Date: Fall 2011
I am a avid deer hunter. The question I want to ask is I can see deer eyes during very low to dark light conditions without a flashlight or any artifical means. The only source of light is the reflection off the moon. I can see the replections at long distances probably up to 100+ yards. I have seen them in many different conditions cloudy nights ect. Yesterday morning before sunlight 2 deer walked right under me. They looked at me and there eyes glowed very bright. Can deer adjust the sensitiveity of there eyes? Nobody
that I talk to say they can see them without a flashlight. Am I going crazy or is there a answer to this.
Particularly in the case where the deer were *under* you, their eyes are going to reflect back available light from the night sky, which in relation to the background behind them could appear fairly bright. Deer, along with many nocturnal animals such as owls, some mammals (but not humans), possess a "tapetum" or "tapetum lucidum", which is a highly reflective layer behind the light sensors in the retina. In this way, light particles that miss a light receptor are reflected forward, thus doubling the chance of being detected. Also, deer pupils can open quite large, around four times as big as a human, so they let in/out a great deal of reflected light. So to answer your question, yes, deer adjust the sensitivity of their eyes like we do... the pupils open up wide when it is dark and maximize the visibility of the "eyeshine" you see. To answer your other question, no, you are not crazy.
As something to ponder, I have wondered if the tapetum is a compensation for most retinas being built "backwards". In other words the front of most retinas is covered in nerve cells and other tissue and many light particles never reach the receptor cells behind them. Only Cephalopods (octopus, etc.) have retinas built the “right way” (light receptors in front), and none of them have tapeta.
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Update: June 2012