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These dogs specialize in pursuing prey, keeping it in sight, and overpowering it by their great speed and agility. They must be able to quickly detect motion, so they have keen vision. Sighthounds must be able to capture fast, agile prey such as deer and hare, so they have a very flexible back and long legs for a long stride, a deep chest to support an unusually (compared to other dogs) large heart, very efficient lungs for both anaerobic and aerobic sprints, and a lean, wiry body to keep their weight at a minimum.
The typical sighthound type also has a light, lean head, which is referred to as being dolichocephalic in its proportions. This shape can create the illusion that their heads are longer than usual. Dolichocephaly is actually a characteristic of wolves and other wild carnivores, and is related to the need for stereoscopic vision in active hunting. For reasons not yet understood, most domesticated animals including dogs (other than the sighthounds) have become brachycephalic—short-headed—and have partially lost this aspect of their field of vision (see cephalic index).
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Sighthounds,
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Scent hounds (Scenthounds) are a type of hound that primarily hunts by scent rather than sight. The Scenthound breeds are generally regarded as having some of the most sensitive noses among canines.
Scenthounds specialize in following a smell or scent. Most of these breeds have long, drooping ears. One theory says that this trait helps to collect scent from the air and keep it near the dog's face and nose. They also have large nasal cavities, the better to process scent. Their typically loose, moist lips are said to assist in trapping scent particles.
Most of these breeds have deep, booming voices and use them actively when running, and especially when following a scent trail. Although this can be a nuisance in settled areas, it is a valuable trait that allows the dog's handler to follow the dog or pack of dogs during a hunt even when they are out of sight, such as when following a fox or raccoon through woodland.
Scenthounds do not need to be as fast as sighthounds because they do not need to keep prey in sight, but they need endurance so that they can stick with a scent and follow it for long distances over rough terrain. The best scent hounds can follow a scent trail even across running water and even when it is several days old.