WHITE RIVER JUNCTION – And you thought you had trouble telling one butterfly species from another. Tiger swallowtails, which are rather large, yellow butterflies with black tiger stripes, flutter over the hills and valleys of eastern North America each spring and early summer, sometimes in great numbers. But figuring out which tiger is which has baffled lepidopterists for more than three centuries.
The first New World butterfly to be painted by a European artist was a tiger swallowtail. John White painted one on Roanoke Island, North Carolina, in 1587 while serving as the expedition leader of Sir Walter Raleigh’s third trip to America. Despite exaggerating the wing shape, his details were relatively accurate.
Linnaeus, the father of modern taxonomic naming, classified tiger swallowtails as a single species in 1758 (Papilio glaucus). But Linnaeus had actually named the species from a black-colored female, a rarer version of the group that looks just like the more common yellow females, except for a darkly pigmented wing background. These dark females generally occur from Massachusetts to Florida, the southern portion of the tigers’ domain.
Why are there dark-colored females? They are thought to have evolved to mimic the dark color of the foul-tasting and poisonous Pipevine Swallowtail butterfly. When an otherwise palatable species evolves to closely resemble an unpalatable cousin, this phenomenon is called Batesian mimicry.
Linnaeus also observed the more common, yellow-colored female tiger swallowtail but decided it was another species altogether. Making matters even more confusing, a contemporary of Linnaeus documented a male tiger swallowtail, which is also yellow, but decided it, too, was its own species.
In the 1800s, biologists realized that the three species were only differentiated by color and lumped them together under the common name, Eastern Tiger Swallowtail. Starting in at least 1906, however, lepidopterists noticed that the more northern populations were smaller and had slightly different markings. Some began recognizing this as a subspecies of the Eastern Tiger Swallowtail and called it canadensis.
In 1991, biologists from Michigan State University announced that they had enough evidence to declare canadensis as a species of its own: the Canadian Tiger Swallowtail. Their evidence included genetic differences, color and size differences, caterpillar food-plant use, lack of black-colored females in canadensis, and only a very narrow hybrid zone between the two species. It is now widely accepted that Eastern Tiger Swallowtails are found southward and Canadian Tiger Swallowtails are found northward.
The range of the Eastern Tiger Swallowtail just barely makes it north into Vermont and New Hampshire. Most of the tiger swallowtails we see around here, therefore, are Canadian Tiger Swallowtails.
Generally, Canadian Tiger Swallowtails are flying around from the beginning of May until the end of June (later in higher elevations), while Eastern Tigers fly from June into October. So if you see a tiger swallowtail sailing over a meadow from mid to late summer, it just might be an Eastern. But even up close, they look very similar. The Eastern is larger, with the underside marginal forewing band broken into yellow dots separated by black borders. On the underside of the Canadian Tiger’s hind wing, the black line nearest the body is very wide. Minute details for sure. Even worse is that, in the hybrid zone between species, there are many that appear intermediate. In Vermont and New Hampshire, we are in the thick of the intermediate zone.
But the story doesn’t end there. In 2002, another potential tiger was described by two lepidopterists, Harry Pavulaan and David Wright. “When it became apparent that there were inconsistencies in the natural history of mountain populations versus lowland populations of tiger swallowtails, an intensive effort was made to study the field biology of the mountain populations,” said Pavulaan. They have named the new species the Appalachian Tiger Swallowtail and affectionately refer to it as “Appy.” So far, it is known only from the central and southern Appalachian Mountains. Finally, a fourth tiger swallowtail species, the aptly named Western Tiger Swallowtail, resides in western North America.
Why have tiger swallowtail identities been so hard to pin down? Probably because tiger swallowtails are comprised of sibling species—two or more populations that have become reproductively isolated from one another, yet so similar in outward appearance as to be lumped together even by experts. Careful, intense study of details such as anatomy, biochemistry, and behavior can bring sibling species to light. But there can be many dead ends and evolutionary tricks that confuse biologists.
If you want to wade into the world of tiger identification, now is the time! The lovely Canadian Tiger Swallowtails are flitting over meadows near you.
Kent McFarland is on the staff at the Vermont Center for Ecostudies.
