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Balancing forces in the evolution of flight

Calculating the competing forces that pull on a bird's shoulder during flight …

Birds may make it look easy, but flight is very stressful for them. Or at least for their shoulder joints, according to some recent research. Because the muscles and ligaments that stabilize the shoulder all reside close to the body, they have little leverage to hold the wing in place. As a result, one of the muscles (the pectoralis) in a pigeon's wing needs to exert a force of seven times its body weight simply to hold the wing steady while gliding, according to a model of the wing built by the authors. To actually pull the wing down for flight increases the requirement to 13 times body weight.

This would all be easy if the muscle were pulling against a rigid joint, but a bird's shoulder needs to be open and allow a broad range of movements. As a result, unless the pectoralis's force were balanced, it would pull the wing right out of its socket. The authors of this report tested a number of other structures in the wing for their ability to counter-balance the force of this muscle and found that a ligament (the acrocoracohumeral ligament, or AHL) was ideally located to provide an opposing force. In fact, tests with the actual ligaments of dead pigeons revealed that the AHL could survive strains of nearly 40 times a pigeon's typical body weight.

The authors ask what this ligament is doing when it's not stabilizing a wing? The closest living relatives of birds are the Crocodilians, which primarily flex their shoulders horizontally. In alligators, the ligament extends horizontally, and doesn't even seem to be put under stress during their normal stride—the force generated by the pectoralis seems to be counteracted by other muscles. This raises some obvious questions about how the ligament came to play such a key role in the wing.

Fortunately, the paths and attachments of ligaments leave landmarks on bones which can be detected even in fossils. Ancestors of birds such as therapod dinosaurs and the Archaeopteryx appear to have an arrangement very much like an alligator's. But after that point, other fossil species on the bird lineage indicate that the adoption of a new orientation for the AHL and a more flexible shoulder joint occurred both very gradually and in tandem. The timing of these changes suggest that the last common ancestor of all modern birds (which lived well after their split with reptiles) had an intermediate shoulder where only part of the stress of a vertical wing was transferred to the ligament, but that the process was at least underway.

It's a neat study that suggests that even mechanical engineers might have something to tell us about evolution.

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Channel Ars Technica