Vision in Birds, Butterflies, Cats, and Dogs
Do animals see the world and each other as we humans see them? Do they even see in the same visible spectrum? Is animal eye structure similar to human eye structure? What colors are attractive to different species? How does coloration influence an animal choosing a mate? How do common pets, such as dogs and cats view the world? Are blue-eyed cats always deaf? Do dogs eyes contain more rods or cones? Which species has the broadest spectrum of color vision in the animal kingdom?
I have been curious about animal vision for a long time and wondered how eyes work in other species. As a young child I would look into my Chesapeake Bay Retriever's big brown eyes and wonder what she saw when she looked at me and the rest of the world. I even wondered if my green-eyed cat saw the world in a greenish tinge. Now I know the answers. My project investigates the above questions in common but distinctive species such as birds, butterflies, cats, and dogs.
The dominant sense in the avian species is vision. No other vertebrates have such excellent eyesight. The acuity of a bird's vision is so great that it can see two to three times more sharply than humans. Birds' eyes are so large for their head size that they have no room to rotate their head as other mammals do. This explains why birds are constantly turning their heads, getting their field of vision alligned. In fact, the eyeball is so large that little room is left in the head for the brain or for eye muscles.
Birds have four types of color receptors, cones, and the 4th cone which humans lack allows birds to see into the ultraviolet range. The ability of birds to see into the UV range allows avians to see each other's plumage in a much different way than we humans see their feathers. Certain areas of plumage, such as throat pouches and side feathers, may appear brightly patterned under UV light. In 2001, English scientists did a study on UV light and its effect on several species of birds. It was found that female zebra finches (Australian striped songbirds) were responsive to male finches feathers because of reflected ultraviolet light. The brighter and more reflective the male's plumage, the likelier that a female will pick him for a partner. The blue grosbeak is one of the best examples of a bird that utilizes the ultraviolet spectrum. While female grosbeaks are brown, male grosbeaks are a brilliant blue. The brilliant blue comes from the males' feather structure; after measuring peak feather wavelength (when the most light is reflected) and intensity under a spectrophometer, both peak wavelength and intensity were found to fall in the ultraviolet range. The intensity of UV reflectance varies from male to male, so female grosbeaks can tell a difference while we humans cannot.
Diurnal birds, such as cardinals, robins, goldfinches, grosbeaks, and wrens have retinas which are densely packed with cones; they may have as many as two to five times more cones per square millimeter than are found in the human eye. The cones are distributed not randomly, but in uniform fashion across the retina. The back of a bird's eye is flatter than that of a mammal, thus this aspect combined with even cone distribution lets birds have a wider in-focus area. Pigmented oil droplets enhance cone color vision in diurnal birds. Scientists do not fully understand the function of these oil droplets but they appear to sharpen color perception and to reduce glare. The vision system in birds is likely much better than the human vision system. Diurnal birds do have a small amount of rod cells along the periphery of the retina. A bird's retina contains the pecten, a pleated fin of vascular tissue that goes through the vitreous body into the lens. The pecten provides a blood supply to the retina through its' tiny blood vessels and is believed to improve visual acuity. Diurnal birds' eyes contain a temporal foveae (laterally positioned) and a central fovea, allowing birds to see overhead, in front, and behind the head.
Nocturnal birds such as the different kinds of owls have retinas with only a few cones. Since they are active mostly at night, nocturnal birds' eyes contain a large number of rods, useful in dim light for night hunting. All types of owls have similarly constructed eyes and eye sockets. In small species, owls' huge eyes almost touch inside of the head. Their eyes have 100 times the light gathering power of humans and even the smallest owls have eyes as large as a man's. During the day, owls draw a unique third eyelid over their eyes, similar to a transparent window shade which keeps out the sun.
One of my favorite members of the avian family is the hummingbird. Each summer, my family feeds and observes Ruby-throated hummingbirds in our flower garden as they stop for a feeding visit in Virginia, on their route of migration. Eventually, they will across the Gulf of Mexico to winter in tropical warmth. A ruby-throated hummingbird weighs three grams and consumes about half of its' weight in sugar each day to survive. Hummingbirds are the tiniest of all bird species but are fiercely agressive and will fight for the food source that it wants. Able to fly forward, backward, and even dive upside down, hummingbirds are "aerial specialists." Hummingbirds, like diurnal birds who rely on seeing long distances, have a temporal fovea which is believed to improve binocular vision. They locate their food by eyesight, watching for colorful reddish flowers from which they can feed on nectar.
It is a well known fact that hummingbirds are very attracted to the color red. They seek out red, orange, and purple flowers that are good providers of nectar. Nectar is vital for the process of pollinization, as birds and insects fertilize flowers with pollen that was picked up when sucking nectar from the first flower. Hummingbirds rank among the most important bird pollinators throughout North America. It is no surprise that many of the plants hummingbirds depend on for food are interdependent on hummingbirds for pollination. Fuchsias, for example, are a type of plant that has adapted specifically for hummingbird pollination instead of insect pollination. Specific features, such as long, tubular, and scentless flowers and being red, (or close to red) are attributes which attract mainly hummingbirds instead of "pesky" insects. However, hummingbirds do eat a small amount of tiny insects and spiders in addition to nectar. Since most of the good nectar sources are red, hummingbirds can tell while in mid-flight which ones suit their feeding methods. Knowing which flowers will provide a good food source, as can be told by the red color, saves the humminbird precious energy that otherwise would be spent searching for a food source. Along mountainous hummingbird migration routes in the spring and summer, red wildflowers are abundant. Against the green background of their leaves, red flowers are less likely to attract bees, possible "competition" to the hummingbird, even though they are unable to see colors at the red end of the spectrum.
Hummingbirds have vividly irridescent feathers, with the gorget (throat patch) being the most brilliant, especially in males. Feathers in the gorget contain layers of platelets filled with tiny air bubbles. The reflection and then refraction of light off these bubbles causes the bright reds and purples on the gorget to be seen, changing as the bird is viewed from different angles. Hummingbirds also have small feathers packed closely together on the rest of their body. Their irridescence is caused by light refracting off tiny platelet structures in the feathers filled with air and melanin, similar to that in the gorget. The platelets average about 2.5 microns in length, but thickness and amount of air and melanin inside varies. The vaired amounts of air in the platelets account for a refractive index from the red to the blue end of the spectrum, as seen on the birds' feathers. Hummingbird plumage colors are structural instead of pigmented, whereas most birds' feathers are pigmented. The feathers shimmer with color like oil on water.
While the irridescent feathers are not only beautiful, they are useful in attracting a mate. Male hummingbirds catch the attention of females through elaborate flying displays of plumage and short vocalizations. Daring aerial displays of swing dives, moving from side to side in a swinging fashion, are performed by males. Often during these displays, males will exaggerate their bright plumage by puffing out their iridescent crown and throat feathers to further impress females through magnificent visual effects. After mating, the male returns to his territory and the female takes on the responsibilities of nesting and raising the young.
Similar to birds, butterflies are able to see in the ultraviolet spectrum; unlike birds, butterflies have the broadest spectrum of color vision known to exist in the animal kingdom. The eye of a butterfly is among the most complex in the animal kingdom. A compound eye is located on each side of the butterflies' head and is made up of many little eyes pressed together into one. The tiny individual eyes are called facets, and are made up of six sides. Thousands of facets make up the two compound eyes. Unlike human vision, where we see one image, butterflies see thousands of small images at a time. Underneath the facets is a crystal cone that extends inwardly and forms a transparent rod. When light enters this rod, it has already been reversed twice, making its' rays parallel so that light enters the rod in a straight line. Compound eyes aid in seeing into the UV, detecting movement, and seeing varied colors. The side location of their eyes enables them to see in different directions at one time, useful in detecting predators. However, butterflies cannot see detail from a distance and can only recognize the fine patterns of other butterflies from a few feet away.
The varied colors of flowers are beneficial in attracting butterflies to pollinate them. In order of attractiveness, butterflies seek out purple, pink, yellow, white, blue, and red nectar flowers. We humans need to remember that the way a butterfly sees a color is different from how we see it, since we are unable to see in the UV. For example, a human looking at a Black-eyed Susan sees yellow and brown; a butterfly sees bright blue and silver. Bee balm, Butterfly bushes, Purple coneflowers, Goldenrod, Petunia, Mint, Verbena, and Zinnias are all common examples of butterfly nectar sources that can be planted easily on the yard. We have several of these plants in our yard; the Purple coneflowers seem most attractive to the butterflies.
Each species of the butterfly has unique wing colors and markings. The wing scales are either pigmented by natural chemicals made in the body during metamorphosis or they are structural, acting as prisms which refract sunlight due to microscopic striations on particular scales. It is the structural colors which are irridescent. The colors yellow, white, brown, black, and red are made from pigment; greens, blues, silver, and gold are structural colors. There are some exceptions, as a few butterflies have wings with both pigments and structural colors, like the Purple emperor, thus the entire spectrum of visible light is covered. Coloration is very useful for male butterflies when selecting a mate because they can tell a female of a certain species by coloration. Males such as the Tiger Swallowtail will fly around an area, displaying their colorful wings and looking for a female. Males are more likely to have brighter colors than females.
For as long as I can remember, my family and I have watched for migrating Monarch butterflies in late summer and early fall, flying through our backyard on part of their long southwestern migration journey to Mexico. Monarchs east of the Rocky Mountains migrate 2,000 or more miles to the 10,000 foot high mountains west of Mexico City, beginning in late summer/early fall and wintering there until March. The Monarchs' navigational skills are not exactly understood by scientists; perhaps an internal compass leads them to their wintering grounds. They huddle together on trees throughout the winter, sometimes as dense as 4 million butterflies per acre. In the spring, the Monarchs mate and the female instinctivly heads northeard to deposit her eggs in a milkweed plant. While milkweed (pink) is a favorite food source for these butterflies, other plants such as goldenrod (yellow), zinnias (yellow, pink), lantana (yellow, red) and cosmos (pink, white) are very attractive to Monarchs.
In the wild, a cat may be both prey and predator, but a pet feline still has the same eye characteristics of a wild one. Cats have excellent peripheral vision and their protruding eyes give them a wider angle of vision than human eyes. Cats see clearly in only 1/6 the light we humans need and their pupils can be dialated wide enough so that they take up 90% of the eye area. The pupil may dialate whenever the cat becomes angry, excited, or frightened as the slit pupil becomes an elongated oval. In sunlight, the pupils close almost all the way, to protect the eye. Feline eyes have both rods and cones, with more rods for vision in dim light. A cat's vision is best between six to 18 feet in front of it. A unique feature of cat vision is that the animal is able to see well in both day and night, as their pupil is able to change from an elongated oval slit to a round circle almost the same size as the cornea.
A common myth is that cats are color-blind. Through tests and trials, scientists have determined that cats are able to differentiate between green, blue, and yellow, but not red. Feline cone cells are sensitive to blue and green but not to red. Color in itself is not very meaningful to cats. The world from the feline view is seen in a soft focus; a cat cannot hone in on details because of the large lens, which functions to gather as much light as possible. Motion-detecting rod cells in the retina make cats' eyes very attuned to motion, more than human eyes. Perhaps the most wonderful aspect of the cat's eye is the tapetum lucidum, a layer of reflective cells found behind the retina. Through the bouncing of light off the tapetum lucidum and back to the retina, rods and cones have a better ability to receive information.
In domestic cats, the range of eye color can vary from hazel to pure green and it is not governed by coat color. Blue eyes are the only color linked to coat color. In cats whose coats contain much white, blue eyes are the result of a type of albinism which has led to a lack of pigmentation in both the coat and iris. Commonly deaf, blue-eyed cats lack the pigment necessary for coat color as well as the fluid necessary in the ear receptor for hearing. The blue eyes of Siamese cats are not a sure sign of deafness, but instead are associated with poor 3-dimensional vision. Long ago, Siamese squinted to compensate for this disability. Breeding has removed the squint and it appears that no visual acuity has been lost.
In the dog family, the range of eye shape and size is very wide. For example, Pekinese have round, bulging eyes that can be overexposed to wind, water, and dust; Bloodhounds have problems with their palpebral fissure (the space between the eyelids when they are open fully) being too wide. The eyes are positioned nearer to the sides of the face than human eyes, so dogs have a broader range of vision than we do. Dogs are able to see about three-quarters of the way around them while humans are only able to see the equivalent of a half-circle. Boston Terriers, one type of broad headed dogs, have a better field of vision than "arrow" headed dogs such as the Greyhound.
Dogs' eyes contain both rods and cones, but their ratio of rods to cones is much higher than in humans and they have more retinal photoreceptors. Similar to cats, they may be able to see in conditions which seem completely dark to us. In a study by an experimental psychologist at the University of California, it was concluded that canine color vision is similar to a human's with deuteranopia, which is red-green color blindness. This 1989 study used a computer-controlled device which measured "spectral thresholds, analyzed wavelength discrimination and delivered beef-and-cheese-flavored food pellets," to a poodle and two greyhounds in several behavorial experiments. Scientists have also concluded that dogs are nearsighted and do not have sharp, focused vision. Their vision could be compared to a middle-aged person with bifocals. Not having the best vision does not really trouble dogs because they rely on their strong sense of smell for many activities.
If this investigation of vision in the animal world and becoming informed that some animals have the ability to see in the ultraviolet spectrum has left you wishing that you could too, well, there is a way you can, somewhat. This way is through cataract surgery, maybe a surgery that your grandmother has had recently. Cataract surgery replaces the natural lens of the eye with a plastic lens, which UV rays can pass through. Human cone cells may respond a little to near-ultraviolet light. John Endler, a researcher (from the University of California who is working to develop a camera that will see the world the way birds do) had this to say about his mother after her cataract surgery, "It happened to my mother. She started to see the nectar guides on flower petals." While we will never be able to truly see what birds, butterflies, cats, and dogs see through their vision, we can imagine using our understanding of light, light sources, color, and knowledge of the eye.
National Audubon Society http://www.audubon.org
Brewer, Jo and Sandved, Kjell B. Butterflies. New York: Harry N. Abrams, Inc., 1976.
Carlson, Delbert G. and Giffin, James M. Dog Owner's Home Vetinary Handbook. New York: Macmillan Publishing Company, 1992.
Dennis, John V. and Tekulsky, Mathew. How to Attract Hummingbirds and Butterflies. San Ramon, California: Ortho Books, 1991.
Dunning, John Jr., Chris Elphick, and David Sibley. National Audubon Society: The Sibley Guide to Bird Life and Behavior. New York: Chanticleer Press, 2001.
Fogle, Bruce. The Encyclopedia of the Cat. New York: Dorling Kindersley Limited, 1997.
Griggs, Jack. All the Birds of North America. New York: HarperCollins Publishers, Inc, 1997.
Johnsgard, Paul A. The Hummingbirds of North America. Washington, D.C.: Smithsonian Institution Press, 1997.
Laksy, Kathryn. Monarchs. Orlando, Florida: Harcourt Brace and Company, 1993.
Mikula, Rick. Garden Butterflies of North America. Minocqua, Wisconsin: Willow Creek Press, 1997.
Roth, Sally. Attracting Birds to Your Backyard. Emmaus, Pennsylvania: Rodele Press, 1998.
Smythe, R.H. Vision in the Animal World. New York: St. Martin's Press, 1975.
Stokes, Donald and Lillian. The Hummingbird Book. New York: Little, Brown and Company, 1989.
Stokes, Donald and Lillian, and Williams, Ernest. The Butterfly Book. New York: Little, Brown and Company, 1991.
Stokes, Donald and Lillian. Bird Gardening Book. New York: Little, Brown and Company, 1998.
Berger, Cynthia. "Seeing Color in a New Light." National Wildlife. Dec/Jan 2002: pages 14-21.
Weiss, R. "Orange you Glad? Fido sees the Blues." Science News. 30 September 1989: page 215.
Thank you, Dr. Bordley, for having us do this project; I've found the answers to questions I have been wondering for awhile and now I know how to construct a website. Also, I want to thank the staff from the ATC lab who helped me insert pictures and deal with a few problems.