The Retina of Ansorge's Cusimanse (Crossarchus ansorgei): Number, Topography and Convergence of Photoreceptors and Ganglion Cells in Relation to Ecology and Behavior

Abstract

The family Herpestidae (cusimanses and mongooses) is a monophyletic radiation of carnivores with remarkable variation in microhabitat occupation and diel activity, but virtually nothing is known about how they use vision in the context of their behavioral ecology. In this paper, we measured the number and topographic distribution of neurons (rods, cones and retinal ganglion cells) and estimated the spatial resolving power of the eye of the diurnal, forest-dwelling Ansorge's cusimanse (Crossarchus ansorgei). Using retinal wholemounts and stereology, we found that rods are more numerous (42,500,000; 92%) than cones (3,900,000; 8%). Rod densities form a concentric and dorsotemporally asymmetric plateau that matches the location and shape of a bright yellow tapetum lucidum located within the dorsal aspect of the eye. Maximum rod density (340,300 cells/mm²) occurs within an elongated plateau below the optic disc that corresponds to a transitional region between the tapetum lucidum and the pigmented choroid. Cone densities form a temporal area with a peak density of 44,500 cells/mm² embedded in a weak horizontal streak that matches the topographic distribution of retinal ganglion cells. Convergence ratios of cones to retinal ganglion cells vary from 50:1 in the far periphery to 3:1 in the temporal area. With a ganglion cell peak density of 13,400 cells/mm² and an eye size of 11 mm in axial length, we estimated upper limits of spatial resolution of 7.5-8 cycles/degree, which is comparable to other carnivores such as hyenas. In conclusion, we suggest that the topographic retinal traits described for Ansorge's cusimanse conform to a presumed carnivore retinal blueprint but also show variations that reflect its specific ecological needs.