A dog's eyes dilate at times of stress or excitement, as well as to see better at night. How much better is canine night vision than that of humans?

We lose the night vision contest because dogs have more rod cells, which are the most sensitive to low light. Along with cats, frogs, and owls, dogs also have a special mirror-like layer called the tapetum lucidum that reflects light back out (and also the reasons why Rover's eyes seem to glow in the dark).

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