BIRDS have more expertise in structural engineering than anyone has given them credit for. Researchers in Manchester and New Zealand have shown that birds鈥� eggs are designed to be much easier to break out of than into.
Most of an egg鈥檚 strength comes from its shape. But another factor is the relative resistance to fracturing of the shell鈥檚 outer and inner surfaces. When a force is applied locally to a thin piece of brittle material, the opposite surface stretches and deforms. The weaker this surface, the easier the material is to break. Once cracks start to form, they soon spread through the material鈥檚 entire thickness, however strong the surface to which the force is being applied.
Paradoxically, the only previous attempt to measure the relative strengths of an eggshell鈥檚 surfaces suggested that eggs are designed to make life difficult for a hatching chick. In 1966, Cyril Tyler of the University of Reading cut strips from the shells of hens鈥� eggs and then measured the force required to snap them in either direction. He concluded that the outermost layers of an eggshell are actually more resistant to fracturing than its inner surface.
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But Kenneth Entwistle of the University of Manchester Institute of Science and Technology argues that Tyler鈥檚 experiments revealed little about the strength of a shell in normal circumstances. 鈥淲hen you cut strips out of a brittle material, you damage the edge and weaken the structure,鈥� he says. So Entwistle, his colleague Sylvester Ochieng Abuodha and Heather Silyn-Roberts of the University of Auckland, devised a way to measure the relative strengths of an eggshell鈥檚 surfaces without weakening them. First, they cut hens鈥� eggs in half and held the blunt end between two plastic rings 鈥� one 7 millimetres in diameter placed inside the shell and the other 15 millimetres in diameter placed outside. By applying force to the smaller ring, Entwistle and his colleagues could measure the fracture strength of the outer surface. To do the same for the inner surface, they squeezed uncut eggs between two 7 millimetre diameter rings. The inner surface of an eggshell turned out to be 34 per cent stronger on average than its outer surface (Proceedings of the Royal Society B, vol 262, p 169). 鈥淭his makes it easier for the chick to get out,鈥� says Entwistle.
The explanation probably lies in the microscopic structure of eggshells. Silyn-Roberts and her Auckland colleague Roy Sharp have found that an eggshell鈥檚 inner surface consists entirely of calcite crystals arranged in a honeycomb-like pattern. The outer layers, however, are broken up by a network of organic material which presumably weakens their structure.