Dogs' value to early human hunter-gatherers led to them quickly ing ubiquitous across world cultures. Dogs perform many roles for people, such as hunting, herding, pulling loads, protection, assisting police and military, companionship, and, more recently, aiding handicapped (disabled) individuals. This impact on human society has given them the nickname "Man's Best Friend" in the Western world. Most breeds of dogs are at most a few hundred years old, having been artificially selected for particular morphologies( 形态学) and behaviors by people for specific functional roles. Through this selective breeding, the dog has developed into hundreds of varied breeds, and shows more behavioral and morphological variation than any other land mammal. Compared to equally sized wolves, dogs tend to have 20% smaller skulls, 30% smaller brains, and also have smaller teeth. The skin of domestic dogs tends to be thicker than that of wolves for use as clothing due to its greater resistance to wear and tear( 疾驰)in harsh(hard) weather. Dogs tend to be poorer than wolves at observational learning( 观察学习), being more responsive to instrumental conditioning. For dogs, other members of their kind are of no help in locating food items, and are more petitors. Dogs display much greater tractability( 温顺)than tame( 驯服的) wolves, and are, in general, more responsive toward positive conditioning and rewards. Unlike tame wolves, dogs tend to respond more to voice than hand signals.
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