HALL, MI; ROSS, CF: Eye Shape and Activity Pattern in Amniotes

Most vertebrates rely on vision to interact with their environments. The ability to see well depends on absolute, not relative, eye size, since all animals must utilize the objective qualities of light. All visually dependent animals are selected to have a large overall eye size. However, those animals adapted to scotopic (low light) and photopic (plentiful light) conditions utilize different eye shapes. Scotopic-adapted animals need to maximize the amount of light collected. Therefore, the anterior portion of their eyes is enlarged to increase light gathering. Photopic-adapted animals are faced with an abundance of light and instead require a large eye to enhance visual acuity. Therefore, the posterior portion of the eye is enlarged both to increase image size and to maximize the number of photoreceptors over which that image is projected. To test the hypothesis that activity pattern in reflected in eye shape, the corneal diameter and axial diameter measurements of 1,200 vertebrate eyes were compared to see if eye shape reflects known activity patterns. Animals measured include birds, lepidosaurs, turtles, and mammals. Activity pattern is reliably reflected by eye shape across amniotes. Visually dependent scotopic animals have a larger cornea relative to axial diameter, and photopic animals have a larger axial diameter relative to the cornea. Despite variation among amniotes in structures such as tapeta, colored lenses and corneas, photoreceptor structure and placement, and iris and pupil size, it is significant that eye shape reliably reflects activity pattern.