On a weekend in early August on a country road in Door Co., Wisconsin Mike Draney and I were surprised to find ourselves in a cloud of butterflies. Driving along County Road EE in Door County we were scaring up clouds of Clouded Sulfur butterflies (Colias philodice). We stopped and found that they were gathering by the hundreds in the wet mud of the stormwater ditches. We collected several dead butterflies, likely the victims of car traffic, and found that every individual we collected was a male.
Congregations of butterflies at mud puddles, animal dung, or carrion were first recognized in the early 1900s, but surprisingly, scientists still don’t know a lot about why they do it. It was long suspected the behavior was related to nutrients and salts. Flower nectar, while high in carbohydrates, is extremely low in nutrients like salts and amino acids. Experiments by chemical ecologists S. Smedley and T. Eisner conducted in 1995 confirmed that butterflies do prefer saltier puddles. And since them numerous studies have confirmed that butterflies are seeking sodium or proteins and sometimes other nutrients in the puddles that are scarce in nectar.
Females are not usually seen at puddles, even though it makes sense that they would also be in need these scarce nutrients to produce healthy eggs. In fact it has been demonstrated that males transfer some of the nutrients they collect at puddles to females as “nuptial gifts” during mating that she will then use to provision her eggs. In some cases may give away almost half of their total reserves. The gift is important because larvae that are born with higher amounts of nutrients probably have an advantage in surviving especially in low sodium environments. (See Molleman et al. 2005 for more information)
But why so many of this species? Clouded Sulphur caterpillars feed on legumes like alfalfa and clover, and there can be as many as 3 generations in favorable years, so it isn’t surprising to expect to see large numbers of them in agricultural areas in mid to late summer.
But Sulphurs aren’t the only butterflies you might see visiting puddles, many other species of terrestrial arthropods that have low salt diets including Blattodea, Diptera, Diplopoda, Hemiptera, Hymenoptera and Orthoptera have also been observed to puddle.
References:
Molleman F, Grunsven RHA, Liefting M, Zwaan BJ, and Brakefield PM, 2005. Is male puddling behaviour of tropical butterflies targeted at sodium for nuptial gifts or activity? Biological journal of the linnean society. 86: 345-361
Smedley and Eisner, 1995. Sodium Uptake by Puddling in a Moth. Science. 270: 1816-1818