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TETRACHROMACY: why, in comparison to hummingbirds, all humans are colorblind

TETRACHROMACY: why, in comparison to hummingbirds, all humans are colorblind
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Photo by Birger Strahl on Unsplash

 

To explain the uniqueness of hummingbirds’ vision compared to humans, Mary Caswell Stoddard of Princeton University (USA) uses an understandable analogy — the difference between color blindness and normal vision. And we, in this analogy, obviously do not distinguish as many colors and shades as hummingbirds.

 

The human eye contains three types of cones. They are differentiated by their sensitivity to light with different wavelengths: violet-blue, green-yellow, yellow-red. The presence of cones and rods, which are sensitive in the emerald green part of the spectrum, gives humans color vision. The study found that hummingbirds have an additional type of cones — to distinguish ultraviolet color.

As Mary Stoddard notes, theoretically, the hummingbird eye is capable of even more — to distinguish combined colors, for example: ultraviolet + green and ultraviolet + red. However, in practice, this is very difficult to test because human eyes are unable to recognize such combinations of colors.

To study birds’ perception of the color palette of the world, Stoddard’s research group created conditions that simulate their natural habitat. The experiments were conducted at Rocky Mountain Biological Laboratory in Colorado.

Scientists worked with three-colored selasphorus (lat. Selasphorus platycercus) — birds of the hummingbird family. This species of hummingbird is notable for the fact that with an intense flapping of its wings, which creates a quite audible humming sound. Interestingly, with a wingspan of 12 to 14 cm, the birds are from 83 to 97 mm long, and they weigh from 3 to 4 g.

The three-colored selasphorus prefers flowering lowlands and highland landscapes overgrown with juniper, pine, oak, cypress, and fir. Some individuals nest at altitudes of more than 3,000 meters above sea level. These birds are not afraid of people and settle near them. The three-colored selasphorus feeds on small insects and flower nectar, preferring red-colored plants.

Mary Stoddard’s team conducted experiments that proved the hummingbird’s ability to see color combinations unimaginable to the human eye, as well as distinguish unique hues artificially created by scientists.

«The most accurate perception experiment could have been done in the lab, but we feared losing sight of the bigger picture — how birds use their ability to distinguish colors in their everyday existence. Hummingbirds are perfect for our experiment because they need to distinguish the brightest and most colorful colors of the flowers that attract them to nectar», Stoddard explains.

 

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The team she leads has been conducting research for three years with scientists from Princeton, Harvard, the Universities of Maryland, and British Columbia. They created several custom LED tubes programmed to display a wide range of colors, including non-spectral colors such as ultraviolet + green. The experiment was carried out in the alpine tundra, where three-colored selasphorus are frequent visitors, breeding at high altitudes.

The researchers usually got up before dawn and set up two feeders, one with sweet water and the other with regular water. Near each feeder, they placed an LED tube. The tube next to the sweet water emitted one color, while the one next to the regular water — another.

The researchers periodically swapped the tubes so that the birds could not tell from the familiar color of the tube where the nectar and plain water were. Control experiments were also conducted to prove that the tiny birds did not use odor or other unintentional cues in their search for sweetness.

In just a few hours, the hummingbirds learned to distinguish the color they needed, the «useful» color for them. Using this pattern, the researchers recorded more than 6,000 visits to feeders in a series of 19 experiments.

The scientists’ eyes did not distinguish ultraviolet + green from pure green, for example, but the hummingbirds persisted in choosing ultraviolet + green because it was associated with sweet water.

Benedict Hogan, one of the team members, was eventually forced to admit that we are left to guess how hummingbirds actually perceive color. Is ultraviolet + red a color combination for them, or an entirely new color?

And most curious of all. Studies have shown that tetrachromacy, i.e., the presence of four cones to distinguish colors, is characteristic of birds, fish, reptiles, and, as far as we can claim today — dinosaurs.

 

Original study: Wild hummingbirds discriminate non-spectral colors (Proceedings of the National Academy of Sciences)

 


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