One of the most beautiful aspects of biology is the biodiversity that exists on our planet, the enormous number of forms and adaptations, which has allowed every corner to be colonized. In this biological model, birds are no exception.
Despite the fact that we think of a chicken egg as the ‘gold standard’ of birds, the reality is very different, not all of them have the same shape or the same color. We have known for a long time that the size of the maternal oviduct – the hole through which the eggs exit – is a determinant of the final shape of the egg, as is the thickness of the egg membrane, which is located immediately under the shell. .
Let’s leave the rest of the birds and fix our attention on the chicken. Its reproductive system is made up of ovaries and oviduct, and it is after the twentieth week when it reaches sexual maturity and the laying of eggs begins.
The ovaries are located in the abdominal cavity near the kidney, and have a clustered appearance. About ten days before ovulation, a phase of rapid growth of the yolk occurs within the ovarian follicle (vitellogenesis) and successively concentric layers are incorporated, whose color depends on the pigments contained in the food consumed.
When the follicle reaches maturity, the yolk is released, which will be captured by the oviduct. It is a 60-70 cm long tube that connects the ovary with the cloaca, through which the egg will emerge 24-26 h after entering the oviduct.
The hen with the green eggs
The eggshell is formed in the uterus, in the so-called shell gland. It arrives there approximately five hours after ovulation has taken place and remains for another twenty hours before being expelled.
It is estimated that it is around ten hours after ovulation when the shell formation process begins, made up mostly of calcium carbonate crystals. What activates the formation of the shell is the presence of sodium, an ion that is secreted by the uterine glands.
Now, we have all observed that there are eggs of different colors, this is due to the presence of certain pigments (ovoporphyrin), derived from the metabolism of hemoglobin, and which vary according to the breed of laying hen. In general, those chickens that have white earlobes lay eggs of that color, while those that have them reddish or brownish lay brown eggs.
In addition to genetics, the color of the egg is determined by the age of the hen and the stress to which it is subjected, but not by its diet. There is a typical chicken from Peru, Chile and Ecuador –the Araucanian chicken- that lays eggs of much more colorful colors, ranging from blue to green. Apparently the origin of this hue is due to the fact that more than five centuries ago a virus infected a species of chickens native to South America, which caused a genetic mutation that caused a certain pigment -biliverdin- to accumulate, which is responsible for last of color
In other words, it is false that nutritional quality or flavor varies by color, nor that brown eggs are more natural than white eggs, nor has it been proven that color influences shell thickness.