Crocodiles can't fly. Birds can. Scientists have finally figured out why

This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The closest living relatives of crocodiles and alligators are, perhaps surprisingly, birds. Both are archosauriforms, a group of species that originated about 250 million years ago near the beginning of the Triassic period.

The group soon split into two lineages: One led to crocodiles (Pseudosuchia), while the other led to dinosaurs and then birds (Avemetatarsalia). This means there is a combined 500 million years of evolutionary change between modern birds and crocodiles. The eons have erased almost all evidence of shared ancestry.

Crocodiles are cold-blooded (called ectotherms by physiologists). Their body temperatures are determined by the environment, and they have low metabolic rates (the speed at which their bodies turn food into energy). They appear sluggish, except for occasional brief flurries of powerful activity while drowning prey or fighting among themselves.

Birds are warm-blooded (or endotherms). They have high metabolic rates, producing energy quickly enough to warm and stabilize their body temperatures and to undertake sustained, strenuous flight. Birds don't fatigue and fall out of the sky.

Even if crocodiles somehow had wings, the metabolic difference in power alone would mean they could never fly. How did birds and crocodiles become so different? The conventional assumption has been that their common ancestor was more like a crocodile than a bird—but our new research shows the opposite is most likely the truth.

Scientists have assumed that warm-blooded animals evolved from cold-blooded ones, not the reverse. As warm-blooded creatures ourselves, we have had a habit of thinking warm-blooded animals are physiologically superior to cold-blooded ones. By this logic, if a group of animals evolved to become warm-blooded, it could never revert to a cold-blooded state without quickly going extinct.