Brown, white, blue… Why are the eggs different colors?

Some scientists have suggested for decades that it was a camouflage strategy, others that the colors mitigated the effects of ultraviolet light on the embryo's DNA.

Colors for all tastes. And for the colors, eggs. From brown to white, the colors of their shells can be so varied that the subject has long left scientists perplexed.

We can eat them all, yes (even if it is a very nutritious food and rich in proteins, there is no need to gorge ourselves), but the question remains: why is this one brown, why is that one white, why are some more or less brown, why are some even bluish? Each color has been the subject of dozens of popular theories, but beyond these theories, there is a scientific explanation.

Some scientists have suggested for decades that this is due to an innate camouflage strategy against potential predators. Others have always pointed out that the colors could mitigate the damaging effects of ultraviolet light on the embryo's DNA, or even help the birds recognize their own eggs. The answer is broader and, at the same time, simpler.

A Matter of Days

First of all, we must not forget that, like pregnant people, a chicken has thousands of eggs. These are the ones that will later become the egg yolks of the food industry.

About 10 days before the hen enters the ovulation period, the rapid growth phase of the egg takes place inside the ovarian follicle. It is at this time that “concentric layers of yolk are incorporated, the color of which varies according to the type and concentration of pigments contained in the food eaten by the hen during this process”. This is how it grows.

According to Jacinta Bowler for Cosmos MagazineThe yolk then enters a zone called the magno (although before this it passes through another zone called the infundibulum, where the yolk is covered with membranes that protect it from the water coming from the white), where the albumin is formed, known in our daily life as the white, or “the white part of the egg”.

A matter of hours

Next, the skin of the egg's outer membrane is quickly created in an area called the isthmus. Now it's the shell's turn: when the egg reaches the uterus or shell gland, about 5 hours after the onset of ovulation, it stays in the uterus for up to 18 hours. It will remain there for 18 to 22 hours.

It is during this time that the response you are expecting occurs. “In the uterus there are two areas with different secretory cells. The part closest to the isthmus is tubular in shape, 2 cm long, where the egg remains for 5 hours and where, in addition to the hydration of the egg white, the fibers of the outer membrane of the testis are organized in the nuclei of the mammillary layer. This influences the subsequent fixation of the calcium carbonate crystals and, consequently, the strength of the future shell”, explains the research institute.

Layer by layer, the shell will form by calcification over the next 20 hours. Indeed, calcium plays a fundamental role in the color of the shell and, more generally, in its formation. And where does the animal find it? Well, in the food it eats regularly. In general, up to 2 grams of this element are found in each shell.

The importance of calcium

The other large percentage of the organic matrix of the uterine matrix is the uterine fluid itself, which contains the proteins necessary to make it possible. “The organic part represents 2% of the total shell and consists of a mixture of proteins and glycoproteins (70%) with 11% of polysaccharides. This matrix integrates into the growth of the calcite columns, giving elasticity and consistency to the shell.”

It is during the last two hours of egg formation that porphyrins are deposited, resulting from the metabolism of hemoglobin and responsible for the coloring of the shell.

As you can see, the exact color of the eggshell has not yet been determined. For this, it is necessary to wait until the last two hours of egg formation, when the porphyrins, derived from the metabolism of hemoglobin and responsible for the coloring of the shell, are deposited. In any case, the color will depend on the breed of the hen, i.e. it is more related to genetics than to the food.

A journalist with a degree in genetics, Bowler points out that several physiological mechanisms allow the concentration of Ca++ ions in the hen's blood to remain relatively constant and high. “With the instinctive purpose of obtaining an even deposition of the eggshell, during the laying period the hen has a greater appetite for calcium, that is, she consumes more of it, to deposit it in the shell of the forming egg.”

The answer lies in the breed

In addition, a study published last year in the journal Nature Ecology & Evolution showed that birds living in cold climates with open nests tend to have eggs with darker shells.

The study authors speculate that this darker pigmentation allows the egg to maintain its internal temperature longer, since they have less time to expose themselves to sunlight.

In any case, while Australorp, Barnevel, Buckeye, Delaware, New Hampshire Red and Sussex hens lay “brown” eggs, Andalusian, Dorking, Fave-Rolle, Lakenvelder and Leghorn hens lay white eggs. Marans lay reddish-brown eggs. As for the curious blue eggs, only three breeds lay them: the Araucana or Mapuche, the Dongxiang and the Lushi.