Glowing Animal Eyes
Hello, I'm the OUTDOOR PROFESSOR from DiscoveringTheOutdoors.com/
Here's your outdoor tip on why animal eyes glow in the dark.
Have you ever caught a deer in the headlights of your car at night and seen a luminescent glow in its eyes? Have you gone along a river or lake at night shining the shoreline with a spotlight and seen a spooky glow of eyes seeming to stare at you through the dense darkness?
This phenomenon is caused by a series of special adaptions designed to help animals see in low light. The shine from the eyes is caused by a layer of tissue, Tapetum Lucidum, located behind the retina that reflects light back through the retina giving the receptors a second chance at the rays of light. This increases the available light providing a kind of night vision for the animal. Also, when a light shines, it reflects a bright glow that can be seen from a distance.
When light enters the human eye, it passes through the pupil that regulates how much light strikes the retina at the back of the eye. Receptors there called rods and cones receive the light. Working best in low light, rods help you to see movement and broad details. Cones work in bright light to help you see fine details and colors. Humans have more cones while night animals typically have more rods. Nocturnal predators have almost no cones relying on sudden movement to spot prey and guide their attacks.
Animals such as deer are largely active during dawn and dusk so their eyes have larger pupils than animals that are mostly active during the day. This increases the glowing eye effect since in the dark, the pupil can expand to cover most of the front of the eye. This opens much of the shiny layer beneath the retina increasing the glowing effect. The eye shine of animals will vary within the same species. Genetics, diet, injury, and other factors make the glowing eyes of one animal different than that of another.
Animals that have the tapetum lucidum include deer, dogs, cats, cattle, horses and ferrets. Humans and other primates do not along with squirrels, kangaroos, and pigs. And all glowing animal eyes do not glow the same color due to different substances such as bioflavin or zinc in the tapetrum. Varying amounts of pigment in the retina and age can also change the glow’s color. Cats’ eyes often glow bright green with Siamese cats' eyes often glowing bright yellow. Some dogs have eyes that glow turquoise.
In photographs animals’ eyes shine back at us. Human eyes only glow bright red in photos. This is because human’s lack the reflective layer that many animals have in their eyes. The retina by itself does not have the reflective power of the tapetum lucidum that adds color to the reflection. The flash coming through the human eye is simply showing the color of the red blood vessels crisscrossing in the eye reflecting back from the retina.
So when you see those glowing eyes on your next nighttime adventure, you’ll hopefully understand a bit more about where the shining comes from.
This is the OUTDOOR PROFESSOR from DiscoveringTheOutdoors.com/
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References-Additional Reading
The Science of How Eyes Glow in the Dark
http://io9.com/the-science-of-how-eyes-glow-in-the-dark-1600183971
Why do animal eyes glow in the dark?
http://www.npr.org/templates/story/story.php?storyId=96414364
Why do deer’s’ eyes glow?
http://animals.mom.me/deers-eyes-glow-7877.html
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