The typical image of a butterfly involves sipping sugary nectar from a flower, but their dietary habits are often surprising. This insect, and some species of moth, sometimes engages in lachryphagy, which translates literally to “tear-feeding.” This unusual practice involves landing on the eyes of other animals, including reptiles, birds, and mammals, to drink their secretions. This act is driven by a deep biological necessity for a specific mineral.
The Drive for Essential Salts
A butterfly’s primary food source, floral nectar, is rich in carbohydrates but deficient in other nutrients, especially sodium (Na). Sodium is fundamental for physiological processes, including muscle contraction and nerve impulse transmission. Without sufficient sodium, a butterfly’s ability to fly, navigate, and even survive is compromised.
Tears, in contrast to nectar, are a highly concentrated source of sodium chloride, along with other trace minerals and proteins. Since many animals excrete excess salt through their tears, these eye secretions become a readily available mineral supplement. This dietary imbalance creates a strong drive for butterflies to seek out alternative, non-nectar sources to balance their mineral intake.
The need for sodium is especially pronounced in male butterflies, which exhibit this mineral-seeking behavior far more frequently than females. During mating, the male transfers a package of sperm and nutrients, called a spermatophore, to the female. A substantial portion of the sodium the male collects is incorporated into this “nuptial gift,” providing the female with the necessary resources to ensure the viability and quality of her eggs. This transfer of sodium directly increases the reproductive success of the pair.
Unexpected Mineral Sources
Tear-drinking is one example of a broader behavior called “puddling,” where butterflies aggregate to draw liquids from moist, non-floral substrates. The most common form of puddling involves sipping moisture from damp soil, mud puddles, or riverbanks, which contain trace minerals leached from the earth. This behavior is common in areas where environmental sodium is naturally scarce.
The quest for sodium and other minerals leads butterflies to an array of unusual sources. They are frequently observed feeding on animal dung and urine, which are rich in salts and nitrogenous compounds. They will also readily imbibe from rotting fruit, or land directly on humans and other mammals to sip sweat. Sweat, like tears, is a salty excretion that provides a concentrated dose of the scarce mineral.
The Mechanics of Drinking
To consume these liquids, the butterfly uses its proboscis, a slender feeding tube formed by two elongated halves called galeae. When not in use, this organ is kept tightly coiled beneath the head. When a liquid is detected, muscles and hydraulic pressure work together to uncurl and extend the proboscis toward the fluid source.
The two halves of the proboscis interlock to form a central channel, the food canal, which functions much like a straw. Liquid is drawn up this channel using a combination of a sucking pump located in the head, and capillary action. Capillary action is enhanced by microscopic grooves and pores along the outside of the tube, allowing the butterfly to wick up thin films of moisture.