Tarsiers, some of the world's smallest primates, have the largest eyes relative to body size among mammals (Figure).2 Human eyes would have to be the size of apples to match tarsier eye proportions. Surprisingly, they are named for their extremely long feet and not for their enormous and fascinating eyes, which take up about half of the relatively tiny face. Each eye is larger than the animal's brain or its stomach. Tarsiers originate from the Philippines and nearby islands and are mostly active at night. However, they lack a reflective tapetum lucidum characteristic of most nocturnal animals so having very large eyes helps to maximize their sensitivity in low light. Unlike many other nocturnal animals who have slit-shaped pupils, tarsiers have humanlike round pupils that are able to constrict very effectively to about half a millimeter.3 Their eyes have retinae with both rods and cones and a fovea, which is unusual for nocturnal animals. The large crystalline lens aids in dim light.4 Tarsiers cannot rotate their huge eyes; instead, they are able to turn their heads nearly 360°.
Rozenbaum I, Faschinger C. Small Primate, Big Eyes. Arch Ophthalmol. 2008;126(4):542. doi:10.1001/archopht.126.4.542