Tetrachromacy

Some animals see many colors.

Most people see three kinds of color parts in their eyes. But some animals have four parts. These animals are called tetrachromats.

Many birds and fish are tetrachromats.

Birds can see a special light. This light is called ultraviolet. This extra color helps birds find food. It also helps them find mates.

Some women might see extra colors too. This happens because of how their eyes work. It is a very special way to see the world.

Most people see the world using three types of color parts in their eyes. These parts are called cone cells. But some animals see even more colors. They are called tetrachromats.

Tetrachromats have four types of cone cells. This lets them see a wider range of colors. Many fish and reptiles see this way. Some birds are also tetrachromats.

Birds can see a special kind of light. This is called ultraviolet light. This extra color helps them find food. It also helps them find mates.

Some humans might be tetrachromats too. This may happen to some women. They might have a fourth cone cell type. One study says up to 50% of women could have four color pigments. This could help them see more shades of color. Scientists are still studying how the brain uses this extra information. We do not yet know if the brain can fully use a fourth color channel.

Tetrachromacy is a special way of seeing color. Most people are trichromats, which means they have three types of cone cells in their eyes. These cone cells are light receptors that help us see different colors. A tetrachromat has four different types of these cells instead. This extra cell allows them to see a much wider range of colors.

This ability is very important for many animals in the wild. It helps them find things that others might miss.

How does this extra vision work? In a tetrachromat, the retina has four types of cone cells. Each type is sensitive to a different part of the light spectrum. This means they can see colors that look identical to us. They can also see wavelengths of light that humans cannot see. For example, many birds can see ultraviolet light.

This extra light helps them find food or pick a mate. The four types of cells work together to create a four-dimensional color space.

Scientists have studied how color vision changed over a long time. The common ancestor of all vertebrates was likely a tetrachromat. Later, a common ancestor of mammals lost two of those cone types. This happened during what scientists call a nocturnal bottleneck. Some primates later evolved a third cone type. Today, we see tetrachromacy in many fish, reptiles, and birds.

Even some fish, like goldfish, use this special vision.

There are interesting facts about humans and tetrachromacy. Most humans only have three types of cone cells. However, some researchers believe some women might be tetrachromats. One study suggested that 15% of women might have a fourth cone. Another study suggested that as many as 50% of women could have four pigments. In 2010, neuroscientist Gabriele Jordan identified a woman who could detect many more colors than others.

This woman was a functional or "true" tetrachromat.

This way of seeing connects to how our own eyes work. Most of us use three channels to see colors like red, green, and blue. Tetrachromats add a fourth channel to their vision. This is similar to how a bird uses ultraviolet light to see patterns. While humans cannot see ultraviolet light because our eye lenses block it, some animals thrive with it. Some animals even have more than four types of receptors. For instance, mantis shrimp have 33 different types of pigments!

Tetrachromacy is a specialized condition of color vision. It occurs when an organism possesses four independent channels for conveying color information. This means the eye contains four distinct types of cone cells. These cone cells are high-intensity light receptors located in the retina. Most humans are trichromats, meaning we only have three types of cone cells. For a tetrachromat, the sensory color space is four-dimensional. This allows them to distinguish between many more colors than a trichromat.

The mechanism of tetrachromacy relies on the spectral sensitivity of these receptors. Each of the four cone cell classes responds to a different portion of the light spectrum. When light hits the retina, these cells trigger signals based on the specific wavelengths they detect. To achieve true tetrachromatic vision, the body also needs a post-receptoral mechanism. This system must be able to compare the signals from all four receptor classes. This process is often described by the opponent process theory. In humans, the three existing channels work through an opponent process to create color. It is still being studied if a fourth channel can function the same way.

Different species exhibit various levels of color vision complexity. Many birds, fish, and reptiles are known tetrachromats. For example, goldfish are typical tetrachromats.

Some birds, like the zebra finch, use ultraviolet wavelengths to help with foraging and mate selection. These birds can see light in the 300–400 nm range. Their vision is further enhanced by pigmented oil droplets in their photoreceptors. These droplets filter light before it reaches the visual pigment. Some animals even reach higher levels, such as pentachromacy. Pentachromacy requires at least five different classes of photoreceptors. Pigeons are believed to be pentachromats, and some lampreys may also possess this ability. Invertebrates can have even more, like the mantis shrimp with 33 different pigments.

The history of color vision shows a changing evolutionary path. The common ancestor of all vertebrates was likely a tetrachromat. However, a common ancestor of mammals lost two of its four cone cell types. Scientists believe this loss occurred during a period called the nocturnal bottleneck. Later, some primates evolved a third cone to regain more color detail. This evolutionary history explains why most mammals today are trichromats or even dichromats.

In humans, tetrachromacy is a subject of intense scientific study. Most researchers believe it may exist in a small percentage of the population. This is often linked to the X chromosome, which carries the genes for cone pigments. Females who carry recessive alleles for color vision deficiency (CVD) might actually be tetrachromats. This happens through a process called X-inactivation. One study suggested that 15% of women might have a fourth cone type. Another study estimated that up to 50% of women could have four photopigments. In 2010, neuroscientist Gabriele Jordan identified a woman who was a "true" tetrachromat. This woman could detect a much wider variety of colors than standard trichromats.

There are interesting physical limits to how we see light. While some animals see ultraviolet light, humans generally cannot. This is because the lens of the human eye blocks most light between 300 and 400 nm. The cornea also blocks shorter wavelengths. However, people with aphakia, or a lack of a lens, can see near-ultraviolet light. They often describe it as whitish blue or whitish violet. This occurs because the three types of cones are somewhat sensitive to those wavelengths.

Tetrachromacy connects to many broader biological and optical concepts. It demonstrates how the complexity of a sensory system can change through evolution. It also shows how genetic inheritance, specifically through X-linked alleles, affects how we perceive the world. The study of these pigments helps us understand the relationship between light physics and biological processing. Understanding how the brain and optic nerve handle new color channels remains a key area of research. We do not yet know if the brain can fully utilize a fourth signal if it is presented with one.