A whitetail buck grazed on a hillside in Washington County. Two does nipped grass just above him. The driver of a red pickup truck spotted the deer, 150 yards away, and braked. As the truck stopped, all three deer raised their heads and stared intently toward the intruding vehicle. The driver wondered: "Did those deer see my truck as an alien object? Or, had they simply heard the crunch of tires on gravel and sensed danger when the sound stopped?"
Such encounters happen all the time between humans and deer. Hunters, especially, ponder what deer can see and how to dress in the woods to avoid detection.
Ongoing research shows that hunters have over-simplified how deer see, especially in assuming that deer are "color blind."
In a series of studies at the University of Georgia, a research team anesthetized nine deer and recorded electrical responses in the whitetail's eyes to light shone on their retinas. While the retina of the human eye is equipped with a high density of "cone cells" that support color vision across the broad range of hues perceived as blue to red, deer have relatively few cones. Human eyes also have a "filter" which protects the eye from ultra-violet light and helps humans see objects in fine detail.
Deer eyes, though, are packed with far more "rod cells," enabling deer to see well in low light. From a human viewpoint, deer "sacrifice" broad color vision and detail for acute sight at night, dawn and dusk.
But it is wrong to say that deer are "color blind."
"What we discovered is that deer are not color blind, although they do see color differently than we do," said Brian Murphy, CEO of the Quality Deer Management Association and a wildlife biologist who participated in the University of Georgia research.
"Deer are essentially red-green color blind like some humans. Their color vision is limited to the short [blue] and middle [green] wavelength colors. As a result, deer likely can distinguish blue from red, but not green from red, or orange from red."
Murphy explained that a deer probably perceives a hunter's blaze orange coat as neutral gray, which, if color were the only visual clue, might always blend in well with natural backgrounds. But the ultra-violet radiance of clothing can "give it away" in a deer's eye, even if color does not.
"It's too simplistic to say that wearing orange will not impact your hunting success," Murphy said. "There is color, then there is the UV radiance off that color, no matter what the color is."
He explained that a deer's ability to see well in the short-wavelength "blue" light of night also enables deer to "pick out" ultra-violet emissions from hunters' clothing. Similarly, a highly reflective material such as a slick raincoat is likely obvious to a deer's eyes, regardless of color.
Hunters became widely aware of UV brightness in recent years, even washing their camouflage clothing in UV "killers."
Murphy points out that some field conditions nullify the UV concern. Bright sunlight, for instance, overwhelms UV reflection, making it appear visually neutral. Murphy also reminds, however, that most hunter-deer encounters happen when deer are most active -- at dusk and dawn -- so precautions against UV radiance are probably warranted by serious hunters.
Regarding UV "killer" washing agents, Murphy leaves that up to hunter's choice.
"Should you purchase such a product? This is difficult to answer. Hunters have been successfully harvesting deer for hundreds of years without the aid of such products. However, armed with our latest knowledge it remains possible, even likely, that such a product may help," Murphy said. "On the other hand, it definitely can't hurt."
Murphy reminds hunters to think of deer vision as the marvelous sense that it is.
"A deer's eye is particularly specialized for what they do," he said. "It is well adapted to life as a prey species, having evolved with animals that wanted to eat them every day. Just consider the placement of the eyes, which allow a deer to detect predators in a 310-degree arc."
Murphy said that some of the most amazing things about deer vision are not that obvious.
"The anatomy of their head and eyes gives them the ability to focus on something moving in the distance and never move a muscle," he said.
"They can remain motionless and watch a predator cross a long stretch. We humans would have to move our head constantly to do what they do. It's just one more of their survival techniques."
First Published November 22, 2009 5:00 AM