Toddlers aren't the only ones fond of mud puddles. Butterflies and moths often gather at puddles in large groups. I witnessed about 30 tiger swallowtail butterflies around a puddle on a woods road one spring, their yellow, black-veined wings twitching slightly, contrasting with the brown mud. Another time I saw a crowd of swallowtails around a pile of damp wood ashes in my yard.
This curious behavior is known as puddling. Although butterflies and moths get most of their nutrition from flower nectar, puddling provides another way to obtain nutrients, and replenish fluids. The insects use their long tongues, called proboscises, to deliver the fluid or other material into their mouths.
Some species puddle on scat and carrion. This can be shocking to human sensibilities. I was a bit taken aback the first time I saw a beautiful tiger swallowtail on a fresh pile of manure. Kent McFarland, a conservation biologist at the Vermont Center for Ecostudies, recalled seeing a harvester butterfly on a pile of coyote scat.
Fermenting fruit is another butterfly favorite. We have an old apple tree that shades our driveway. In the fall I have to pull my car in slowly so as not to hit all the white admirals -- black-winged butterflies with white bands and red and blue dots. They seem to love the crushed, rotting apples.
What nutrients do butterflies obtain through puddling? Salt is probably the most important one. Sodium tends to be scarce in the adult butterfly diet and is essential for reproduction and flight.
Many species in our region puddle. In addition to the swallowtails and admirals, these include members of the whites and sulphurs family, as well as a number of the brushfoots, including mourning cloaks, question marks, and eastern commas.
Although both male and female butterflies are known to puddle, the activity is primarily a male behavior. One reason appears to relate to reproduction. In experiments, scientists attracted male butterflies to sand soaked with sodium. At least in some species studied, males have been found to transfer sodium to females during mating. The males delivered the sodium via the spermatophore, a package which contains the sperm, making a "nuptial gift." Females incorporated the element into their eggs, and at least in one species studied, the presence of sodium increased egg production. Receiving sodium from males frees females from the risks of predation that puddling on the ground can entail.
Another reason for the predominance of male butterfly puddling may be the differences in energy requirements between the genders. Sodium is critical for neuromuscular activity in butterflies. Males may have a greater need for sodium because they spend more time in flight than females and tend to be quicker.
Why do butterflies cluster in groups while puddling? It may be a simple case of social facilitation -- one butterfly finds the resource, others see it and follow. Clustering also reduces the risk of predation to any one individual. These congregations may also make males more conspicuous to females looking for mates.
Puddling behavior is not unique to the Northeast. It is common around the world, although feeding preferences vary among butterfly species, as do nutritional needs. In the tropics, you can get amazing hordes along rivers, said McFarland. Certain exotic moths engage in weird puddling behaviors. A moth found in Madagascar sucks tears by inserting its proboscis into the closed eyelids of roosting birds. Cases of moths drinking human tears have been reported from Thailand. Some species of Russian moths are known as "vampire moths" -- they drill their barbed tongues under the skin of sleeping animals (including humans) to suck blood. Fortunately, our New England butterflies and moths refrain from such activities.
It's easy to make a puddling area in your flower garden. Place a shallow basin of water containing a pinch or two of salt on the ground. Also try putting out a tray of fruit that is past its prime (though be forewarned that this may also attract bees). Watch for butterflies and see if you can spot their long tongues through your binoculars.
Susan Shea is a naturalist, conservationist, and freelance writer who lives in Brookfield. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org.