Birds may have had to crouch before they could fly

Fossils of avian ancestors show progressive redistribution of weight toward front

Today’s birds may have inherited their distinctive crouching posture from ancestors whose hind limbs had to compensate for the weight of increasingly beefy forelimbs, aka wings. New findings, though preliminary, suggest that changes both fore and aft may have been important in the evolution of flight.

UPRIGHT ANCESTOR Forelimb growth over millions of years made members of the bird lineage more front-heavy, forcing modifications to the hind limbs that led to the crouched posture seen in modern birds, research suggests. Before that change, the dinosaur ancestors of birds stood upright, with the feet right under their center of mass. Vivian Allen

FORWARD SHIFT Over millions of years, the creatures became more front-heavy. With no other anatomical adjustments, that change would have put the body’s center of mass in front of the feet and left the animal off-balance. Vivian Allen

CROUCHING POSTURE To compensate for the forward weight shift, the upper bones in the hind limbs bent backward, becoming almost horizontal to the ground. That kept the feet under the center of mass, stabilizing the creatures’ bodies and giving them a crouched posture. Vivian Allen

Members of the dinosaur lineage that led to modern birds stood upright, with stubby, clawed forelimbs suited to tearing at flesh. But over millions of years, the bodies of creatures on the bird lineage became more front-heavy as their forelimbs grew larger and turned into heavily muscled wings, researchers found. To compensate for that extra weight, their femurs bent backward, becoming horizontal to the ground. Scientists previously thought that birds’ ancestors became front-heavy, and thus crouched, to balance their increasingly smaller tails, but they hadn’t explored the role of changes in the front of the body.

A team led by John Hutchinson of the United Kingdom’s Royal Veterinary College digitally reconstructed the bodies of 17 bird ancestors — mainly dinosaurs, including the famed Archaeopteryx — for which they had mostly complete skeletons. Then the researchers digitally added flesh around the creatures’ bones and analyzed the reconstructions to see how mass was distributed along the creatures’ bodies.

The researchers confirmed that tails did get shorter in the more recent species. But that didn’t fully explain why the creatures became front-heavy. Growth of the forelimbs played a far bigger role, the researchers concluded. This shift appeared to speed up around the time that flight likely came about.

Although a crouched posture may be good for perching, takeoff and landing, the researchers caution that the link between the crouch and the evolution of flight is still unclear. “I think this is a missing puzzle piece that future studies will be able to fill in using models like ours,” Hutchinson says. The findings are reported April 24 in Nature.

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