by Perry McCarney

Created on: February 28, 2009   Last Updated: November 27, 2009

The omnivorous sugar glider (Petaurus breviceps) has forward facing eyes, as all predators do. This provides binocular vision which enables depth perception and the judgment of distances when attacking prey; it is even more essential to these small marsupial's because their main form of locomotion is to volplane (glide) from tree to tree.

Because they are nocturnal hunters, their eyes are large in comparison to their head size and bulbous, they protrude significantly. Because sugar gliders are prey to larger predators, their eyes are wide-set, enabling them to both focus on their prey or aerial destination and have extensive peripheral vision. This combination can sometimes be mistaken for the side of the head positioning that typifies herbivorous prey animals.

Sugar gliders have excellent eyesight which may equal or excel the domestic cat's, but it is achieved through a quite different eye structure. While cat's eyes shine in the dark due to a reflective layer of cells behind the retina called the tapetum lucidum, a sugar glider's eyes gleam no more than our own, as neither of us have this reflective layer. Instead, sugar gliders have an avascular retina similar to that of the sharper-sighted raptors (birds of prey), rather than the vascular retina most mammals have.

The light entering a cat's eye first passes through the neural layer of their vascular retina, which consists of nerve cells and blood vessels, before impacting on the rods and cones of the photoreceptor layer beneath. Avascular retinae do not have a neural layer, the first thing the light reaches are the photoreceptor cells that decipher it. The photoreceptor layer in the avascular retina of the sugar glider is highly complex, more so than that of comparative marsupials with vascular retinae. With their large, protruding eyes maximizing the light caught, highly expandable pupils to pass it in and complex avascular retinae to process it, sugar glider's manage to see very well indeed.

Unless, that is, they have eye problems. And these can develop while they're in their mother's pouch, before ever having opened their eyes. The culprit is their mother's diet. What their mother eats is going to effect the nutrition they receive from her. If their mother receives a diet too high in fat, something highly unlikely in the wild, the fat is passed on to the young in the mother's milk. This can lead to the formation of fat deposits in their eyes which can distort the light reaching the retina or block it

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