Principles and Applications of Domestic Animal Behavior (Cabi) by Price Edward O

Author:Price, Edward O. [Price, Edward O.]
Language: eng
Format: epub
Publisher: CABI
Published: 2008-10-08T16:00:00+00:00

Fig. 11.4. Threatening and submissive postures in the dog. Note the features that are opposites (e.g. ear and tail positions, contour of the spine, general posture) (Darwin, 1872).

Visual acuity

Visual acuity involves the degree of detail that can be seen. Humans have very good visual acuity relative to most domestic animals. This is because our retinas have proportionately more cone cells to rod cells. Cone cells in the retina of the eye enable us to perceive both well-defined and color images. Rod cells are more sensitive to light, which favors night vision, but they provide images that are coarse and poorly defined. The images are not out of focus, they just lack detail.

Nocturnal animals like domestic cats (F. domestica ) have retinas with proportionately more rods than cones. In addition, the pupils of the cat’s eyes can enlarge 170% in the dark to capture more light for the retina. As a consequence, they have excellent night vision at the expense of visual acuity and color vision. Cats can see well at light intensities approximately ten times lower than the ideal light intensities for humans.

Dogs (C. familiaris ) are visual generalists; their visual acuity is reasonably good at both high and low light intensities. Some breeds of dogs are myopic (near-sighted). One study showed that 53% of German shepherd dogs could not focus well at distances over 0.5 m. Rottweilers also tend to be myopic, as are about 10% of Labrador retrievers. The visual acuity of dogs and horses (E. caballus ) is about the same (approximately 20/60 to 20/70 on an optometrist’s scale for humans).

Color vision

Colored objects do not emit their characteristic color, per se. Rather, they absorb visible light of all frequencies impinging on them except for light in the frequency range perceived as the color observed. Grass is ‘green’ to us only because: (i) light in other portions of the color spectrum is absorbed by grass; and (ii) we have ‘green’ cones in the retina of our eyes which send signals to our brain that are interpreted as ‘green’. Animals which do not have ‘green’ cones see grass as a different color than we do.

Color vision in humans and many other primates is highly developed due to a proportionately large ratio of cones to rods in the retina. Primate vision, including our own, is trichromatic. Humans, Old World primates and apes and some New World monkeys possess three types of cones in the retina capable of processing wavelengths in the blue, green and red range of the color spectrum, respectively. Trichromatic color vision occurs when the signals from these three cone types are compared in the retina and brain. For example, ‘blue’ light stimulates the ‘blue’ cones a lot; the ‘green’ cones a little; and the ‘red’ cones not at all. This sensory mixture (in the brain) yields what we call ‘blue’. ‘Green’ light stimulates the ‘green’ cones a lot and the ‘blue’ and ‘red’ cones a little. This mixture yields ‘green’. ‘Red’ stimulates the ‘red’ cones a lot, ‘green’ cones a little and ‘blue’ cones not at all.

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