Birds The Reason Insects Are Smaller Today

The world's largest dragonfly today can have wingspans of up to twenty centimeters. 300 million years ago, the griffinfly could have a wingspan of up to 28 inches, or up to 75 centimeters long. The reason our skies today may not be buzzing with insects three times the current size may be thanks to birds.

Around 150 million years ago during the Jurassic period, birds first began appearing alongside dinosaurs. The reason for this evolutionary divergance could have been because of the time it takes a bird to reach sexual maturity versus the time for a dinosaur. In any case, the appearance of birds correlates with the decrease in the size of insects.

The team conducted the study by categorizing 10,500 insect fossils from the last 320 million years.

Traditionally, insect size has correlated with the oxygen levels in the atmosphere. 

"When oxygen went up, insects got bigger. And when oxygen went down, they got smaller," said co-author Matthew Clapham.

According to experts, 300 million years ago, the oxygen levels in the atmosphere were 30 percent compared with 21 percent today. This extra oxygen would have allowed larger insects like the griffinfly to power larger bodies.

However, when birds came onto the scene 150 million years ago, the insects got smaller even though oxygen levels were rising.

"The change in insect size is gradual," Clapham told LiveScience. "This gradual change fits quite nicely with the gradual evolution in birds at the time."

Despite the earlier evolution of the pterosaurus and other flying dinosaurs, insect size was not affected by them. Researchers think this had to do with the manuverability of the dinosaurs. And since small things are more manuverable, the smaller insect has a better chance of avoiding a bird than a larger, more visible and bulky one.

Another reason could also be that some insects like the griffinfly and dragonfly are predatory. Being the same size as some birds, they might have competed for similar prey, with the birds eventually outperforming them.