Around 300 million years ago, dragonflies with the wingspans of hawks flitted above coal-producing swamps. Such giants don't exist today, partly because oxygen levels in the atmosphere are much lower. But another reason is that the evolution of birds and their increasing agility in the air forced flying insects to shrink, according to a new study.
Like all multicellular animals, insects fuel their metabolism by taking in oxygen. Unlike creatures with lungs, however, insects draw in air through holes in their shell-like exoskeletons. The oxygen diffuses from those holes to the creatures' tissues through a dense network of tubes. Because diffusion becomes less efficient as the tubes get longer, the atmospheric concentration of oxygen is a powerful constraint on body size, especially for active insects that fly. The higher the concentration, the larger the insects can grow.
But new analyses of fossils have revealed another apparent constraint on insect size: the presence of predators. Several previous studies, especially those in the past 5 years, have hinted that factors other than oxygen levels influenced insect size.
For their new study, paleontologists Matthew Clapham and Jered Karr of the University of California, Santa Cruz, sorted more than 10,500 fossils of flying insects that lived in the past 320 million years into 10-million-year intervals. In each interval, they compared the largest wing length, which presumably belonged to the largest insect that lived during that particular period, to the average oxygen concentration in Earth's atmosphere during that time.
Results of the study, which the researchers report online today in the Proceedings of the National Academy of Sciences, show that for the first 170 million years or so of flying-insect evolution, wing length grew and shrank in step with variations in average oxygen concentration. Then, between 140 million and 130 million years ago, wing length dropped even though oxygen levels began to rise. Probably not coincidentally, says Clapham, this 10-million-year interval is the first one after the evolution of Archaeopteryx, long considered one of the first birds, and other early birds. Many of these creatures probably preyed on flying insects. Even though these early birds were presumably clumsy in flight, they drove insect evolution. The larger the insects, the less maneuverable they were in the air -- and the less likely they were to escape predators, the researchers contend. Indeed, one group of large-bodied insects that probably glided rather than flew—relatives of today's dragonflies—disappeared from the fossil record and apparently went extinct soon after birds evolved.
Over time, birds became more capable flyers, and as a result, insects shrank substantially again sometime between 90 million and 60 million years ago. "By the end of the Cretaceous period [about 65 million years ago], birds looked quite a lot like they do today," says Clapham.
A simple comparison illustrates the trend, he says. For the last 100 million years or so, the concentration of oxygen in the air has hovered around 21%, and the largest insect of this period had wings between 6 and 7 centimeters long, says Clapham. But late in the Triassic period—between 210 million and 220 million years ago, when oxygen levels were similar to those of today—one species closely related to today's grasshoppers had wings ranging from 15 to 18 cm long, he notes.
The team's findings "support, but certainly don't prove" the hypothesis that the evolution of birds triggered the downsizing of flying insects, says Jon Harrison, an evolutionary biologist at Arizona State University, Tempe. Plenty of other factors, including temperature, exert a strong influence on insect size and oxygen requirements, he notes.
"This is a great paper, and their argument makes sense," says Conrad Labandeira, a paleoentomologist at the Smithsonian Institution National Museum of Natural History in Washington, D.C. However, he adds, it would also be interesting to see if long-term variations in the atmospheric concentration of carbon dioxide had any apparent effect on insect size. That's because high levels of carbon dioxide typically render plants less nutritious, which would in turn tend to stifle insect growth. "Insects need two things," he notes. "They need to respire, and they need to eat."