Zebras, horses and other equine species put substantial stress upon their central forefoot bones, particularly the third metacarpal, bones with remarkable strength despite having holes in them for blood vessels to pass through. The presence of a hole (or foramen) in a structural element offers the potential for it to act as a site of stress concentration and initiation of cracks, yet these foramina do not weaken the bone nor act as fracture initiation sites. Hence the foramen in the third metacarpal of equine species has been of interest to engineers to learn how to design openings in structures in a way that avoids cracking. The key features investigators have found that minimize cracking at these sites are: their location in regions predominantly experiencing compression, their elliptical rather than round shape (oriented parallel to the long axis of the bone and the lines of force), the ‘softening’ of the material discontinuity by increased compliance of the tissue surrounding the opening that shifts peak stresses away from the foramen edge, and a ring of increased stiffness reinforcing the foramen at some distance from it to absorb those stresses shifted inward from the compliant foramen edge. Many human-made structures, such as airplane wings, need to have holes in them to accommodate wires, fuel lines or hydraulic system elements and hence inspiration from the design of foramina in bones could have wide application.
"The third metacarpus bone in a horse's leg supports much of the force conveyed as the animal moves. On one side of the cucumber-size bone is a pea-size hole where blood vessels enter. As a rule, drilled holes weaken structures, causing them to break more easily than solid structures when pressure is applied, but nature has found a way of circumventing this rule…the (horse) bone was configured in such a way that it pushed the highest stresses away from the foramen into a region of higher strength…the bone's hole is also tougher than a typical drilled hole—more resistant to initial cracks growing to catastrophic lengths." (DeGaspari 2003:45)