Why do many nocturnal snakes have vertical slit pupils?

Nocturnal snakes possess vertical slit pupils primarily as an adaptation to manage varying light intensities. This pupil shape allows the eye to open wide in low-light conditions, maximizing light intake to improve night vision, which is essential for nocturnal activity.12

Conversely, the vertical slit can constrict more effectively than a round pupil when exposed to bright light. This constriction protects the snake's sensitive, dark-adapted retina from potential damage during the day. While diurnal species typically have round pupils, the vertical slit serves as a specialized mechanism for snakes that must navigate both dim environments and brighter conditions.12