St. Louis, MO--Researchers at Washington University School of Medicine in St. Louis have mapped the retina of the chicken's eye and found it superior in several ways to that of the human eye.1
While the human retina has cones sensitive to red, blue, and green wavelengths, avian retinas also have a cone that detects violet and near-UV. Their retinas also have a specialized receptor, called a double cone, that the researchers believe helps birds sense motion.
Most avian cones have a lenslike drop of oil within the cone that is pigmented to filter out all but a particular spectral range of light. The researchers used these drops to map the location of the different types of cones on the chicken retina. They found that while the different types of cones were evenly distributed throughout the retina, two cones of the same type were never located next to each other.
Joseph Corbo, an assistant professor of pathology and immunology and of genetics, speculates that extra sensitivity to color may help birds in finding colorful mates, as well as berries and other fruit.
Mammals' nocturnal heritage
Birds likely owe their superior color vision to not having spent a period of evolutionary history in the dark, says 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.
Night vision relies on rods, which flourished in the mammalian eye during the time of the dinosaurs. Daytime vision relies on cones, which 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.
Corbo plans followup 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.
1. Y. A. Kram et al., PLoS One, Feb. 1, 2010.