Visual system in a diurnal marsupial, the numbat (Myrmecobius fasciatus): retinal organization, visual acuity and...

The visual system of the numbat (Myrmecobius fasciatus), a highly endangered diurnal marsupial, has been investigated both anatomically and behaviorally. The ganglion cell layer, examined in cresyl-violet stained wholemounts, contained 832,800 ganglion cells; the number of ganglion cells corresponded to optic axon counts. An area centralis was located in the mid-temporal retina, where cells formed a bilayer, but there was no evidence of a visual streak. Visual acuity, estimated from counts of peak ganglion cell density (8,100/mm(2)) and measurements of posterior nodal distance (7.84 mm), was found to be 6.3 cycles per degree. The value was similar to that of 5.2 cycles per degree estimated by behavioral tests. Sectioned material revealed the presence of numerous oil-droplets in a cone-dominated retina. A rich retinal vasculature of the end-artery type of paired arteries and veins formed beds in the ganglion cell and inner nuclear layers. Measurements of visual fields revealed a binocular overlap of 80 degrees in the horizontal plane, and a total field of view approaching 240 degrees of visual angle. Monitoring of pupillary responses with an infrared video camera showed that the numbat possesses a remarkably wide, static pupil. Our results are discussed in relation to the ecology of the species and its phylogeny.