Although the list of things I know about bird biology is short, one thing I felt confident about was that bird skeletons are lightweight. I've seen for myself the thin-walled, hollow bones found in many bird species, and this always made intuitive sense: flying is certainly easier if you're lighter. Betsy Dumont at the University of Massachusetts recently decided to look more deeply into this apparent truism by asking just how lightweight bird bones are.
Using museum specimens, Dumont measured bone density across a wide variety of perching birds, rodents and bats. She chose to focus on the cranium, humerus and femur, which represent the largest components of the feeding and locomotor systems of these animals. Rodents were chosen for comparison as they are of a similar size to birds and also maintain relatively high metabolic rates. Bats were studied because outside birds they are the only vertebrates capable of generating powered flight, and bats too are purported to have lightweight skeletons. To measure bone density, Dumont weighed the bones to find their mass and measured the bone volume using helium displacement, which is like using water displacement, except that helium molecules are considerably smaller than water molecules and so can infiltrate much smaller spaces in bone, leading to better density estimates.
It turns out that humeral density is not distinguishable among the groups, averaging just above 2.0 g cm–3 in birds, rodents and bats. Femoral density is lowest in birds, near 1.8 g cm–3; however, cranium density is highest in birds, averaging nearly 2.2 g cm–3 (for comparison, rodent crania have densities averaging below 1.8 g cm–3). In short, different bones within and across major vertebrate clades vary with respect to density, a fact that may not raise many eyebrows. What is surprising though is that average bone density, which accounts for femora, humeri and crania and is weighted to reflect the relative contribution each bone makes to the total mass of its respective skeleton, is highest in birds and lowest in rodents. Flying animals appear to have denser bones than terrestrial counterparts of similar size.
In bones, differences in density generally mean differences in mineral content. Higher mineral contents are linked to greater stiffness and strength. Dumont thinks that such material properties may be central to understanding why bone density is relatively high in birds and bats. Along with flight, birds and bats have also evolved small body size, and one way to design a small skeleton while preserving its strength is to make its bones more dense. Investigating links between the material properties of bone and the evolution of flight and small body size in birds and bats is going to require much more work – if only we could get bone density measurements from transitional forms in the fossil record like Archaeopteryx. In any case, the next time you happen to handle a bird skull, don't worry so much about it breaking. What may appear to be a fragile, lightweight structure, is likely both heavier and stronger than you think!
