Why Egg Yolk Is Yellow

Why egg yolk is yellow (or red) (Blount et al. 2000) -- Egg yolk in birds is colored yellowish-red by carotenoids. Until recently, there has been no adaptive explanation of why many egg-laying animals provision their eggs so richly with carotenoids. It now appears that, in developing birds, carotenoids protect vulnerable tissues against damage caused by free radicals. Athough embryonic tissues depend on oxidizable, unsaturated fatty acids in yolk, their abundance makes the tissues susceptible to peroxidation caused by reactive oxidative metabolites and by free radicals, which are produced as normal by-products of metabolism. Protection against lipid peroxidation in young birds is afforded by the actions of yolk-derived carotenoids and other antioxidants, like vitamin E. Antioxidants also protect passively-acquired antibodies (IgY; see above) against break-down. Thus, maternal investment in egg composition, including carotenoids, might have a greater influence on offspring viability than has been realized. The use of carotenoid pigments in the sexual displays of female birds might indicate their ability to produce high quality eggs and chicks.

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• albumen
  
  
  
  • 90% water & 10% protein
    
  • the embryo's water supply, but also serves as a 'shock-absorber' to help protect the embryo
    
  • buffers embryo from sudden changes in temperature
    
  
  
• shell membranes
  
  
  
  • attached to the shell are two membranes, the inner and outer shell membranes. They protect the egg from bacterial invasion and help prevent rapid evaporation of moisture from the egg.
    
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Keratin fibers from the outer shell membrane can be seen above, attached to the
calcium carbonate crystals that make up the main shell structure.
(Source: http://www.rit.edu/~tld0898/SEM.html)

• shell
  
  
  
  • protects the embryo
    
  • contains thousands of pores (see diagram below) that permit gas exchange
    
  • generally white in cavity-nesters & colored and patterned in open nesters (see Ecology of egg colors below)
    
  • color is added to the eggshell from pigments secreted by cells in the wall of the uterus
    
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Thousands of tiny pores like the one pictured above, cover the shell, providing a passage for gas exchange.
(Source: http://www.rit.edu/~tld0898/SEM.html)