Adaptive Features of the Eye to the Ecological Habit of the Short-Eared Owl Asio flammeus and Japanese Scops Owl Otus semitorques

Abstract

In this study we investigated the eye morphology and retinal topography of two owl species in relation to their visual environment. Although Short-eared Owl Asio flammeus is larger and weighs more than Japanese Scops Owl Otus semitorques, its eye dimensions (weight, corneal diameter, and axial length) are all smaller than the scops owl's. Owl retinas were examined in Nissl-stained whole-mount preparations. The total number of retinal ganglion cells (RGCs) was greater in the Japanese Scops Owl (4,703.0×103 cells) than in the Short-eared Owl (2,346.8×103 cells). The eye morphology of, and the number of RGCs in the Japanese Scops Owl indicate that it is more adapted to a nocturnal habit. An area of high-density RGCs was horizontally distributed in the temporal retina of the Short-eared Owl, with a peak density of 17.4×103 cells/mm2. In the Japanese Scops Owl's temporal retina, there was an oval-shaped arrangement with a peak density of 23.1×103 cells/mm2. These distributions indicate that whereas Short-eared Owl is adapted to open habitats, Japanese Scops Owl is adapted to enclosed habitats. The RGCs of both species were classified into three categories (small, medium, and large) based on the size and appearance of somas. In both species, medium-sized cells predominated, and the proportion of large-sized cells was smallest. The distribution and high-density areas differed among these groups, suggesting adaptation to the visual environment. The high-density areas of these groups cover the nasal visual fields, which include the overlap for binocular vision.