Giant Prehistoric Insects Shrank to Escape Developing Birds

Insects with wingspans of more than two feet ruled the skies until 150 million years ago, when predatory birds evolved into skilled flying bug-zappers, according to a study.

The insects originally grew in tandem with rising oxygen levels until birds developed, the research found. The study, published in the Proceedings of the National Academy of Sciences, suggests the bugs then got smaller as a way to maneuver away from the avian predators.

Oxygen levels before birds became a factor surged as high as 30 percent, compared with today’s 21 percent. After fast- flying birds evolved in the early Cretaceous period, about 55 million years ago, the bugs began to shrink even though oxygen levels continued to rise. While there’s no direct evidence linking cause and effect, the birds must be considered an obvious culprit, said Matthew Clapham, a study author from the University of California, Santa Cruz.

“It was really surprising how well oxygen and body size follow each other until birds evolve,” Clapham, who is an assistant professor of Earth and planetary sciences at the school, said by telephone. “If you look at the oxygen levels today, it’s about the same as the mid-Triassic, and the largest insects then had wings that were almost three times as big.”

The study looked at maximum body sizes from fossils, Clapham said. The average insect remained small. And although bats may have eaten some of the bigger insects, they seem less likely to have had a greater effect than birds. That’s because the larger insects were dragonflies, which are active during the day, and bats are active at night, he said.

Flight Agility

“Big things are less maneuverable,” Clapham said. “So as the birds got more agile at flying and moving at low speeds, they got better at catching insects.”

The researchers found a weak link between body size and the evolution of the pterosaurs, flying reptiles which appeared about 230 million years ago, during the Triassic period. The smaller pterosaurs may have dined on insects, Clapham said.

However, the link between the two isn’t clear because there’s a 20 million year gap in the record after the pterosaurs, and a drop in oxygen levels complicates the analysis.

The data was compiled from over 10,500 wing lengths from insect fossils. The researchers used oxygen levels from the “Geocarbsulf” model developed at Yale University in New Haven, Connecticut, by Robert Berner that uses known factors of oxygen, carbon and sulfur in considering the atmospheres of each prehistoric period.

The study was funded by the National Science Foundation and UC Santa Cruz.