The sight of butterflies flocking onto the heads of yellow-spotted river turtles in the western Amazon rain forest is not uncommon, at least if one is able to sneak up on the skittish reptiles. But the reason why butterflies congregate onto the turtles may be stranger than you think: to drink their tears.

The butterflies are likely attracted to the turtles' tears because the liquid drops contain salt, specifically sodium, an important mineral that is scant in the western Amazon, said Phil Torres, a scientist who does much of his research at the Tambopata Research Center in Peru and is associated with Rice University.

Unlike butterflies, turtles get plenty of sodium through their largely carnivorous diet.