Clones and Caterpillars
by Cory Christopher, Director of Conservation
I love pawpaw trees. And it's probably not for the reason you suspect.
Among the leaves of this unassuming native tree lives one of my favorite creatures. Hungry and armed with chemical weaponry, the caterpillars of the zebra swallowtail butterfly (Protographium marcellus) are the only North American member of the tropical kite swallowtail group of butterflies.
Their relationship with the pawpaw (Asimina triloba) would make an incredible sci-fi flick, except that it’s absolutely true.
Travel back with me to the Miocene Epoch (around 5 to 23 million years ago), when Ohio was much warmer than it is today.
Members of the Annonaceae—the plant family to which the pawpaw belongs—thrived in our climate. Giant, ground-dwelling mammals like giant sloths and mammoths ate the fruit and dispersed their large seeds. Over time, temperatures cooled and these giant mammals disappeared.
Without seed dispersers, none of the Annonaceae were able to survive outside the tropics. None that is, except for the ancestors of our pawpaw.
Our pawpaw managed to survive when other Annonaceae did not because they were able to adapt to a life without seed dispersers. Here is how I imagine it went: at some point in the past, a pawpaw tree happened to have a proclivity for suckering—that is, it sent new shoots up from its roots, copying itself many times over.
This growing population of duplicate trees did not need seed dispersers to successfully reproduce—the trees simply cloned themselves. Importantly, however, they retained the ability to reproduce sexually through cross-pollination with other pawpaws.
Freed from their dependency on seed dispersers, these pawpaws survived the loss of giant mammals when other Annonaceae did not. And, because our pawpaw was able to adapt to a changing environment, so, too, did the tropical zebra swallowtail, a species that was, and still is, absolutely dependent on the pawpaw.
Have you ever crushed a pawpaw leaf?
If so, you remember the smell. If not, just know they stink. The smell is due to a mixture of chemicals, and among these are the acetogenins. This group of chemicals is commonly produced by members of the Annonaceae and serves to inhibit insects and other herbivores from eating the leaves.
In general, the defense works pretty well, but our friend the zebra swallowtail is not put off. In fact, the zebra swallowtail actually sequesters the acetogenins in its body, rendering it toxic to animals that would otherwise eat it—similar to how monarchs use toxins from milkweeds.
But the plot thickens. Other compounds in the pawpaw called turpenes are also sequestered by the zebra swallowtail caterpillar. These chemicals can then be sprayed into the air from the caterpillar’s osmeterium, a forked organ that the caterpillar literally pushes out of its forehead when it’s alarmed.
The smell of these aerosolized turpenes repels predators. Let that sink in for a bit. An already toxic caterpillar fervently forces a foul, fetid, forked appendage out of its forehead to frighten would-be foes.
I always imagine that scene from Alien when the creature pops out of Kane’s chest. If the caterpillars weren’t so cute, they’d be terrifying.
After a couple months of feasting on pawpaw, these plump little stink trolls resurrect into colorful, flying hippies and leave the forest to sip nectar and find love.
Oddly, the adult butterfly does not pollinate pawpaw flowers, instead preferring flowers of milkweed, blackberries and dogbane, to name a few.
The story of the pawpaw and swallowtail has always intrigued me. On the one hand, you have a tropical plant that shouldn’t be here, except that one of its ancestors was good at cloning itself.
On the other hand, you have a highly specialized butterfly species that should be restricted to the tropics, except that the one family of plants it relies on has a single representative species in Ohio.
Together, these two species have survived because they’ve both adapted to different environmental pressures.
But why doesn’t the swallowtail pollinate the pawpaw? Why doesn’t the pawpaw develop stronger toxins that kill even the swallowtail?
I have so many questions.
On the surface, the pawpaw-swallowtail relationship seems precarious, if not one-sided. But, like so many one-sided relationships, I suppose, as long as it’s advantageous to one, and not overly costly to the other, it works—at least for a while.
This is the epitome of evolution. For now, though, let’s not complicate the romance of this relationship with too much scientific doubt. Just as I do when I watch a sci-fi movie, I will ignore the scientist in my head and simply enjoy the show.