Communication and Evolution of Amazon River Dolphins

Two species of freshwater dolphins inhabit the Amazon River Basin, the pink river dolphin, Iria geoffrensis and the grey river dolphin, Sotalia fluviatilis. These two species are particularly interesting because although they inhabit the same freshwater system, their evolutionary histories are very different. The pink river dolphin is thought to be a much older species that evolved from a species of dolphin found in the Pacific before the Amazon River formed. It is thought that a cove formed on the pacific coast of what is now Peru. This cove was eventually closed in and became a large lake in which a group of dolphins was trapped. The lake gradually changed from salt to fresh water and the pink dolphins adapted with it to live in fresh water. When the mountain range on the west coast of Peru formed, the lake became part of the Amazon river system. It wasn’t until this happened that the grey river dolphin became a part of the picture. This species is thought to have travelled up the river and adapted to fresh water more recently. This is the history of the two dolphin species that I learned over the summer when I volunteered with the conservation organization Operation Wallacea in upper Amazonia.

I was very interested to find an article written by the Umass professor Jeff Podos that discusses how the differences in vocalizations between the two species can help answer questions about the evolutionary history of the amazon river dolphins and oceanic dolphins. This paper, “Vocalizations of Amazon River Dolphins, Inia geoffrensis: Insights
into the Evolutionary Origins of Delphinid Whistles” addresses the question of whether whistles, a well-know method of communication within the delphinid family was uniquely derived in this family or whether it began to evolve in a common ancestor with Iria geoffrensis. Whistling is not found in the Monodontidae or Phocoenidae family, so if it is not a unique derivation of the delphinid family, it must have been lost in these two groups. Podos found that pink river dolphins do make vocalizations. However, they are both structured and used differently than the whistles of delphinids. Pink river dolphin vocalizations were found to be much less common than grey dolphin vocalizations and they occurred in bouts. They were most common during hunting activities. These vocalizations were lower-frequency and longer than delphinid vocalizations and they consisted of a series of short notes rather than a single note. Podos concluded his paper by saying that it is still not clear when the evolution of dolphin whistles occurred but that it is likely to be a unique derived characteristic of the delphinid family.

-Lauren Lynch