Albumen, the fluid in bird eggs, protects the chick by being elastic and incompressible.
"The egg's shock absorption, which has received little investigation, is based on the fact that the embryo is surrounded by the albumen, an elastic gelatin-Iike substance of high water content. The result is a propitious combination of properties: a liquid that cannot be compressed, only displaced, and an elastic substance. When the embryo is pushed against the shell by some forceful impact, the liquid must flow past it and transform the destructive energy into heat. The shock absorption of the egg is further improved by an air cushion located at the thick end of the egg--the same end as the center of gravity. In a falling body the center of gravity moves to the lowest possible point, so in an egg the embryo falls on the air cushion. The air pocket in the egg has another mechanical function. It prevents temperature fluctuations from cracking the shell." (Tributsch 1984:22)
"However ordinary it may seem to us, the egg of a chicken has about fifteen thousand pores resembling dimples on a golf ball. The spongy structure of smaller eggs can only be observed under the microscope. These spongy structures give eggs added flexibility and increase their resistance to impact…An egg is a miracle of packaging. It supplies all the nutrients and water that the developing foetus needs. The yolk of the egg stores protein, fats, vitamins and minerals, and the white works as a reservoir of fluid…The developing chick needs to inhale oxygen and exhale carbon dioxide. It also requires a source of heat, calcium for its bone development, protection of its fluids, protection against bacteria and physical impact. The eggshell provides all of these for the chick, which breathes through a membranous sac that develops in the embryo. Blood vessels in this sac bring oxygen to the embryo and take carbon dioxide away." (Yahya 2002:69)
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