Deep in the western Amazon rainforest, salt is especially scarce. It's far from the Atlantic ocean and cut off from windborne minerals by the Andes Mountains.
Carnivorous animals, like turtles, have little trouble getting enough sodium because meat contains plenty of salt. But herbivores can have a harder time. Monkeys, for example, eat dirt while parrots lick clay.
Geoff Gallice, a graduate student of entomology at the Florida Museum of Natural History, says the butterflies can also get salt from animal urine, sweaty clothes and even sweaty people.
Scientists aren't clear whether the drinking butterflies help or harm the turtles, saying their vision could be impeded, but noting that they don't seem to mind.
Bees also like to drink turtle tears, but bees seem to annoy the turtles more than butterflies with all their buzzing.
Scientists plan to test the turtle tears to see if there are any other minerals and nutrients the butterflies are lapping up.