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Luna moth, Actias luna (female) |
Here's a beautiful moth that teaches us a great deal about evolution, how it works and how it can end up producing things that look decidedly maladaptive, even sometimes a little ridiculous and unnecessarily ostentatious. But perhaps the most important lesson here is how it has nothing to do with us and our perception of what looks intuitively to us to be maladaptive and unnecessary. In fact, in this case, what it has to do with is bat perception.
I'm referring to those long streamers on the ends of the hind wings which twirl around more or less randomly as the moth flies. They surely make flight more difficult, don't they?
Normally with maladaptive features like these, for example the male peacocks elaborate tail and gaudy plumage, female sex selection plays a big role with the male gaining benefit from his enhanced ability to attract females which outweigh his reduced ability to escape predators. In this case, however, while there is some sexual dimorphism, both sexed have streamers and they don't appear to play any part in courtship displays. The male's broader antennae suggest that, like many nocturnal moths, female luna moths use scent and pheromones to attract a mate so selection, if any, favours the males best able to follow the scent and find the female.
So, why would a moth have appendages on it wings that make flight more difficult?
The answer is one of the other features of evolution which can produce something apparently pointless - an evolutionary arms race. An evolutionary arms race is responsible for trees expending vast amounts of energy growing massive trunks to get their leaves above the other trees - which are growing massive trunks for the same reason. In the long run, no trees gain from this race as there are limits to the height they can grow a trunk and they all end up where they started, no better off when they were short. The competition for light drove them to heights of stupidity. Similarly, cheetahs have evolved speed to catch gezels because gezels have evolved speed to outrun cheetah. If gezels had been designed for cheetah food the designer would have made them slow. If gezels had been designed to eat grass and produce more gezels then why design cheetahs? Unless the designer is a sadist who likes watching blood sports and enjoys the death agony of gezels - the same designer that designed cancer and parasites and tsunamis and earthquakes to hurt us, so beloved of Ray Comfort, Ken Ham, Michael Behe, et al, and their credulous followers.
The arms race the luna moth has been involved in for 60 million years is with bats. Unlike cheetahs, bats don't use speed to catch food, they use echolocation and moths don't use speed to escape them because bats can fly much faster than they ever could. But nocturnal bats are almost entirely dependent on accurate echolocation to strike their prey in flight, so anything which makes the echo less precise and fuzzy, or garbles it in some way, cuts down the bat's chance of making a hit and improves the moth's chance of escaping.
And this is exactly what the streamers on the luna moth's wings do. Unless we look at a flying luna moth through the perception of an echolocating bat, the streamers make no sense at all and look intuitively maladaptive.
Significance
Bats and moths have been engaged in acoustic warfare for more than 60 million y. Yet almost half of moth species lack bat-detecting ears and still face intense bat predation. We hypothesized that the long tails of one group of seemingly defenseless moths, saturniids, are an anti-bat strategy designed to divert bat attacks. Using high-speed infrared videography, we show that the spinning hindwing tails of luna moths lure echolocating bat attacks to these nonessential appendages in over half of bat–moth interactions. Further we show that long hindwing tails have independently evolved multiple times in saturniid moths. This finding expands our knowledge of antipredator deflection strategies, the limitations of bat sonar, and the extent of a long-standing evolutionary arms race.
Abstract
Adaptations to divert the attacks of visually guided predators have evolved repeatedly in animals. Using high-speed infrared videography, we show that luna moths (Actias luna) generate an acoustic diversion with spinning hindwing tails to deflect echolocating bat attacks away from their body and toward these nonessential appendages. We pit luna moths against big brown bats (Eptesicus fuscus) and demonstrate a survival advantage of ∼47% for moths with tails versus those that had their tails removed. The benefit of hindwing tails is equivalent to the advantage conferred to moths by bat-detecting ears. Moth tails lured bat attacks to these wing regions during 55% of interactions between bats and intact luna moths. We analyzed flight kinematics of moths with and without hindwing tails and suggest that tails have a minimal role in flight performance. Using a robust phylogeny, we find that long spatulate tails have independently evolved four times in saturniid moths, further supporting the selective advantage of this anti-bat strategy. Diversionary tactics are perhaps more common than appreciated in predator–prey interactions. Our finding suggests that focusing on the sensory ecologies of key predators will reveal such countermeasures in prey.
It's quite easy to understand how this evolved by small steps too, as each mutation which lengthened the streamer a little would have increased its effectiveness at garbling and confusing the echo. Eventually, a limit would be reached at which the length of the streamers began to make difficulty in flying outweigh the advantage of avoiding being eaten, so an evolutionary balance would have been reached. The fact that bats have never evolved the ability to overcome this, reflects the different evolutionary pressure from simply missing a meal when alternatives are available, and avoiding being a meal - at which point your genes are eliminated from the genepool.
Creationists will look at this and decide the luna moth looks that way because that's what their magic designer wanted them to look like. Evolutionary biologists, and people without creationists' essential ignorance can see the real wonder in the luna moth's streamers and understand exactly why it has them. They can also understand that luna moths look the way they do because they have spent 60 million years in the presence of echolocating bats which are trying to eat them and those who can avoid getting eaten generally leave more offspring. Just like every other living thing they've been fashioned and shaped by their environment acting on small variations so those best able to survive and reproduce pass these variations on to more descendants than those without them.
No magic needed, but the result, which is far too stupid to have been intelligently designed, has produced this amazing little planet and the great thing is that we can now understand how it got that way.
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