If there’s a passionvine in your neighborhood, you may have noticed a big showy butterfly, flame-orange with silvery spots like a spatter of mercury on the underside of its hindwing, hanging around. That would be a Gulf fritillary, which is actually a longwing rather than a true fritillary.
Someone once proposed renaming it the silver-spotted flambeau, which didn’t catch on. In Hawaii, where its larval host plant is ubiquitous, it’s called, sensibly enough, the passionvine butterfly.
By whatever name, it’s one of the few North American members of a mostly tropical group. The butterflies can’t tolerate cold winters, and those at the northern end of the range stage mass southward migrations. Common in the Southeast (hence the Gulf part), its range is limited by that of its host plants. One species of passionvine gets as far north as Arkansas, where we called it maypop and made jam from it, and there are a bunch of wild species in Florida.
California has no native passionvines, though, and the Gulf fritillary didn’t establish itself here until they had been planted as ornamentals. It’s not clear when the butterflies first turned up; one lepidopterist speculated they followed the Southern Pacific tracks, but they might have wandered up from Mexico. After colonizing Southern California, they followed their food plant north, reaching the Bay Area in the 1950s. Art Shapiro at UC Davis says this species is pretty much an urban butterfly, especially common in Berkeley.
When the adult butterflies emerge from their chrysalids in spring, there’s a brief courtship in which the male fans his wings to give his mate a heady dose of pheromones. Then she lays her barrel-shaped eggs on stalks, so ants and other small predators can’t get at them on a passionvine leaf. The caterpillars hatch out and begin to munch.
Flowering plants have a many-sided relationship with animals. A passionvine needs to have its flowers pollinated, its seeds distributed, and its leaves left the hell alone. So it’s evolved colorful fragrant blossoms to attract pollinating insects, and tasty fruit enclosing seeds that will hopefully be deposited somewhere away from the parent vine. And several lines of leaf defense have been developed.
One tropical passionvine has hook-shaped hairs that puncture the soft bodies of caterpillars. Some resort to trickery: their leaves have projections that look like fine places to lay an egg but that are jettisoned by the plant once an egg is deposited. Still others have nectar glands that attract ants, which eat the longwing eggs or larvae, or faux eggs that make the leaf appear to have been preempted.
The most common defense, though, is chemical. Passionvine leaves contain substances called cyanogenic glycosides, precursors of cyanide. This is enough to deter most leaf-eaters, but the evolutionary arms race hasn’t gone far enough to make the leaves unpalatable to the larvae of longwing butterflies.
The butterflies get an advantage from their toxic diet. Experiments have shown that birds find Gulf fritillaries and other longwings distasteful. And it’s in the butterfly’s interest to advertise this. Gulf fritillaries may have evolved their vivid colors for the same reason that deer hunters wear Blaze Orange vests: to maximize their visibility.
That would only work for predators with color vision, of course, which happens to include birds. The idea is that an inexperienced bird will take a bite of fritillary, go “Feh!,” and avoid big orange butterflies from then on. The learning process takes its toll of a few individuals, but the species benefits.
Gulf fritillary caterpillars are also fairly gaudy, at least in their later stages: orange with menacing-looking black spines. The chrysalis, in contrast, is a cryptic object that looks like a curled-up dead leaf.
Other butterflies publicize their bad taste in similar ways: the monarch, whose caterpillar stores up milkweed toxins, white butterflies that feed on mustard, the pipevine swallowtail whose larval diet is, guess what? And there’s an advantage to being orange, or whatever warning coloration: a bird that had tried to eat a fritillary might also pass up monarchs, and vice versa.
This is where mimicry comes in: palatable butterflies which have evolved a protective resemblance to the distasteful ones. In the tropics this gets really complex, as most things do: there’s a whole raft of passionvine-feeding longwing species whose colors and patterns have converged to better spread the message, and free riders pretending to be poisonous.
Warning coloration is not just a butterfly thing: it occurs in amphibians, sea creatures, and at least one bird, the hooded pitohui of New Guinea. It’s a nice bonus that the colors that function as keep-away signals to predators are also pleasing to humans. If you have room and a sunny exposure, you might consider planting your own passionvine to attract Gulf fritlliaries. Just remember not to eat the leaves.