Like a theater curtain, a slit pupil can close as much as necessary to prevent too much light from entering the eye. For humans and other animals active mainly in the daytime, visual sensitivity is not as important as the ability to see small details in bright light. So the eyes of humans, lizards, ground squirrels, and most birds have a design suited for operation in bright light without the special protection of slit pupils. In dim light, our eyes are not as sensitive as those of nocturnal animals. Of all pupil shapes found in vertebrates — animals with backbones — round pupils are the most common. Non-circular pupil shapes found in vertebrates include not only the vertical slits familiar to us from cats’ eyes, but also horizontal slits, crescents, heart shapes, and keyhole shapes.