Nursing Entrance Exam (NEX) Practice Exam

If a mammal is nocturnal, how has it likely evolved in terms of eye structure?

1. As many cones as there are rods to capture the colors at dusk/dawn

2. An absence of cones since nocturnal mammals only need rods to see black/white at night

3. Fewer cones and a greater concentration of rods to collect the minimal light for night vision

4. More cones than rods to enable them to see the muted colors at night

The correct answer is: Fewer cones and a greater concentration of rods to collect the minimal light for night vision

Nocturnal mammals have evolved specific adaptations in their eye structure to maximize their ability to see in low light conditions. Among these adaptations, a greater concentration of rods in their retinas is crucial. Rods are photoreceptor cells that are highly sensitive to light and enable vision in dim environments. They do not provide color information, which is less critical for animals that are active at night, but they excel in detecting light contrast and movement. Having fewer cones is beneficial for nocturnal mammals, as cones are responsible for color vision and function optimally in bright light conditions. Since the primary need for nocturnal mammals is to navigate and find food in darkness, an eye structure abundant in rods greatly enhances their night vision capability while conserving energy and resources that would be required for a larger number of cones. This adaptation illustrates the evolutionary trade-off; these mammals prioritize the ability to see in low light over the capacity to perceive a wide range of colors, which is less relevant in their typical nighttime activities. Thus, the evolution in eye structure of nocturnal mammals reflects a specific need for enhanced vision in dark environments.