In nocturnal mammals, there are fewer cones and a greater concentration of rods.

• A. There are more cones and fewer rods
• B. There are equal numbers of cones and rods
• C. There are no rods
• D. There are fewer cones and greater concentration of rods

Answer: (D)

The key idea is that night-active animals optimize their retinas for very low light. Rods are incredibly light-sensitive and drive vision in dim conditions, while cones need brighter light and provide color vision and sharp detail. So nocturnal mammals end up with more rods and fewer cones, enhancing their ability to detect faint light but reducing color vision and fine detail. This arrangement matches the statement describing fewer cones and a greater concentration of rods. If there were more cones, or no rods, their vision in the dark would be poor; equal numbers would not reflect the specialization for nighttime seeing.