The Outdoor Journal by Kyle Carroll

Often when I get back to the house from running or from a long bike ride, I'll toss my sweaty clothes over the deck rail so they can dry out before I put them in the dirty clothes. On a pretty regular basis I will come back to retrieve my sweaty duds to find several butterflies on the sweatiest articles of clothing. You've probably had a similar experience. Sometimes you see the same behavior at a mud puddle, on a dead animal, or even on a manure pile. Why do butterflies do that?

While butterflies mostly feed on nectar from flowers, they also need other nutrients like salt. Butterflies will often supplement their diet by visiting moist sources of nutrients including puddles, scat, urine, and even carcasses.
Deven Bonwker on THE WILDLIFE blog explains it this way. “..We all know that butterflies drink nectar from plants. That sweet nectar is essentially pure sugar water—..but if sugar water was all that butterflies ever drank, they’d come up short on a lot of other vital nutrients to their survival....more broadly, this behavior is characterized by a butterfly (and some other insects) actively seeking out a moist surface such as a mud puddle, rotting plants or animals, excrement, and even blood, sweat, and tears, where they will use their long straw-like “tongue” called a proboscis to suck up the sodium and amino acid dense fluids.” 

All this salt licking and mud sucking is usually associated with the male of the species. The behavior is thought to aid in butterfly reproductive success when some of the sodium and amino acids are transferred to the female and eventually aid in the survival of her eggs.

So if anyone ever asks you, “Why do butterflies do that?”, now you know.