Why do rats have poor vision?

Answer 1

Part of the reason why rats have such poor eyesight is because generally the eyes of small and nocturnal animals are far-sighted, which signifies a lack of need for optical quality. Like most nocturnal animals, rats depend less on vision than do diurnal (active in the daytime) ones. Rather, rats have sacrificed much visual capacity and instead rely more on olfactory (smell), auditory (hearing), and tactile (touch) abilities. This phenomenon is clearly illustrated in the observance of certain strains of rats that undergo spontaneous retinal degeneration soon after birth. Compared to normal, sighted rats, these animals show no differences in behavior under daylight conditions. Compare this behavior to that of a blind bird or kitten, both of which would be virtually helpless without sight.

Although visual acuity is poor, the rat has a large cornea and pupil, as well as a powerful lens. Coupled with the rod-dominated retina, these components result in a very light-sensitive organ that is a good detector of movement. It has been demonstrated that neither the sight nor smell of a cat will produce defensive reactions in a rat, but movement of a cat or an inanimate object (a card, for example), will result in defensive posture. This suggests that motion of the predator, rather than its visual image, is a major factor in the initiation of defense responses in the rat. In certain instances, however, rats do use vision for short-range purposes. For example, wild rats have been observed to discriminate between sizes of sunflower seeds, choosing long over short seeds. Young rats at play with one another use visual rather than tactile (from whiskers) cues to orient themselves before attacking one another's back. In the instance that one rat steals a food pellet from the other, the victim also responds from visual cues.

The rat's world is very blurry. Visual acuity is measured in cycles per degree (cpd), a measurement of the number of lines that can be seen as distinct within a degree of the visual field. Acuity of humans is about 30 cpd, normally pigmented rats is 1 cpd, and 0.5 for albino rats (Prusky et al. 2002, 2000;also see Birch and Jacobs 1979 who found 1.2 cpd for pigmented rats and 0.34-0.43 cpd for albino rats). If we translate Prusky's cpd measurements into vision chart measurements, a normally pigmented rat has about 20/600 vision, and an albino rat has about 20/1200 vision.

Rat acuity can also be measured by examining the density of ganglion cells in the retina. The denser the ganglion cells, the higher the acuity at that point of the retina. In the rat, the densest area of ganglion cells (defined as the region encompassing 75% of maximum ganglion cell density) is 52.8º wide and is located slightly above and temporal to the optic disk. The maximum density of this area is 6,774 cells/mm2. This isn't very dense -- the densest area of the human retina, the fovea, has up to 38,000 cells/mm2 (Curcio and Allen 1990). The low density of ganglion cells of the rat's retina suggests a maximum visual acuity of 1.5 cpd, which is consistent with the measures found in behavioral acuity experiments (Heffner and Heffner 1992).

Depth of focus: Combined with poor visual acuity, rats have an enormous depth of focus. Depth of focus is the range of distances at which an object is in equivalent focus for an unaccomodated eye. In humans, the depth of focus is from 2.3 meters to infinity (Campbell 1957). In rats, the depth of focus is from 7 centimeters to infinity (Powers and Green 1978), which may be due to the small size of the rat's eye and its poor acuity (Green et al. 1980).

One consequence of this difference in depth of focus is that humans perceive blurriness after a change of about 1/3 diopter, but rats require a 14 diopter change to perceive any blurriness (Powers and Green 1978).

Answer 2

so they can see better and so they can see if people are coming