![]() Cathemeral birds exhibit an intermediate morphology between nocturnal and diurnal birds, with relative cornea sizes that are larger than those of diurnal birds but smaller than those of nocturnal birds. This configuration increases the posterior nodal distance of the eye, which increases retinal image size and enhances visual acuity. ![]() Conversely, diurnal birds and lizards exhibit smaller relative cornea sizes compared with nocturnal species, and diurnal birds with smaller corneal diameters also consistently exhibit an absolutely longer axial length of the eye. This configuration increases the light-gathering capacity of the eye at maximum pupillary dilation, thus improving visual sensitivity under scotopic light conditions. Among both birds and lizards, nocturnal species nearly always exhibit a larger corneal diameter than a diurnal species with the same axial eye length. By comparison, the size of the cornea relative to eye length (henceforth ‘relative cornea size’) usually varies in a highly predictable manner with activity pattern. For example, both lizards and non-primate mammals exhibit little interspecific variation in axial eye length according to activity pattern. Axial eye length itself, a common measurement of overall eye size, does not always vary predictably with activity pattern among vertebrates. As a result, increased eye size may simultaneously benefit diurnal species by increasing visual acuity and nocturnal species by improving visual sensitivity. Similarly, absolutely larger eyes may gather more light than smaller eyes. photoreceptor size, spacing and retinal connectivity) are held constant, absolutely larger eyes have greater resolution than absolutely smaller eyes. Theoretically, all vertebrates could benefit from having larger eyes regardless of their activity pattern. Prior research has demonstrated that the functional demands of vision under high-light (photopic) and low-light (scotopic) conditions select for divergent eye morphologies in nocturnal, cathemeral and diurnal species because the vertebrate eye cannot be optimized for high-quality vision under both photopic and scotopic conditions. Activity pattern-the time of the day when an animal is awake and active-determines the amount of light available for vision, and as such is a key selective influence on the evolution of the vertebrate visual system. The relationship between eye morphology and activity pattern in different vertebrate groups has been the subject of numerous recent comparative studies. Our results provide additional evidence for a nocturnal ‘bottleneck’ in the early evolution of crown mammals. The only mammalian clade that diverges from this pattern is anthropoids, which have convergently evolved eye shapes similar to those of diurnal birds and lizards. Additionally, most diurnal and cathemeral mammals have eye shapes that are most similar to those of nocturnal birds and lizards. We find that the eye shapes of cathemeral mammals completely overlap with nocturnal and diurnal species. Here, we conduct a detailed analysis of eye shape and activity pattern in mammals, using a broad comparative sample of 266 species. However, a recent study has argued that new statistical methods allow eye shape to accurately predict activity patterns of mammals, including cathemeral species (animals that are equally likely to be awake and active at any time of day or night). By contrast, several studies have concluded that many mammals exhibit typical nocturnal eye shapes, regardless of activity pattern. Conversely, diurnal vertebrates generally demonstrate smaller corneas relative to eye size as an adaptation for increased visual acuity. Nocturnal vertebrates typically have large corneas relative to eye size as an adaptation for increased visual sensitivity. Most vertebrate groups exhibit eye shapes that vary predictably with activity pattern.
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