Dear Journey North,
The monarch is engaged in a behavior called "Mudpuddling." It is a common behavior in many butterflies, although I have almost never seen it in monarchs.

I suspect there are salts* on the road that may have contained some sodium and potassium chlorides.

Professor Thomas Eisner and his students at Cornell University have done several studies on the behavior. For some reason, male butterflies (and some moths) are attracted to these wet spots to imbibe the salts present. They may pass a lot of water through their bodies in order to concentrate the salts. Eisner showed that males of some species then pass these salts onto the females when they mate with them in "nuptial gift" behavior.

Here in Virginia, I frequently see the pipe vine swallowtail, the tiger swallowtail, and the spicebush swallowtail drinking from damp dirt along the road or the edges of mudpuddles. Interestingly, the behavior almost always involves males only.

There are wonderful pictures of tropical river sandbars covered with thousands of butterflies of numerous species. In the tropics, mudpuddling occurs naturally on areas where fish have died on sandbars.

In a somewhat macabre behavior, the swallowtails here in Virginia will often mudpuddle in spots where cars have run over and squashed a group of the same species of butterflies that were previously mudpuddling. Squashed toads and frogs are also attractive.

Professor Lincoln P. Brower
Research Professor of Biology
Sweet Briar College
(Distinguished Service Professor of Zoology Emeritus, University of Florida)

* Editor's Note: Salt is put on this road in the winter to melt ice. Also, the road is sprayed with calcium chloride to control dust. Evidently the calcium chloride makes a salt solution when it absorbs moisture from the air and the road.