Sciurus niger
(fox squirrel)

Image Caption:
Pictured is the eye of a juvenile male fox squirrel.  The organ had a mass of  
1 0.5 g, a height of 13.6   0.5 mm, a width of 14.2 0.5 mm, resulting in a diameter of about half that of the 25 mm human eye (Ndabahaliye, 2002). A tinted lens probably protect the retina from the bright sun.  Assuming the shorter diameter eyeball translates into a shorter focal length and by using photographs to estimate the squirrel aperature to be less than 5 mm in bright light, compared to the human aperature of about 2.7 mm, I can estimate the ratio of light that would strike the squirrel's retina comparted to the human.  The squirrel's retina would receive somewhere around 3 times more light then a human's if the squirrel had no pre-retinal absorbance.

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Sciurus niger rufiventer (Fox Squirrel) Eye

Works Cited:

Ndabahaliye , A.  (2002).  Diameter of a Human Eye.  Physics Factbook.  Retrieved April 23, 2009, from  (Link)

Image Location: United States, South Dakota, Brookings
Image Date: 2008APR18
Image Species: Sciurus niger rufiventer
Sex: Male
Age: Juvenile
(EXIF Info is accurate – stamped in UTC).

Web Page and Pictures By Hannah Leitheiser
Updated: 2009APR22