QUESTION: We often drive the back roads looking for deer at night. We've seen plenty in our headlights and even more when shining the fields. I've always wondered, why do their eyes shine in headlights, but not a human's? - J. Morrill
ANSWER: Eons of natural selection have fine-tuned the whitetail's eyes to be most effective when they are most active, in the low light of dawn and dusk.
Like humans, a deer's eyes have two types of photo-receptor cells: rods and cones.
In daylight, cones help discern the detail and color of objects, and humans have more then deer. In dim light, visual sensitivity is achieved primarily through rods, which outnumber cones 20:1 in deer. This gives them much better vision in low light than humans.
Unlike us, deer have a shiny, blue-green colored membrane attached to their retina (the back of their eye) called the tapetum lucidum. This membrane functions like a mirror, reflecting light that has already passed across the rods. Because the light crosses the rods again, it makes them twice as effective at seeing in the dark.
The light reflected from the back of the eye and the membrane is the eye shine you see when illuminating deer. You'll notice this in other animal species, as well.
Editor's Note: In some states, purposefully shining headlights into a field at night is considered animal harassment and is illegal even if you have no weapon in your vehicle. Check with your state's DNR before doing this.