Mammalian Optics for Vision

Abstract

Mammalian eyes exhibit striking diversity in the optical systems involved in capturing and focusing light onto the retina, including the orientation of the orbits, eye size and shape, pupil shape and dynamics, lens structure, accommodative ability, and variation in the color, distribution, and presence of the tapetum lucidum. Research on this diversity suggests that mammalian eye structure reflects complex interactions between optical constraints, ecology, and evolutionary history. Eye anatomy represents a fundamental trade-off between visual sensitivity (i.e., the ability to detect photons in dim light) and spatial resolution (i.e., the ability to resolve spatial details), as features that enhance sensitivity in dim light generally come at the expense of spatial resolution under brighter conditions. Many mammals experience dramatic differences in ambient light levels associated with the time of day that they are active and the habitats they use, which have selected for optical structures that can carefully balance the sensitivity–resolution trade-off. In addition, ecological tasks, such as visually guided hunting, predator detection, or locomotor needs, have further fine-tuned this balancing act. Importantly, however, the diversity of mammalian eye structures has also been influenced by evolutionary history and phylogenetic inheritance, which can limit the extent to which optical systems can evolve in response to ecological pressures. All mammals regardless of activity pattern, for example, retain some features of eye structure typical of other nocturnal vertebrates, a proposed legacy of a prolonged period of nocturnality in early mammalian evolutionary history. Mammalian optical systems thus reflect the interplay of physical constraints, ecological demands, and evolutionary history.