Hiker sweat, alkali metals, and butterfly sex

Yesterday, Sunday, August 11, Kevin and Cathy went hiking on a day trip to Windham High Peak and Burnt Knob in the northeastern Catskill Mountains. Up near the summit of the peak, we got an intriguing lesson in ethology from Mother Nature herself.


Hoary Comma butterfly, Polygonia gracilis, alighting on Kevin's arm

While coming up over the last ledge, an insect started fluttering close around the two of us. Catherine startled, and Kevin said, "Don't worry, it's just a butterfly." The creature started flying very close—somewhat annoyingly, actually—until it finally came to rest on Kevin's arm. It proceeded to extend its proboscis and start licking his arm. It's a rather strange sensation for a butterfly to be holding onto you with its sticky feet and probing you with its proboscis!


Underside of the butterfly. Note the extended proboscis licking Kevin's skin.

It turns out that this behaviour is well known among butterflies. The food that caterpillars eat (leaves, mostly) is very poor in sodium and rich in potassium. Similarly, the flower nectar that sustains adult butterflies is also sodium-poor. Throughout their lifecycle, butterflies therefore must struggle to maintain their electrolyte balance, eliminating K⁺ and accumulating Na⁺. Many species share a common adaptation to this problem, which is integrated in their sexual behavior.

The males will fly to some wet source of sodium: ones that are frequently seen include carrion, piles of scat, wet soil where a large animal has urinated, and puddles on the sides of roads (where road salt accumulates). There, they will process large quantities of water, absorbing sodium. (Some male butterflies and moths can drink and void several dozen times their body weight in a few hours, extracting the sodium as the water passes through.) The male then presents the sodium as a gift to the female during the mating, and the female's body passes most of the sodium on in the eggs to the offspring. The caterpillars thus start their lives with a reserve of sodium that can see them through their early development.

So, what we're seeing in these pictures is a horny male butterfly stocking up on gifts for the ladies before going out on the town, in hopes of getting the opportunity to pass on his genes. He sees a sweaty hiker as a fine source of sodium!