While watching two dogs risk life and limb during a fight in his kitchen,David Carrier began to ponder how the artificial selection pressures that humans have imposed on dogs have affected the limb bones of different breeds. Carrier suspected that there is a trade-off between properties that make limb bones suitable for fighting and those that make bones suitable for running. Teaming up with Travis Kemp, Kent Bachus and John Nairn, Carrier compared the limb bones of stocky pit bulls, bred for fighting prowess, with those of slim greyhounds, bred for high-speed running(p. 3475).
The team expected greyhounds to have stiff bones that can transmit muscular forces, and pit bulls to have tough bones that resist breaking. To find out how canine limb bones hold up under pressure, Carrier and his team placed greyhound and pit bull bones into a loading machine and measured the forces and deformation exerted on each bone until each bone snapped. The team found that greyhound limb bones were much stiffer and coped with higher stress before deforming than pit bull bones. But pit bull bones were more circular in shape and absorbed more than twice as much energy as greyhound bones before breaking; their limb bones are tougher than those of greyhounds. The team's results suggest that the limb bones of animals specialized for running are shaped differently and composed of bone tissue with different mechanical properties than are the limb bones of animals specialized for fighting. `There appears to be a trade-off between bone requirements for running versus fighting', Carrier concludes.