How Ratites Ended Up on the No-Fly List

An extensive DNA analysis offers new insight into the evolution of flightless birds.

From shy, chicken-sized kiwis to 200-pound, lion-kicking ostriches, the ratites—flightless birds—are a diverse bunch. Unlike their airborne relatives, ratites have primitive, reptilian palates and flat, unkeeled sternums that make it impossible for flight muscles to take anchor. Because they are comparatively poor long-distance travelers, their evolution into such widely dispersed and varied forms has long presented an ornithological mystery. But a group of researchers from Toronto think they have found the answer to the phylogenetic puzzle in the DNA of a drab, ground-dwelling bird called the tinamou.

For years, biologists reasoned that the ratites must share a common ancestor that lost its ability to fly before the continents drifted apart, which would explain their presence in places as far-flung as Argentina, South Africa, and New Zealand. But tinamous, a family of 47 species in Mexico, Central America, and South America, complicated that hypothesis.

Like the ratites, tinamous have a reptilian palate. But they also have a keeled sternum; although they do their best to avoid it, tinamous can fly. To determine exactly where tinamous fit in, the researchers analyzed nearly 1,500 DNA sequences from ostriches, emus, tinamous, moas (an extinct ratite), and other more distantly related species.

Published in Molecular Biology and Evolution, the surprising results show that tinamous and moas are more closely related to each other than they are to any other species, which means that tinamous and moas diverged after they split from the other ratites. It's very unlikely that tinamous became flightless at some point in their evolutionary history and later took to the skies once more. A more probable explanation is convergent evolution—each group of ratite independently lost its ability to fly as an adaptation to its cursorial lifestyle.

Previous studies had attempted to elucidate the ratite-tinamou relationship, but none used such extensive DNA samples, which is what Allan Baker, Senior Curator of Ornithology at the Royal Ontario Museum and the paper's lead author, says makes these results so conclusive.

The next step is to further untangle the relationships among ratites. It's not an easy task, but the researchers—like the birds they study—have hit the ground running.