Visual capabilities in a crepuscular marsupial, the honey possum ( Tarsipes rostratus ): a visual approach to ecology

Abstract

Behavioural and anatomical analyses were undertaken to determine the visual capabilities of a small nectivorous Australian marsupial, the honey possum Tarsipes rostratus. It is proposed that, despite a crepuscular lifestyle and a low visual acuity, vision contributes substantially to the honey possum's perception of its environment. Visual acuity, determined using a discrimination task, revealed a slightly higher performance in daylight (0.63 cycles/degree) than in moonlight and starlight (0.60 cycles/degree). Ultrastructural analysis of the outer retina revealed a rod to cone ratio comparable to that found in diurnal retinae (20:1 in the central retina). Two types of oil droplets were distinguished on the basis of size. The degree of neural convergence was also similar to that found in a diurnal retina, with a ratio of 12 photoreceptors:1 ganglion cell. The remarkable total field of view, approaching 240° of visual angle and including a binocular overlap of 80° in the horizontal plane, provides an efficient predator detection capacity. The visual capabilities of the honey possum are discussed in relation to its lifestyle and the associated visual requirements.