River Spirits

Tracking prey deep in the forest, freshwater dolphins make the most of the Amazon’s prodigious annual flood.

This story appears in the June 2009 issue of National Geographic magazine.

The dolphins are swimming through the trees. Bending sinuous bodies, they glide through branches and curl like snakes around fluted trunks. As frog-green fish dart through the leaves, the dolphins, pink as bubble gum, snap at them with long, toothy beaks.

This is not some kaleidoscopic dreamscape from a novel by Gabriel García Márquez; this is the wet season in the upper Amazon, downstream from Iquitos, Peru. The river has flooded the rain forest, luring freshwater dolphins to hunt in the woods.

The Amazon dolphin, Inia geoffrensis, parted company with its oceanic ancestors about 15 million years ago, during the Miocene epoch. Sea levels were higher then, says biologist Healy Hamilton of the California Academy of Sciences in San Francisco, and large parts of South America, including the Amazon Basin, may have been flooded by shallow, more or less brackish water. When this inland sea retreated, Hamilton hypothesizes, the Amazon dolphins remained in the river basin, evolving into striking creatures that bear little resemblance to our beloved Flipper. These dolphins have fat, bulbous foreheads and skinny, elongated beaks suited to snatching fish from a tangle of branches or to rooting around in river mud for crustaceans. Unlike marine dolphins, they have unfused neck vertebrae that allow them to bend at up to a 90-degree angle—ideal for slithering through trees. They also have broad flippers, a reduced dorsal fin (a larger one would just get in the way in tight spots), and small eyes—echolocation helps them pinpoint prey in muddy water.

At up to 450 pounds and eight feet in length, the Amazon dolphin, or boto, is the largest of the four known species of river dolphin. The others live in the Ganges in India and the Indus in Pakistan, in the Yangtze in China, and in the Río de la Plata between Argentina and Uruguay. All river dolphins are superficially similar, says Hamilton, yet the four species don't belong to the same family. DNA studies by Hamilton and others have shown that river dolphins evolved from archaic marine cetaceans (the order that also includes whales) on at least three separate occasions—first in India, later in China and in South America—before modern marine dolphins themselves had emerged as a distinct group. In an example of what's known as convergent evolution, geographically isolated and genetically distinct species developed similar characteristics because they were adjusting to similar environments.

Every spring the Amazon dolphins get to leave the confines of their river channel for a taste of their former habitat. At the Mamirauá Reserve in western Brazil, where Tony Martin of the University of Kent in England has studied the dolphins for the past 16 years, two Amazon tributaries flood thousands of square miles of forest for half the year, converting it into a vast tree-canopied sea. Martin and his Brazilian colleague Vera da Silva have found that female dolphins in particular stray far into the forest—perhaps to take refuge from aggressive, bright pink males. The females are mostly gray; the males' pink color, Martin and da Silva believe, is scar tissue.

"The males beat the hell out of each other," says Martin. "They are brutal. They can snap each other's jaws, tails, flippers, lacerate blowholes. The large males are literally covered with scar tissue." Only a small percentage of males turn bright pink, Martin says, and those are the ones females are most attracted to—at least during the mating season, when the water retreats back into the river channel and both sexes are thrown together.

Being pink is not the only strategy males have for impressing females. They also sometimes pick up weeds or a piece of wood with their beaks, twirl in a circle, and smash the object on the water. Locals long believed the dolphins were just playing, but Martin discovered that only males carried objects, and only in the presence of females. What's more, they were 40 times more likely to get into fights when engaged in such ostentatious behavior. No other mammals besides humans and chimps carry objects for display, says Martin. "It's like a guy showing off—the equivalent of having a Ferrari," he explains.

River dolphins have no predators, except for humans. In December 2006 the Yangtze river dolphin, called the baiji, succumbed to pollution, propeller blades, dams, and overfishing; it became the first cetacean to be declared "functionally" extinct—meaning the species cannot renew itself, even if a specimen or two still exist. "Losing the baiji is like taking a chain saw to the cetacean tree of life," says Hamilton. "We've lost 20 million years of independent evolution." The Ganges river dolphin is also in grave danger; only a few thousand remain, and they live in some of the most polluted rivers on Earth.

The Amazon species probably has the best prospects; though its numbers are uncertain, Martin thinks at least 100,000 are left. But the trend is worrisome. At the Mamirauá Reserve, Martin's study population has declined by half over the past seven years. Fishermen hunt dolphins for catfish bait, he says, and they also kill them accidentally in their gill nets.

Once that would have been unthinkable. In Amazonian folk wisdom the boto is an encantado—an enchanted being, a shape-shifter that sometimes takes on human form, coming out of the river to beguile men and women and lead them into its magical underwater city. Some say it wears a hat to hide its blowhole and its bulbous forehead. The stories are incredible to modern ears, and in a way, that's unfortunate. Because to survive in today's world, the boto may need to enchant a wider audience.