River dolphins can whistle, too, sort of.

In the most elaborate attempt so far to eavesdrop on Brazil's pink river dolphins, researchers have detected what may be a counterpart to seafaring dolphins' whistles.

Jeffrey Podos of the University of Massachusetts in Amherst took underwater microphones when he joined Brazilian researchers in the Mamiraua Reserve. They intended to record what scientists had loosely called whistles from Inia geoffrensis.

The project proved trickier than expected, Podos says. The researchers couldn't see their own hands in the murky water, much less a dolphin in the depths. The scientists could still make recordings, but they had to abandon their plans for watching what a dolphin was doing as it vocalized.

During 5 weeks in the field, however, the researchers did in fact record intriguing sounds when dolphins swam by. The diverse calls included whistlelike sounds. However, Podos describes them as "highly distinct" from the whistles of marine dolphins. The sounds differ so much, he says, that he now suspects they evolved only after the lineage of river dolphins diverged from marine dolphins.