The first pictures of that newborn baby that surfaced in our family’s chat group showed a charming face with a surprised look and large, dark gray eyes.
Her shape was similar to her father’s brown eyes, but the color was closer to her mother’s green eyes.
But on the second birthday, the images revealed a happy child with eyes the same dark brown as his father. The dark gray of the first photographs was completely lost.
We can consider that the color of our eyes is one of the physical characteristics that define us, as particular to us as the shape of our nose or the size of our ears.
This trait can often also make a lasting impression on us – eye color can even influence the level of trust we place in someone.
But surprisingly, our eye color doesn’t always remain constant throughout our lives. In fact, many outside influences can change it, from injuries to infections and sun damage. And sometimes these changes appear spontaneously.
Evidence indicates that whether or not a baby’s eye color changes depends largely on the color itself.
A study conducted by ophthalmologist Cassie Ludwig of the Byers Eye Institute at Stanford University in California (United States) followed 148 babies born at Lucile Packard Children’s Hospital in California, recording the color of the iris at birth.
About two-thirds of the babies studied were born with brown eyes and a fifth with blue eyes.
Two years later, Ludwig and his colleagues concluded that of the 40 blue-eyed babies in the study, 11 had brown eyes by age two, three had hazel eyes (greenish-brown), and two had green eyes. And of the 77 newborns with brown eyes, nearly all (73) still had brown eyes at age two.
So it appears that blue eyes are more likely to change color than brown eyes during the first few years of life. Because?
One indication is the fact that the eyes of babies who actually changed color tended to get darker, not lighter.
In Ludwig’s study, one-third of the babies’ eyes changed color within the first two years of life, and the change to darker colors was more common. The eyes of only five (3.4%) of the 148 children in the study got clearer with age.
The tendency towards darkening can be caused by the accumulation of a protective pigment in the iris, as we will see later. And this healthy and relatively common color change is more restricted to early childhood.
study with twins
In another US study, which followed more than 1,300 twins from childhood to adulthood, eye color typically stopped changing by age six. But in some cases (10-20% of children studied), it continued to change throughout adolescence and into adulthood.
Among non-identical twins, eye color was more likely to diverge at older age than among identical twins.
This indicates a genetic element in the propensity to change eye color, according to David Mackey, professor of ophthalmology at the University of Western Australia’s Lions Eye Institute.
what parents expect
Curious about the phenomenon of eye color change, Mackey concluded that these two studies form roughly all the research ever done on eye color changes in childhood. And his experience indicates that not a few parents expect their babies’ eyes to change color.
“I would hear parents and their friends say, ‘Yes, the baby was born with blue eyes, but that will change in the next few years,'” he says.
“I just sit there thinking I just can’t find data on any of this. I found these two studies, which are relatively small, but demonstrate that eye color does change.”
Although data is limited and was collected in only one country (the United States), eye color changes appear to be more common among people of Northern European, Oceanian or mixed ancestry.
There are parallels with the changes sometimes observed in the hair color of these populations throughout childhood.
“You have pictures of kids who are blonde as babies but have dark brown hair when they grow up,” says Mackey. “Hair pigment can gradually increase over time, probably because the cells that produce pigment are actually growing in number and migrating to that region.
melanin effect
He suggests something similar could happen with eye color, with greater amounts of pigment accumulating during the months or years after birth.
“The main pigment of the eyes is melanin and the way in which the melanin is distributed gives the different eye colors,” he said.
“Simply put, you have blue eyes; some people talk about green eyes, but actually they are a variant of blue; you have the combinations of hazel and green, then you have brown, which can be light and dark. All this is related to the amount of melanin that is there”, explains Mackey.
Higher levels of melanin may have beneficial functions when sunlight is intense. As in the skin, the pigment offers protection from damage caused by the sun.
In iris with little melanin, the blue color comes from the way the collagen fibers at the bottom of the iris diffuse light, in the same way that the sky appears blue due to the way light is diffused by the atmosphere.
Why some children’s eyes express more melanin over time remains a mystery, Mackey said.
“We don’t really know what influences these color changes,” says Mackey, but there could very well be an environmental factor at play.
“You could almost say that for everything there is an interaction between genetics and the environment, even for what we think is fully genetic or fully environmental,” according to Mackey. “But what environmental factors could cause that influence? We really don’t have these data for the general population.”
The blue-eyed reindeer
Humans aren’t the only animals that experience eye color changes. Perhaps the most dramatic change is the seasonal change in reindeer eyes, which range from turquoise/golden in summer to dark blue in winter.
This change doesn’t happen in the iris, but in a mirror-like structure that sits behind the retina and is called the tapetum lucidum. It reflects light to give the retina a second chance to catch it.
Humans lack tapetum lucidum, which is most commonly found in animals that are active at night and need to see in the dark.
As the reindeer’s iris dilates in the dark of the Arctic summer, it increases the pressure inside the eye. This pressure causes the collagen fibers in the tapetum lucidum to move closer together so that they can reflect blue light more. And capturing more of the Arctic’s deeply saturated blue light helps reindeer see through the darkest winter months.
“Many animals have tapetum lucidum,” says Glen Jeffery, professor of neuroscience at the Institute of Ophthalmology at University College London. “The reindeer is the only animal that is able to make this change to dark blue.”
Color changes caused by injuries
While many of the eye color changes are harmless, they could also be related to something more serious, such as injury, infection, or sun damage.
One of the most well-known injury-caused eye changes was David Bowie’s left eye. The noticeable difference between his dark left eye and light blue right eye was caused by a punch to the head that left his left pupil permanently dilated.
This condition is known as anisocoria. The punch didn’t change the color of Bowie’s iris. The enlarged pupil was responsible for making his left eye darker.
But it is possible for an injury to change the color of the iris, according to Mackey. “This can happen if you have a lot of blood inside the eye, which can cause partial spots. Or you can simply have the pigment spread and deposited all over,” he explains.
Most commonly, the reason is an infection. A famous case of true heterochromia, in which the color of the iris is different in each eye, is that of the Ukrainian actress Mila Kunis, who has a brown right eye and a green left eye. Kunis’ heterochromia results from an infection of the iris, which has destroyed some of the pigment in your left eye.
“Some infectious diseases can cause the pigment to disappear,” says Mackey. One is Fuchs’ heterochromic cyclitis, caused by a viral infection – often rubella, also known as German measles.
“The virus likes to live in the eye and it can break out at some point in life, causing you to lose the pigmentation there,” he said.
Other viruses can also develop inside the eye and sometimes affect pigmentation. In an extremely rare case, an Ebola survivor experienced a color change in one eye from blue to green when the virus persisted in his eye fluid after it had been cleared from other parts of his body.
Sometimes the change in eye color does not affect the entire iris, causing only a few spots.
Light spots, known as Brushfield spots, can appear on the iris of people with Down syndrome, while brown spots, called Lisch nodules, are a common sign of the genetic condition neurofibromatosis type 1.
And just like on the skin, moles and freckles can appear on the iris and other parts of the eyes.
“You can have a mole that does nothing,” says Mackey. “But for some people, they can grow into tumors, which can be a serious problem.”
In fact, most eye color changes are a benign source of fascination for those who witness them, but Mackey advises that it’s always worth carefully monitoring any changes that might be unwanted.
Read the original version of this report (in English) on the BBC Future website.
This text was originally published here
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