 The small red fox "(Vulpes vulpes)" has vertical slit pupils whereas their large relatives, the gray wolf "(Canis lupus lupus)" and domestic dogs "(Canis lupus familiaris)" have round pupils.One explanation for the evolution of slit pupils is that they can exclude light more effectively than a circular pupil.This would explain why slit pupils tend to be found in the eyes of animals with a crepuscular or nocturnal lifestyle that need to protect their eyes during daylight.