The eye of the crepuscular rodent rock cavy (Kerodon rupestris) (Wied, 1820)

Introduction: The rock cavy (Kerodon rupestris) is a crepuscular Caviidae rodent belonging to suborder Hystricomorpha, inhabiting semiarid areas ofthe Brazilian Northeast region. Material and Methods: Previously anesthetized young adult rock cavies were submitted to transcardiac perfusion. The extraocular muscles and components of the eyeball were dissected and measures were taken. Some eyes were embedded in paraffin and sectioned to study the retinal structure. Results: The rock cavy has lateral eyes housed in a well-constituted bony orbit and endowed with well differentiated extrinsic musculature. Descriptive and morphometric anatomical study of the rock cavy eye showed axial diameter mean values of 10.67 ± 0.55 mm and an equatorial diameter of 11.64 ± 0.69 mm. It has a vertical slit pupil and a crystalline lens with a mean axial diameter of 5.4 ± 0.03 mm, which corresponds to about 45% of the eye axial diameter. The posterior nodal distance and the retinal magnification factor were estimated in 6.74 mm and 118 μm/degree, respectively. Conclusion: Anatomical features of the rock cavy eye suggest that it was designed to acquire greater light sensitivity, at the expense of a reduced resolution, compatible with a vision under mesopic conditions, beitting an animal with a pattern of predominantly crepuscular activity.