How sturdy does an animal have to be to avoid breakage? And can the question be answered without, say, crash-testing beagles?

It can, according to anatomist Andrew Biewener of the University of Chicago, who applied principles of mechanical engineering to animal anatomy. He found that mammals have a constant safety factor of between two and four, whatever their weight. That is, bones, muscles and connective tissue can handle two to four times their normal peak stresses before breaking. In last week's issue of Science, Biewener speculates that natural selection probably favored this range as a trade-off between skeletal durability and the energy cost needed to build, maintain and transport a more heavy-duty structure.

It was once assumed that larger, weightier animals had a different bone shape to handle the higher loads, but Biewener found that terrestrial mammals, large or small, are geometrically similar. Big animals tend to bend their legs less under stress, increasing their limbs' mechanical advantage and aligning joints more closely with ground forces. In animals of similar size, structural differences result from biomechanical strain. For instance, dogs run nearly twice as fast as goats, and thus have thicker bones, but the force per unit of bone area is nearly identical. This suggests that the skeleton has a signaling mechanism that can sense stress and relay instructions to bone-building cells about how thick a bone is needed to handle normal loads.