![]() ![]() Originally, biologists thought that the yellow pigment served simply to reduce chromatic aberration and provide better contrast to the image i.e. Tree squirrels also have a pale yellow pigment in their lenses that absorbs ultraviolet light. ![]() ![]() Preliminary study of the Grey squirrel retina suggests that albinos don’t suffer the same decline in visual acuity as that of other albino mammals. The reason for this contrariety is unknown, but it has been suggested that the squirrels may have been responding to the smell or brightness of the biscuits, rather than to colour. The foregoing does, however, raise the question of how, if Greys are protanopic, they managed to discriminate between the red and green biscuits offered to them by the University of Exeter’s Ian MacDonald? MacDonald found that not only could his subjects discriminate between red and green in a foraging situation, but they could also discriminate colour hue (intensity), which seems unlikely given the retinographic and photopigment data. The comparatively sparse rod cells also suggests that these animals have relatively poor scotopic (dusk) and nocturnal (night-time) vision compared to, for example, a wood mouse or fox, both of which have more rods. Overall, this implies that squirrels have good vision and the ability to see colours, albeit that their pallet is reduced over ours and red objects will have a yellow-green hue. Grey squirrels (and probably Reds) have a two-tiered retina: one tier in the centre of the retina is composed of rod cells that are used in low light and provide monochrome vision, and the outer tier is composed of colour-sensitive cone cells – the rod to cone ratio in Greys is about 2:3. More recently, electroretinographic studies suggest that squirrels do possess dichromatic (two-colour) vision, with colour discrepancy similar to a human protanope (i.e. This implies that squirrels are sensitive to blue and green wavelengths, but cannot decipher red. Work on squirrel spectral sensitivities by Barbara Blakeslee, Gerald Jacobs and Jay Neitz during the late 1980s, however, found that Greys had a peak rod sensitivity of 502nm ( note: rods only sense light intensity, not colour) and peak colour sensitivities at 444nm (blue) and 543 nm (green). Studies of light-sensing chemicals in the retina called photopigments implied the presence of pigments sensitive to long (red) and medium (green), but not short (blue), wavelengths of light. Early retinal studies suggested that it was composed predominantly (if not entirely) of cones, implying good visual acuity and the ability to see colours. The concept of colour vision in sciurids has been studied, with some mixed results. A male Red squirrel (Sciurus vulgaris) at Forest How. They will approach an observer to within a couple of feet, if she/he is standing very still, although the slightest movement can have them dashing for cover. Squirrel vision is based heavily on movement. Squirrels, like humans, also have a blind spot caused by the optic nerve passing through the retina and, in The Eurasian Red Squirrel, Bosch and Lurz note that it forms a small strip at the top of their field of view. The result is that while we can only focus on a single (central) area at a time, the periphery remaining blurred, a squirrel can clearly see what's next to, and above, it without moving its head. We humans, by contrast, have a single point on the retina, called the fovea centralis (or simply fovea), where our colour and detail sensing cone cells are most densely packed and hence our vision is sharpest. Morphological examination of the squirrel eye suggests excellent, wide-angle vision with exceptional focussing power across the whole retina. Simple observation of squirrels chasing through the tree tops suggests that they are equipped with good eyesight.
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