All cats have the innate need to scratch on hard, vertical surfaces in their environment. There is a scent signal produced by the secretion of certain glands which exists between the claws. When done in the outdoors, humans rarely notice the claw marks left on tree trunks.
The most important sense that dogs possess is their sense of smell. Dogs have anywhere from 200 to 300 million olfactory receptors in their nose, while humans have only about 5 million. This means that a dog’s sense of smell is 40 times more developed than ours. For this reason, dogs rely greatly on smell to perceive and relate to the world around them.