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.