Abstract
Introduction: The eye plays an essential functional role in the visual system of bats. The Egyptian rousette bat, Rousettus aegyptiacus, is a cave-roosting bat from the suborder Megachiroptera. These bats rely on vision for navigation; however, they were previously thought to possess rods only. This study found the presence of rods and cones, which play a crucial role in the vision of these mammals. This proves that these bats have the prerequisite for photopic (daylight) and scotopic (night) vision.
Purpose and Methodology: L-cone (middle-to-long wavelength sensitive cone) and rod photoreceptor densities and their retinal topography were investigated in this study using retinal wholemounts and immunohistochemical techniques.
Results: The estimated mean population of the L-cone photoreceptors was 244,956 cells. The highest peak density of L-cones was found in the temporal region of the retinal wholemount and showed a peak density of 6,586 cells/mm2. High densities of cones were also observed in the nasotemporal areas and the mid-ventral regions. All three specimens showed an elongated horizontal visual streak along the nasotemporal axis. Rod densities measured in selected sampling sites showed peak densities ranging from 480,006 cells/mm2 to 586,674 cells/mm2, the highest density found in the ventral parts of the retina. The lowest density of rods counted was found in the dorsal regions of the retina, at 142,224 cells/mm2. This pattern of rod density distribution is similar to the pattern revealed by cone densities.
Conclusion: The findings of this study indicate that the Egyptian rousette bat Rousettus aegyptiacus retina contains rods and cones. The high density of cones in the temporal retinal region may have a functional role during foraging. The presence of the horizontal visual streak indicates an area of specialisation on the retina to enhance optical sampling in the horizon, possibly facilitating the detection of predators. The retina of Rousettus aegyptiacus is rod-dominated, but a significant population of cones affords a neural substrate for vision at higher levels of luminance.
Keywords: bat; cone; rod; visual streak; retina