Why Don’t Birds Have Four Wings Like Microraptor?

If Microraptor’s four wings were so useful, why don’t birds today have them? Let’s explore some evolution.

Last week after I led a class for kids about the dinosaur Microraptor, an adult came up to me and asked, “Did you go to school for this?” I answered, “Yes.” I get asked this from time to time, especially when I touch on topics about life, the universe, and everything (see Douglas Adams). Usually it’s because they want to ensure themselves that their kids aren’t getting taught things that I just made up. Sometimes it’s because someone disagreed with something I said and if I wasn’t taught in school, they might feel they have more leverage in their argument. This time, however, it was different.

“I just had a question,” he said. “You said that Microraptor’s four wings helped it succeed, and that they made it easier for it to capture food. If that’s the case, why don’t animals have four wings now?”

I don’t often get deep into evolution and inheritance in my classes for kids for several reasons. It’s complicated, it’s controversial, and no one wants me teaching their kids about the birds and the bees—I’m generally at public libraries, and keeping things simple allows me to reach more people. But if people approach me after, I go into much more detail. This man asked me a very good question, and it was one I wish I could have answered more thoroughly without a room full of kids ages eight and under zooming around with Microraptor wings we had just made from construction paper. But because I was distracted and dodging Microraptors, I gave the most horrid answer. I explained that while some birds, like some breeds of chickens, have vestigial “wings” on their legs and feet, we don’t see birds with four wings because… I can’t even say it. I can’t repeat it, because I should go to science jail. (*Whispers in an ashamed voice* “…they didn’t need them.”)

This answer is the epitome of teleology, explaining things in terms of their purpose rather than just how they happened or developed. Evolution doesn’t have a purpose, per se. Like I often tell my son, “There is no why in science.” Things just are. We can try to explain how they happen, but there is no reason other than, in life sciences, “They happened to survive better.” And that’s what I meant when I said they didn’t need the four wings, but I know people don’t always understand the concept of evolution enough to get that. My cheap thirty-second answer might cause confusion for this man further along the line, and so I’m writing this article to make up for it.

The truth is, Microraptor did need those four wings to survive. That’s how she was able to compete with other animals for food. She was able to capture insects to eat while they were in the air. She was also able to glide down and snatch fish from the water. She was able to fill an ecological niche and thrive in it. Not only that, but her four wings helped her escape more easily from predators. Microraptor was a successful dinosaur. So the question of why more animals didn’t have four wings like she did is a perfectly reasonable and thoughtful one. My lame answer, while not exactly wrong, was was over-simplified, however. Microraptor, as a dinosaur that could not fly as birds do today, used four wings to glide and maneuver on the air. One hundred twenty million years ago, it was able to thrive because other animals in its ecosystem couldn’t do this as well. Over time, however, things changed. Specifically, some animals got better at flying. They didn’t only glide, but they could stay airborne for long periods of time using wing-powered flight. They could snatch bugs from the air and fish from the sea and fly off with them instead of having to land immediately to eat them. They could land and take off again from the ground instead of needing to glide from high to low places. This allowed them to sample more food, different food, than an animal that relied on gliding could get. Being able to fly away with food helped protect animals and their meals from scavengers. All of these things allowed flying animals to outcompete animals that could only glide.

Microraptor, with four wings, flourished over time because only the ones with four wings ended up surviving in that ecological niche. That is part of Darwin’s Theory of Evolution through Natural Selection. The Microraptors with smaller wings on their legs did not live long enough to mate and have offspring—either they tended to starve because they didn’t get enough food, or they were caught by predators because they couldn’t escape. The ones with four wings could do those things, and therefore had many baby Microraptors that also had four wings. Once other animals evolved with true, powered flight, however, things changed (though recently, scientists have made an argument that Microraptor was capable of some powered flight, but was not the best at it). Microraptor had more competition for food, so there was less of it. They just weren’t as good at catching it as their flying counterparts. This would reduce their population size. The carnivorous flying animals would also prey on Microraptors themselves, since it was much harder for the gliding dinosaurs to escape from flying ones. This would further decrease their population. In the end, avian dinosaurs—those that would become birds—made Microraptor obsolete. More animals with two wings that were capable of strong powered flight made it to maturity, had more babies, and those gliders with four wings did not. The gene for four wings disappeared over time because four-winged babies stopped being born because fewer gliding four-winged animals reached an age where the could mate.

Of course, this is still an oversimplification. We still don’t know a lot about how some animals evolved different traits, and other environmental factors could have been at play in the demise of Microraptor as well. Even so, it all comes down to how many babies hatched and grew to have babies of their own. The numbers of Microraptors dwindled until they no longer existed. If a trait disappears from the gene pool, it can’t just reappear from no where–if neither parent has genes for four wings, the children will not have that gene either. Eventually a mutation might take place that produces this feature again, but it would not be the same genetic lineage. 

Hopefully I’ve explained this in such a way that it didn’t make things more confusing. Evolution is tricky to explain because the popular vocabulary surrounding it has become very “why” focused (which is inaccurate) rather than “how” focused. Making the shift to understanding how it works can take some mental effort but it’s worth it!

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