After a sweaty morning working in the woods I sat down on the porch to catch a few breezes. I took my soaking wet shirt off and hung it up to dry on the steps. To my amazement, almost immediately a silver-spotted skipper butterfly lighted on the damp shirt and began to drink the sweat! What is going on here?
Butterflies are well known to require sodium salts which are lacking in their vegetable diet and they seek salts in animal feces and damp soil which have minerals. I noticed this same butterfly landing on white spots which might have been bird droppings. But how did it know that my shirt contained sodium from my sweat? Surely it does not recognize a shirt as a source of sodium, but it may well be able to smell and identify a sweaty mammal, which is likely to have excess sodium available to drink?
So if you want to attract butterflies, save up those sweaty shirts and hang them out as a lure. You could even add salty water to a particularly sweaty and smelly undergarment to act as a lure. You can add these to the rotten fruit and other strange objects that people use to attract butterflies to their backyards. You may attract fewer neighbors but more butterflies! By the way this would make a great science project for kids, testing various smells and salts as attractants for butterflies.