Chicken Reading

Chickens 'One-Up' Humans in Ability to See Color

ScienceDaily (Feb. 17, 2010) — Researchers at Washington University School of Medicine in St. Louis have peered deep into the eye of the chicken and found a masterpiece of biological design.
Scientists mapped five types of light receptors in the chicken's eye. They discovered the receptors were laid out in interwoven mosaics that maximized the chicken's ability to see many colors in any given part of the retina, the light-sensing structure at the back of the eye.
"Based on this analysis, birds have clearly one-upped us in several ways in terms of color vision," says Joseph C. Corbo, M.D., Ph.D., senior author and assistant professor of pathology and immunology and of genetics. "Color receptor organization in the chicken retina greatly exceeds that seen in most other retinas and certainly that in most mammalian retinas."
Corbo plans follow-up studies of how this organization is established. He says such insights could eventually help scientists seeking to use stem cells and other new techniques to treat the nearly 200 genetic disorders that can cause various forms of blindness.
Birds likely owe their superior color vision to not having spent a period of evolutionary history in the dark, according to Corbo. Birds, reptiles and mammals are all descended from a common ancestor, but during the age of the dinosaurs, most mammals became nocturnal for millions of years.
Vision comes from light-sensitive photoreceptor cells in the retina. Night-vision relies on receptors called rods, which flourished in the mammalian eye during the time of the dinosaurs. Daytime vision relies on different receptors, known as cones, that are less advantageous when an organism is most active at night.
Birds, now widely believed to be descendants of dinosaurs, never spent a similar period living mostly in darkness. As a result, birds have more types of cones than mammals.
"The human retina has cones sensitive to red, blue and green wavelengths," Corbo explains. "Avian retinas also have a cone that can detect violet wavelengths, including some ultraviolet, and a specialized receptor called a double cone that we believe helps them detect motion."
In addition, most avian cones have a specialized structure that Corbo compares to "cellular sunglasses": a lens-like drop of oil within the cone that is pigmented to filter out all but a particular range of light. Researchers used these drops to map the location of the different types of cones on the chicken retina. They found that the different types of cones were evenly distributed throughout the retina, but two cones of the same type were never located next to each other.
"This is the ideal way to uniformly sample the color space of your field of vision," Corbo says. "It appears to be a global pattern created from a simple localized rule: you can be next to other cones, but not next to the same kind of cone."
Corbo speculates that extra sensitivity to color may help birds in finding mates, which often involves colorful plumage, or when feeding on berries or other colorful fruit.
"Many of the inherited conditions that cause blindness in humans affect cones and rods, and it will be interesting to see if what we learn of the organization of the chicken's retina will help us better understand and repair such problems in the human eye," Corbo says.

Chicken Questions

1) What is the name of the light-sensing structure at the back of the eye?

2) Insights from this research are hoped to help in what area of medicine?

3) Which wavelengths of light can chickens see that humans cannot?

4) Why do researchers hypothesize that birds have superior color vision to mammals?

The Nocturnal Eye

The Nocturnal Eye

What appears as pitch black to a human may be dim light to a nocturnal animal. The reason lies in the structure of the eye itself.

Pupils
Nocturnal animals tend to have proportionally bigger eyes than humans do. They also tend to have pupils that open more widely in low light. So, at the outset, nocturnal eyes gather more light than human eyes do.

Rods and cones
After the light passes through the pupil, it is focused by the lens onto the retina, which is connected to the brain by the optic nerve. The retina is an extremely complex structure. It's made up of at least 10 distinguishable layers, and is packed with more sensory nerve cells than anywhere else in the body. The retina is home to two different kinds of light receptor cells—rods and cones. (Both are named after their relative shapes.) Cones work in bright light and register detail, while rods work in low light, detecting motion and basic visual information. It is the rods that become highly specialized in nocturnal animals. In fact, many bats, nocturnal snakes and lizards have no cones at all, while other nocturnal animals have just a few.

Tapetum
Many nocturnal eyes are equipped with a feature designed to amplify the amount of light that reaches the retina. Called a tapetum, this mirror-like membrane reflects light that has already passed through the retina back through the retina a second time, giving the light another chance to strike the light-sensitive rods. Whatever light is not absorbed on this return trip passes out of the eye the same way it came in—through the pupil. The presence of the tapetum can be observed at night when a pair of glowing eyes reflects back a flashlight or some other light source. (Interestingly, different animals have different color tapeta, a fact that can aid in nighttime animal identification.)

Circular vs. slit pupils
One consequence of having extremely light sensitive eyes, is that they must be adequately protected during the day. Some animals accomplish this with a retractable eye flap. Others rely on their pupils.The circular pupil, because of the way the muscle bunches as it contracts, is the least efficient at closing rapidly and completely. A slit pupil, with two sides that can close like a sliding door, is far better at this task, which is why so many nocturnal eyes have slit pupils. These apertures can be vertical, horizontal, or diagonal.

Questions:
1) Why do cat eyes glow at night?
2) Why do cats have vertical pupils instead of round pupils?
3) What do rods do? What do cones do?
4) Why do nocturnal animals have more rods than cones?