Birds’ evolution led to disappearance of giant insects, study finds

If you think houseflies are a nuisance now, imagine flies with a wingspan of 28 inches. Such gigantic insects were common 300 million years, but fortunately did not survive to pester us today. And the primary reason why? The evolution of birds from dinosaurs, which caused insects to become smaller and more maneuverable to escape predation, new research suggests.

Primordial insects were able to grow so large because of high concentrations of oxygen in the ancient atmosphere, about 30% compared with 21% today. Because larger organisms have greater oxygen requirements, a variety of studies have shown that the size of most species is limited by the amount of oxygen they can take in. Insects are especially vulnerable because they inhale oxygen through tiny breathing tubes that they use instead of lungs.

Planetary scientist Matthew E. Clapham of UC Santa Cruz and his graduate student Jered A. Karr compiled a data set of insect wing lengths from published fossil records, compiling more than 10,500 wing lengths. They then compared wing lengths to atmospheric oxygen content, as obtained from other sources. They reported in the Proceedings of the National Academy of Sciences that, for the first 150 million years of insect evolution, wing sizes went up and down in proportion to the amount of oxygen in the atmosphere. Then, about 150 million years ago, oxygen concentrations rose while insect size began to drop. “And this coincides rather strikingly with the evolution of birds,” Clapham said.

Another drop in insect size occurred between 90 and 65 million years ago, at the end of the Cretacous period. In this case, Clapham said, several factors could be responsible, including the continued specialization of birds, the evolution of bats, and the mass extinction at the end of the Cretaceous. As birds and bats became better fliers and more maneuverable, insects had to also get smaller and more maneuverable in order to escape their predators. At the end of that period, insects were about the size they are today.

So, the next time you’re bugged by bugs, just remember: Without birds, it could be a lot worse.



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