Eileen Garvey Article: Comparative Morphology of the Eye in Primates E. CHRISTOPHER KIRK* The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology Volume 281A, Issue 1Volume 281A, Issue 1, Pages Volume 281A, Issue 1 Published Online: November 2004

Background Components of the eye: –Rods: neurosensory cells which are insensitive to color, absorb light over the entire range of the visual spectrum, and are most effective in relatively low intesities of light, such as shade, dusk or night time. –Cones: neurosensory cells that are senstive to color, absorb light in only one part of the visible spectrum (red, green, blue), and function only in prescene of light that falls into this wavelength. Diurnal species are usually more active during the day when the light levels are in the range of cone mediated vision. Nocturnal species are usually active at night when light levels are in the range of rod mediated vision. However there are many mammalian species that are considered, cathemeral, which means they are active in both light and dark periods.

Background continued In the article, the morphological adaptations for different activity patterns are compared with the gross anatomy of the eye of primates. The size of the cornea places an upper limit on the maximum amount of light that the eye can absorb. Nocturnal species increase the size of the cornea compared to the transverse diameter of the eye so that it is possible to absorb more light. Diurnal species have smaller corneas compared to the transverse diameter of the eye.

Hypothesis The goal of the analysis: –Provide a quantitative description of the relationship between eye morphology and activity pattern in broad sample of primate species. –The researchers asked many questions; for example: Do nocturnal, cathemeral, and diurnal primates exhibit systematic differences in eye morphology as has been predicted by other studies conducted on mammalian eyes?

Methods and Materials Data were collected on eye and cornea size for 147 specimens of 55 primate species Samples were taken from preserved specimens that were frozen directly after the animal died or were preserved in formalin Eyes were removed from the orbit and cleaned Each eye was refilled to prevent it from collapsing A needle was inserted into the optic nerve and then the measurements were determined Activity patterns and mean eye measurements were determined for all included taxa Activity patterns and mean eye measurements were determined for all included taxa

Methods and Materials –Data/calculations included: Activty Patterns D=Diurnal, N=Nocturnal, C=Cathemeral Mean transvers diameter in mm Standard deviation Mean transverse corneal diameter in mm Ratio of corneal diameter and mean transverse eye diameter (C:E ratio) Example of calculations: Table 2

Calculations Table 2: Summary of statistics comparing C:E ratios of primates grouped by suborder and activity pattern Diurnal haplorhine Nocturnal haplorhine Dirunal Strepsirrhine Cathemeral strepsirrhine Nocturnal strepsirrhine Number Mean C:E ratio Standard Deviation

Results and Discussion C:E ratios varied between primates that were of different suborders and had different activity patterns; however within the same subgroup the functional morphology was consistent with previous testing results. –For example: diurnal species had smaller relative cornea size than nocturnal or cathemeral species As expected nocturnal species showed higher C:E ratios compared to diurnal species Haplorhines had diurnal species with significantly lower C:E ratios than the nocturnal species. Strepsirrhines had diurnal species with significantly lower C:E ratios than the nocturnal species Cathemeral strepsirrhines had C:E ratios that were larger than the diurnal strepsirrhines and smaller than the nocturnal strepsirrhines

Summary Eye morphology changed consistently with activity pattern in primate suborders For most suborders tested, such as strepsirrhines, and haplorhines, relative cornea size was largest in nocturnal species and smallest in diurnal species

Discussion Data supports the hypothesis that cornea size is largest for species that are most active in dark periods compared to those that are active in the light Primates have evolved their eye morphologies to maximize their visual sensitivity, in order to adapt to the activity patterns