, one general, nagging question remains in all their minds: Where did all this biodiversity come from?

Turns out river dolphin DNA may hold some clues. Biologist Healy Hamilton, head of the Academy's newly formed Center for Biodiversity Research and Information (CBRI), has been comparing genetic sequences of pink Amazon dolphins in the northern part of the continent and La Plata dolphins in the south to piece together river dolphin evolution. She's found that these two genetically distinct river dolphins were once part of a single ancestral species that evolutionarily diverged around 15 million years ago, a time in Earth's history when global sea levels were at their highest.

Her findings suggest that during the era of high global seas, South America's three main river basins-the Orinoco and Amazon in the north, and the Paraná-La Plata in the south-may have been joined as one complex, interconnected waterway. With the flooding of low-lying regions, areas of high topography essentially became islands, isolating organisms from one another. Over geologic time, this process contributed to the extremely diverse fauna we see today.

Two river dolphin lineages inhabit the fresh and brackish waterways of South America. Three subspecies of the Amazon river dolphin, Inia geoffrensis, are distributed throughout the Amazon and Orinoco. The La Plata river dolphin, Pontoporia blainvillei, is found in and around the Rio de la Plata estuary.


Academy scientist Healy Hamilton uses a modified crossbow to obtain small pieces of tissue from live river dolphins for genetic analysis.
Photo: John Goubeaux
The Amazon river dolphin is only distantly related to marine dolphins. It has a long beakwith many teeth, a bulbous forehead, and a distinct pink coloration.
Photo: John Goubeaux
A artist's rendition of the hypothesized seaway that may have connected the proto-Orinoco, Amazon, and La Plata river basins millions of years ago, when high global sea levels flooded lowlying regions around the world.