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Comparative Study
. 2012 Dec 22;279(1749):4962-8.
doi: 10.1098/rspb.2012.2258. Epub 2012 Oct 24.

Eye shape and the nocturnal bottleneck of mammals

Margaret I Hall  1 , Jason M Kamilar, E Christopher Kirk
Affiliations
Comparative Study

Eye shape and the nocturnal bottleneck of mammals

Margaret I Hall et al. Proc Biol Sci. .

Abstract

Most vertebrate groups exhibit eye shapes that vary predictably with activity pattern. Nocturnal vertebrates typically have large corneas relative to eye size as an adaptation for increased visual sensitivity. Conversely, diurnal vertebrates generally demonstrate smaller corneas relative to eye size as an adaptation for increased visual acuity. By contrast, several studies have concluded that many mammals exhibit typical nocturnal eye shapes, regardless of activity pattern. 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). Here, we conduct a detailed analysis of eye shape and activity pattern in mammals, using a broad comparative sample of 266 species. We find that the eye shapes of cathemeral mammals completely overlap with nocturnal and diurnal species. Additionally, most diurnal and cathemeral mammals have eye shapes that are most similar to those of nocturnal birds and lizards. 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. Our results provide additional evidence for a nocturnal 'bottleneck' in the early evolution of crown mammals.

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Figures

Figure 1.
Figure 1.
Scatter plot of pFDA results illustrating the degree of overlap between the activity pattern groups. Triangles, nocturnal; circles, cathemeral; open squares, diurnal; filled squares, diurnal anthropoid.
Figure 2.
Figure 2.
Boxplots showing the range of the corneal diameter : axial length ratios for birds, lizards, anthropoids and non-anthropoid mammals of varying activity patterns.
See this image and copyright information in PMC

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