The way in which a dog interprets the word is very different from the responses of a human being. A dog’s sense of smell is far superior to that of a human, and is evident in the way a dog uses its nose while out for a walk or investigating an object it has found, it is thought that a dog’s sense of smell is at least 100 times more powerful than a human’s and possibly more for certain scents.
A dog’s hearing is also spurious to that of a human, Dogs seem to be more sensitive to some sounds especially those at high frequencies. Hence the use of “silent” dog whistles, audible to the dog but not to the human ear. This probably also explains the ability of a dog to detect the arrival of a particular car or distinguish the footsteps of its owner, well before these sounds can be recognized by human senses. A dog’s mobile ears help to pinpoint the source of a sound since they can be directed towards it. In vision, the dog is inferior to humans, at least during the day. There is some controversy as to whether dogs are color-blind or not. They probably have some colour vision but it is not good. Compared to a person, a dog has much less visual acuity and sees only moving objects well. At night, however, a dog sees better than a human being. The way a dog’s eyes light up in the glare of car headlight is an indication of this. Light is reflected from a layer at the back of the animal’s eye and passes through the light-sensitive retina twice, doubling sensitivity. Dogs have better peripheral vision, giving a larger visual field. So not only can they hear someone approaching from behind better than we can, but they can also hear them sooner.
Communication between dogs is impressive. Sounds, body signals, and chemical smells are all employed. Facial and body expressions indicate feelings. The eyes are also important. The stare, for example, is a threat signal usually given only by a dominant dog to a submissive one. If a person stares at a dog it will usually look away and perhaps roll on its side or back, indicating submissiveness. Occasionally a dominant dog will respond aggressively to a stare and will need to be reminded by vocal commands who is the head of the household.
The way the ears are held is an important indicator of expression. Ears are held back against the head show submission or fear. Erect ears indicate alertness. These expressions will almost always be combined with some indication from the mouth and lips and from the body. Certain dogs bare their teeth as a sign of aggression, but some also do it to the indicate pleasure, almost as if they were smiling. A tail held high usually indicates alertness. A wagging tail probably indicates excitement, and a tail held low may mean fear or a position of submission.
This may be accompanied by a hunched guilt looking posture, the meaning of which is unmistakable. Communication by body language is complex, and confusion can arise from fear and submission or excitement and aggression have components in common. Some breeds, by virtue of ear shape or lack of tail, are unable to communicate visually as well as others can. However, no one who has seen the wagging rump of an exited Cocker Spaniel can be left in much doubt to its meaning.
Their sense smell is very important. When two dogs first meet they usually smell each other’s face and then their inguinal regions. Scent plays a significant part in territory. When a male dog cocks his leg to mark a prominent object while out on a walk, he is deliberately masking the smell of dogs that have recently passed. Scratching with the back feet seen mainly in male dogs after defecation, leaves a chemical signal, known as a pheromone, from special glands between the toes. Feces may be used as scent markers, and a dog has anal glands that secrete a mixture of chemicals. Dogs also sometimes roll in foul-smelling substances. The strong odor may give extra social status. A bitch in heat gives off special smells from the vagina, also present in the urine, which indicate her sexual status.
Sounds used include barking, whining and howling. Barking is usually done in order to gain attention and was probably originally encouraged for watch dogs during domestication. Whining, often indulged in when a dog is left alone, is a distress call aimed at the owner, hardly ever at another dog. Howling Is probably an earning sound to protect territory.
Source: Encyclopedia of the dog.