Did You Know?
The evolution of the dog-like family over the last 40 million years has been very successful (Wang and Tedford 2008). The wolf (Canis lupus), the largest living member of this family, has evolved unique physical attributes that aid it in hunting food — its vision system is one of these. Wolves are carnivores that chase their prey, often at dusk, dawn, and during the night. They need the ability to see prey clearly at long distances, over a wide field of view, and in poor light. A good way to understand how wolves see is to compare their vision with ours.
We are primates, mammals that include lemurs, monkeys, apes, and us. Our eyes are pointed straight ahead. That is, both eyes see the same field, except for the area blocked by our nose! So, we have binocular vision over most of our 180-degree visual range (Harrington and Asa 2003, Plonsky 1998).
Wolves’ eyes, on the other hand, point 25 degrees apart laterally, so they have a more limited range of binocular vision. However, they have a much wider field of view than we do (about 250 degrees) (Harrington and Asa 2003, Plonsky 1998). See illustration.
The inside backs of both our eyes and wolves’ eyes are covered with retinas, light sensitive membranes that send vision signals to our brains. A lens at the front of the eye focuses images on the retina.
Wolves and people have very different color vision. We, and many other primates, have three-color vision (Jacobs and Nathans 2009). That is, we have three color-sensitive receptors in our eyes called cone cells — roughly blue, green, and yellow.
Wolves have only two such color receptors. The range of hues wolves can discriminate is much reduced compared to us. See illustration.
All mammals have another kind of vision cell that is sensitive to very low light levels, but not to colors. These cells are called rods. We have a depression at the center focal point of our retinas called the fovea, which has a very high concentration of cones, but no rods for night vision. We are adapted for seeing great detail in color, during daylight, in a very restricted field of view. Outside of the fovea we have a mix of cones and rods. Wolves lack a fovea, but have a broad central area with a very high density of rods and a higher density of cones compared to the periphery (but still only about 1/6 of ours). Wolves can distinguish many more shades of gray and see much better in the dark than humans. A wolf has relatively sharp vision across much of its visual horizon without having to shift its gaze (Harrington and Asa 2003).
Detailed studies of the wolf’s ability to sense motion have shown that they can detect movement with both rods and cones considerably better than we can, especially in good light conditions. One could say wolves see the world faster than we do (Harrington and Asa 2003).
Finally, wolves have a multi-layer membrane between the retina and the lens called a tapetum lucidum. This membrane reflects light back through the retina, thereby increasing sensitivity in low light conditions. It is also responsible for the “eye shine” of wolves and dogs at night. Many other mammals have this membrane, but not us.
Again, wolves need the ability to see prey clearly at long distances, over a wide field of view, and in poor light. They need to discriminate many shades of gray and detect any movement across the horizon. The eye of the wolf has evolved over millions of years to supply these needs very well.
Bob MacPherson, PhD, retired scientist.
Citations:
Wang, X. and R. H. Tedford, Dogs: Their Fossil Relatives and Evolutionary History, 2008, Columbia University Press, New York.
Harrington, F. H. and C. S. Asa, Wolf Communication in Wolves: Behavior, Ecology, and Conservation, 2003 pp 96-99, L. D. Mech and L. Boitani, eds, University of Chicago Press, Chicago & London.
Jacobs, G. H. and J. Nathans, The Evolution of Primate Color Vision. Scientific American, April 2009: 56-63.
Plonsky, M., Ph.D. Canine Vision, 1998, http://www.uwsp.edu/psych/dog/LA/DrP4.htm
