![]() Kiltie also reported that diel activity pattern has a strong influence on the relationship between acuity and eye length, but the sample of diurnal mammals in this analysis consisted entirely of anthropoid primates. Comparative studies have provided some support for this prediction in marsupials and strepsirrhine primates. Consequently, diurnal mammals are expected to have higher acuity than cathemeral or nocturnal species. the time of day that a species is habitually active) in particular is predicted to exhibit a strong relationship with acuity because adaptations that enhance visual sensitivity in low light are generally incompatible with high acuity. Ĭomparative studies of eye length and other aspects of visual morphology have linked increased visual acuity to a variety of ecological factors, including diel activity pattern, diet and speed of locomotion. A similar result was obtained by Heesy and Hall using a sample of 14 nocturnal and 11 diurnal mammal species, but the relationship between acuity and eye length was less clearly linear than that shown by Kiltie. For example, Kiltie demonstrated a positive correlation between acuity and eye length in a sample of 14 nocturnal mammals and 3 diurnal anthropoids. However, these studies employed relatively small mammalian samples with limited taxonomic and ecological diversity. Some support for this prediction has been provided by comparative analyses of visual acuity and eye length in mixed samples of mammals and birds. Species with absolutely longer eyes are therefore expected to have higher acuity than species with shorter eyes. As a result, studies that calculate visual acuity based on PND and peak ganglion cell density typically use measurements of eye length to estimate PND. ganglion cell receptive fields), such that eye length should be positively correlated with visual acuity. ![]() Larger retinal images will generally be sampled by a greater number of independent sampling units (i.e. If eye shape is held constant, an increase in eye length produces a longer posterior nodal distance (PND) and increases the size of the retinal image (fig. In particular, eye axial diameter (‘eye length') plays a key role in determining retinal image size. In mammalian eyes, theoretical optics predicts that absolute eye size will influence a number of different aspects of visual functionality. Thus, identifying the factors that influence visual acuity is critical for understanding mammalian sensory ecology. microchiropteran bats, rodents) may reflect an increased dependence on nonvisual senses. At the same time, the lower acuity of some other mammalian clades (e.g. primates) is hypothesized to reflect an increased reliance on vision in meeting basic needs, such as finding food and avoiding predators. The evolution of higher visual acuity in certain clades (e.g. the ability to resolve spatial details), ranging from the low acuity vision of microchiropteran bats and small rodents (0.4-1.0 cycles per degree, or cpd) to the highly acute vision (30-64 cpd) of diurnal anthropoid primates. Mammals exhibit substantial variation in visual acuity (i.e. These results suggest that interspecific variation in mammalian visual acuity is the result of a complex interplay between phylogenetic history, visual anatomy and ecology. Our data also demonstrate that higher visual acuity in mammals is associated with: (1) diurnality and (2) predatory habits once the effects of eye size and phylogeny have been statistically controlled. This relationship conforms to expectations based on theoretical optics and prior analyses of smaller comparative samples. These data confirm that axial eye length and visual acuity are significantly positively correlated in mammals. Here we use phylogenetic comparative methods to explore these relationships in a taxonomically and ecologically diverse sample of 91 mammal species. While a number of prior studies have explored ecological and phylogenetic effects on eye shape, a broad comparative analysis of the relationships between visual acuity, eye size and ecology in mammals is currently lacking. Mammalian species have also been hypothesized to differ in visual acuity partly as a result of differences in ecology. Previous comparative research has attributed interspecific variation in eye size among mammals to selection related to visual acuity.
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