Which statement best describes the retina of a nocturnal mammal?

• A. There are more cones and fewer rods to support daylight vision
• B. There are equal numbers of cones and rods
• C. There are no rods in nocturnal mammals
• D. There are fewer cones and a greater concentration of rods to collect the minimal light for night vision

Answer: (D)

Night vision relies on rod photoreceptors, which are far more light-sensitive than cones and excel at detecting faint light and movement in the dark. A retina with a high rod-to-cone ratio and relatively few cones increases overall light sensitivity and broadens the field of view, helping nocturnal mammals see in minimal light even if color and fine detail are limited. That’s why the statement describing fewer cones and a greater concentration of rods fits best. In bright conditions, cones take over to provide color and sharp detail, so having more cones would not optimize night vision. Some nocturnal animals also have a reflective layer behind the retina that further boosts sensitivity, but the main idea remains that rods dominate to support night vision.