| About 100 years ago, people in northern New Guinea carved daggers out of cassowary bones. The bones of a cassowary, a type of giant bird, were as strong as human femurs, scientists at Dartmouth College discovered recently. Cassowaries weigh 125 pounds and are flightless, so it makes sense their bones would be thick (at least for birds). They do not have the classic hollow bones of flying birds, called "pneumatic" bones in the scientific jargon. I had always assumed that hollow bones were lighter, so flying birds did not have to cart around as much weight. Galileo thought as much in 1638, and that idea persists in modern textbooks. Following a tangent while reporting this story, though, I stumbled upon a different answer. As in many things with science, it's a little more complicated that it first seems. Hollowness probably first evolved to make the bones of flying animals stiffer, not as a way to decrease the mass of their skeletons. In 2010, researchers at the University of Massachusetts in Amherst measured the density of bird bones and mammal bones, and found, for their size, the bird bones were denser on average. Yes, birds have air sacs in their bones — in fact, the sacs are connected to their respiratory system. These sacs have another advantage, making each breath during a flight more energy efficient. As it goes, dinosaurs, too, had the sacs in their bones: "Air pockets for [a] high-energy lifestyle," as Nature magazine put it. — Ben |
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