Many dogs have characteristics that make them seem almost human at times, but they experience the world in a very different way than we do. Understanding their unique point of view helps make a person’s relationship with dogs even more rewarding than it would be otherwise.
The Sense of Smell
A dog’s sense of smell is remarkable. In comparison to humans, dogs have over 40 times the number of scent receptors in their noses, and a large proportion of the canine brain is dedicated to decoding what they smell. Scientists estimate that the canine sense of smell is anywhere between 40 and one million times stronger than ours, depending on the breed and the type of odor tested. Remember this the next time you are out for a walk. Try to be patient while your dog endlessly sniffs the same patch of ground. Who knows what type of information he is gathering?
One of the more fascinating ways that the canine nose is being put to use to benefit people is in the detection of some types of human cancer. For example, a study published in England tested whether dogs were able to determine if bladder cancer was present by sniffing samples of urine. Overall, they did a very good job, but most interestingly, the dogs kept insisting that one of the samples was positive for cancer while the researchers were sure it was not. Finally, the patient was retested and the doctors, not the dogs, were wrong.
Dogs have a good sense of sight, but if we could see through their eyes, we would be shocked at how different everything looks. The retina is the tissue at the back of the eye that converts light energy into nerve impulses to be sent to the brain. Cells in the retina called rods are primarily responsible for vision under low light conditions and for the detection of movement. Dogs have a greater number of rods in their retinas in comparison to people.
Dogs also make use of another ocular structure, the tapetum lucidum, to reflect light within the eye. This is also what causes the eyes of some animals to glow when light shines into them in just the right way. More rods and the tapetum lucidum allow dogs to see in dim light and pick out a moving object much better than we can.
Trade-offs are the name of the game in nature, however. The canine investment in rods comes at a cost: fewer cones — the retinal cells that are involved in color vision and the ability to see fine detail. Dogs are not completely color blind, but studies show they have difficulty differentiating between greens, yellow-greens, oranges, and reds; and greenish-blue colors probably appear grey to dogs. Also, canine eyes are set farther apart than are human eyes, so dogs have better peripheral vision but poorer depth perception than we do.
The standard for human vision is 20/20, but most dogs seem to be limited to about 20/75. To get an idea of what this means, stand 75 feet away from an object. For your dog to see it as well as you do, he would have to be only 20 feet away. Consequently, if you need to get your dog’s attention from a distance, don’t just stand still, try waving your arms, moving back and forth, or calling out his name.
Tomorrow: Hearing, Taste, Touch, and a Sixth Sense
Dr. Jennifer Coates
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