|What animal has the sharpest eyesight?|
Article by Molly Kirk and David Denning
Photographs by David Denning
Look around the room you are in. You probably think you see the world pretty clearly. It is hard for us to image an animal seeing better than we do. But, many do. Raptors, or birds of prey, including the eagles, hawks and falcons can see up to 8 times more clearly than the sharpest human eye. A golden eagle for example can see a hare from a mile away. The reason for their remarkable vision is related to their flying and feeding habits.
Flight depends on vision. A blind-folded bird cannot take off or fly properly because it is unable to orient itself. As predator bird ancestors adopted flight, they needed ways to find generally small prey (such as fish and small rodents) from the air; they needed abilities to see at close range; and they needed to maintain focus at high speeds when in pursuit. The most successful birds of prey in any population were those with eyes better adapted to these demands. Over thousands of generations, natural selection led to populations of predator birds with greater visual capabilities.
How do predatory birds see more sharply than us? First of all they have especially large eyes. Large eyes let in the maximum amount of light, and they also allow for a large image. If a retinal image is spread over a greater number of visual cells, there will be greater resolution to the detail in the image. A bird's eye is so huge it occupies a significant portion of the skull (like in this owl skull), allowing only limited room for the brain.
A flying bird hunting small prey far below must not only be able to distinguish how far away the prey is but also its size, shape, position, and motion. In response to these challenges, raptorial birds evolved precise accommodation and binocularity.
To determine the size and distance of a prey, animals rely on memory as well as visual information. They have to know (remember) how large the prey animal is, and then judge its distance based on the size of the image they see. It's also important to be able to develop a clear image of the prey, no matter what its distance.
The eye automatically focuses at a variety of distances using a natural neuromuscular adjustment called accommodation. In this process, tiny ciliary muscles surrounding the eye alter the curve of the lens so that it will focus on objects that are far or near. Raptor eyes have exceptional capabilities for accommodation. Thus, as a potential prey moves closer or farther from the eagle or hawk, the predator's eyes remain focused by rapidly changing the lens curvature.
Raptors have front facing eyes. This makes possible a binocular vision similar to our own. In binocular vision, the fields of view of the left and right eye overlap. Animals with eyes on the sides of their head (such as fish or rodents) have low binocularity (what each eye sees overlaps very little) but high periscopy (each eye has a wide field of view). The right-eye and left-eye visual fields of a hawk overlap about 90 degrees, (in human vision, this overlap is about 120 degrees). A further adaptation in raptors - the cornea and lens are angled toward the beak to increase the overlapping region even more.
Binocularity allows for stereoscopic vision, which in turn allows for determination of distance. When an organism compares the slightly different images from the right and left eye, its brain automatically determines the distance to the object. Raptors, with their greater amount of visual field overlap, have the greatest abilities to use binocularity to develop a sharp, three-dimensional image of a large portion of their view.
The Second Fovea:
In birds that need accurate distance vision the most, (i.e.: birds of prey and some other species of bird), a second fovea evolved in the lateral part of the retina. A fovea is a small region of the retina where the concentration of rods and cones is the highest and therefore vision is the sharpest. Raptors, with their wide binocular field of view, have BOTH a central and lateral fovea. As a result, a substantial proportion of their visual field projects on the most visually receptive parts of the retina.
Another unique structure in a bird's eye is the pecten. Pecten is a thin, greatly folded tissue extending from the retina to the lens. Predatory birds such as eagles and hawks have the largest and most elaborate pecten of all the birds. The pecten supplies nutrients and oxygen throughout the vitreous humour of the eye, thereby reducing the number of blood vessels in the retina. With fewer blood vessels to scatter light coming into the eye, raptor vision has evolved to be the sharpest vision known among all organisms.