Walking my dog earlier today, the two of us ambled down the driveway (a nearly half-mile potholed and muddy track), over a culvert through which our local creek was flowing dark and muddy, and to the other side. There, a number of dips in the gravel held remnant water from the rain and thunderstorms from a couple days back.
In one area, a small patch of dry gravel seemed alive with the twitching yellow and black wings of Canadian Tiger Swallowtails (Papilio canadensis). These big and flashy butterflies appear in late May in the interior of Alaska, and are welcome additions to the summer fauna.
And there is something about our driveway that lures them in by the dozen. In early summer the Swallowtails arrive in big numbers, some years more, some less, but they are always present in a density I don’t see elsewhere.
A few years ago, we had a banner year when hundreds upon hundreds descended onto the rocks of the driveway, bursting away from us in clouds as they flushed by our walking feet, or the passing tires of our vehicles. Though I’ve not seen that many since, they are always present each June.
Check out this video Amy made a few years ago during that big year:
What are they doing there?
They are “puddling”. Though really, that term is not very accurate. Rarely are they actually gathering around the puddles themselves. Rather, they select a few specific spots where they can be found day after day in groups of a ten, twenty, or even more.
As I read up on the behavior, I learned that the individuals gathering around these areas are almost exclusively males. And they are doing what males everywhere do: making themselves more appealing and productive to the opposite sex.
The Tiger Swallowtails of my driveway are not hydrating themselves against the summer heat, but instead are exploiting areas of concentrated sodium. Sodium is a heavily desired mineral in the lepidopteran world.
Check out this quote from a Canadian Journal of Zoology article from 1982 by Adler and Pearson:
“Older males collected from the field show a significantly lower level of body Na [sodium] than freshly emerged males, whereas freshly emerged females and older, field-collected females show no difference. It is suggested that feeding from soil may help restore losses of Na in males. A single female, through oviposition, may lose nearly 75% of the total body Na with which it emerged.”
There is a lot of confusion here. First, I noted that males are the ones doing the puddling to pick up sodium, and yet they are the ones who lose it over the course of their adult lives, while the females will dump 3/4 of theirs during egg-laying and yet maintain a steady level of sodium in their bodies throughout life.
This is how: male butterflies, the little Romeos that they are, pass off huge volumes of sodium to the females during mating. The spermataphores (the packet containing all the males’ sperm and reproductive material) contains a huge amount of sodium, enough to offset all that the females lose during oviposition.
The poor males meanwhile can’t puddle fast enough to make up for the sodium they are contributing to the reproductive effort and end up with a net loss over the course of their already short lives. Sad.
Still, if it means my driveway is home to a Lepidopteran bonanza each summer, I’ll take it.
I do wonder what exactly brings these critter to my home, over the dozens of other gravel roads and driveways in our neighborhood. But, I’ve got a hunch it has to do with foxes.
You see, we live along a small creek which flows out of the hills north of us, and along the creek there is essentially no development. It’s a convenient, forested corridor reaching up and down the valley between more settled areas on the surrounding hills. As a result, we have a lot of wildlife. Moose, foxes, and coyotes are common and we’ve seen Lynx, a Grizzly, Marten, even a Wolverine on the property; lots of critters.
Many of these, and the foxes in particular, use the comparatively smooth and hard driveway as a part of their regular route. Almost every day, there is a new scat somewhere along my path.
And here is my hypothesis: Fox urine is rich in sodium, the driveway is a regular path marked by at least one and possibly several different foxes. Therefore, the “marking post” urinals used by the foxes on a near-daily basis will be rich in sodium. And that, I believe is why on any given June day, I can find the butterflies in a half-dozen different spots along the driveway, but rarely away from those half-dozen spots.
Fox pee, that’s the answer, and I’m taking bets on it.
Adler, Peter H., and David L. Pearson. 1892. Why do male butterflies visit mud puddles? Canadian Journal of Zoology, 60(3): 322-325