their tears until they've had their fill.

　　The butterflies are likely attracted to the turtles' tears because the liquid drops contain

　　salt, specifically sodium (钠), an important mineral that is rare in the western Amazon rainforest, said Phil Torres, a scientist who does much of his research at the Tambopata Research Center in Peru and is associated with Rice University. Turtles get plenty of sodium through their largely carnivorous (食肉的) diet.

　　Meat contains significant levels of the salt, Torres told LiveScience. But herbivores (食草的) butterflies sometimes struggle to get this extra mineral source， he added.

　　Torres explained the western Amazon rainforest is lower in sodium than many places on earth, because it is over 1,000 miles away from the Atlantic Ocean—a main source of salt. The region is also cut off from the mineral dirt blown towards the west from the Andes Mountains. Most of these windblown minerals are removed from the air by the rain before they have a chance to reach the western Amazon.

　　In fact， the butterflies have other sources to get sodium besides turtles' tears, which include animal urine (尿), muddy river banks, sweaty clothes and so on.

　　People can't help wondering if the process is painful for turtles. Torres said it's not completely clear, but the teary effort probably has little effect on the turtles, other than perhaps making them more vulnerable to their enemies like big cats, since the butterflies can block out their vision.

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