I think it would be too much to say that I had a “moth phase” as a child, but I do have a few memories of being very impressed by some of the big moths I encountered in Maine, New Hampshire, and Nova Scotia, Canada. I mention the latter because my family took a vacation there when I was a kid. We stayed in camping sites that were often packed with silver Airstream trailers. It was there that I first beheld the majestic Luna moth. These things can have a wingspan of up to seven inches, and while I don’t know how big the one I saw was, I was pretty small at the time, so it certainly seemed huge. For those who don’t know, Luna moths are big, bright green, and have a sort of swallowtail thing going on.
They’re quite pretty, and they barely seem like functional creatures. I haven’t really considered them much in the last couple decades, so if you asked me why they had tails like that, I’d probably guess that it had to do with mate preference and sexual selection. It turns out that they’re actually to misdirect bat echolocation, and scientists have known that for around a decade. More than that, recent research gives us some reason to believe that the tails are only for bat-scrambling, and probably provide no addition benefit or cost when it comes to reproduction or avoiding other predators.
“They have projections off the back of the hindwing that end in twisted, cupped paddles,” said Juliette Rubin, a doctoral student at the Florida Museum of Natural History and lead author of both studies. “From experimental work with bats and moths in a flight room, we’ve found that these structures seem to reflect bat sonar in such a way that bats often aim their attacks at the tails instead of the main body.”
Traits that evolve for one specific function can often be co-opted by natural selection for another, and Rubin wondered whether the twisted tails of Luna moths might come with any additional benefits or hidden costs.
Silkmoths have independently evolved tails on multiple occasions across three separate continents, and they can vary significantly in length. Hindwings in some species can extend to more than twice the size of the moth’s wingspan, and the longer the tail, the more likely a moth will successfully thwart a prowling bat.
But far from being drab, utilitarian decoys meant only for sonar-sensing bats, silkmoth tails are often visually stunning, like decorative streamers trailing behind a kite. Across the animal and plant kingdoms, many of the most colorful and alluring structures are used to attract mates or pollinators, and scientists suspected the same might be true of silkmoth tails.
This type of dual function for a single trait isn’t without precedent. The vivid colors of strawberry poison dart frogs (Oophaga pumilio) both deter predators and help males attract mates; male deer and other ungulates use their antlers to fight off rivals and signal their vigor to females; and moths that use clicks or chirruping sounds to disrupt bat echolocation can compose duets using the same sounds during courtship.
Luna moths have neither mouths to produce sound or ears to hear it, but they do have sensitive eyes and powerful scent-detecting antennae. When female Luna moths are ready to mate, they perch in one place and emit a pheromone, a single molecule of which is enough to trigger a male antenna. The males of closely related Indian moon moths (Actias selene) can find females from more than six miles away by following the pheromone plume to its source.
“We don’t know how many males are traveling to a female each night,” Rubin said. “It’s entirely possible she’s able to call in multiple suitors and potentially have her pick.”
Rubin put this idea to the test, setting up mating experiments in which a single, female Luna moth was enclosed in a flight box with two males: One with normal hind wings, and one with its tails removed.
Initially, the data seemed to suggest that females had a preference for males whose wings remained intact, but additional controlled experiments demonstrated this was more likely an artifact of the tail removal. In trials where both males had their wings clipped, and one had the tails glued back on, there was no difference in their mating success.
Personally, I think that if they did have mouths with which to scream, we might not be so cavalier about lopping bits off them, and taping them back on. I realize that moth wing tails aren’t really analogous to my own extremities, but still.
It seems as though this study had a pretty small sample size, so some salt is required, but it is interesting. They also tested whether the tails help with avoiding birds during the daytime. Many moths rely on camouflage to survive during the day, and are only really active at night, so the researchers tested whether the tails affect their ability to hide. They did this by making fake Luna moth bodies by wrapping mealworms in pastry dough, and attaching clipped and unclipped wings to them. These bait moths were then hidden in an aviary by being partially covered with leaves, and the researchers then put Carolina wrens into the enclosure to see whether the presence of moth tails affected how well the wrens found the “moths”.
I think it’s likely that the wrens can’t smell the pastry dough, so odds are decent that the makeup of the bait moth’s “body” wasn’t a huge problem (you may not know this, but moths are not, in fact, two mealworms in a pastry dough trench coat). I do wonder what exposure these particular wrens have even had to moths in their lifetimes, though, given that they live in an aviary. I also wonder whether other birds might snack on them more regularly?
This isn’t the most compelling research report I’ve ever read, but as I said at the beginning, I think it does give us at least some reason to believe that the tails really are entirely bat-focused. For most of my life, I’ve lumped them in with the elaborate plumage of various tropical birds, both in terms of function, and in terms of increased risk of predation. Apparently I’ve had it all backwards. They provide a clear benefit to survival, during the short window in which that matters, and apparently have nothing at all to do with mating.
For me, I think this is a good reminder that even when I can get a clear look at something, I’m not always seeing what I think I’m seeing. We make conclusions based on what we already know of the world, and that can, very often, lead us astray.
Also, is pastry dough OK for wrens to eat? It seems like it might not be.
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