Dogs are mammals, usually to be kept as pets, for work on farms or for the police. Some dogs are trained to be rescue dogs and join teams such as mountain rescue.[4]

They have been bred by humans from ancestral wolves. They were the first animal to live with humans.[5]

There was a lot of different types among wolves in the Late Pleistocene.[1] The dingo is also a dog, but many dingos have become wild animals again and live in the wild, away from humans (parts of Australia).[6]

Today, some dogs are used as pets, and others are used to help humans do their work. They are popular pets because they are usually playful, friendly, loyal, and listen to humans. Thirty million dogs in the United States have been registered as pets.[7] Dogs eat both meat and vegetables. Often mixed together and sold in stores as dog food.[8] Dogs often have jobs including police dogs, army dogs, assistance dogs, fire dogs, messenger dogs, hunting dogs, herding dogs, or rescue dogs. They are sometimes called "canines" from the Latin word for dog - canis. Wolves are also canines. A baby dog is called a pup or puppy. A dog is called a puppy until it is about one year old.[9]

Dogs are sometimes known as "human's best friend" because they are kept as pets, are usually loyal, and like being around humans. Dogs like to be petted, but only when they can first see the petter's hand before petting. One should never pet a dog from behind.[10]

August 26 is National Dog Day worldwide.[11] While March 26 is National Puppy Day in the United States.[12]
Appearance and behaviour

Dogs can smell and hear better than humans but cannot see well in color because they are color blind. Due to the structure of the eye, dogs can see better in low light than humans. They also have a larger field of vision.[13]

Like wolves, wild dogs travel in groups called packs. Packs of dogs are listed by rank, and dogs with low rank will submit to other dogs with a higher rank. The highest ranked dog is called the alpha male. A dog in a group helps and cares for others. Pet dogs often view their owner as the alpha male.[14] 

Different dog breeds have different lifespans. In general, smaller dogs live longer than bigger ones.[15] The size and the breed of the dog change how long the dog lives on average. Breeds such as the Dachshund usually live for fifteen years, Chihuahuas can reach age of twenty. On the other hand, the Great Dane has an average lifespan of six to eight years; some Great Danes have lived for as long as ten years. An American Bulldog lives for around 15 years. Bigger dogs will have smaller lives than smaller dogs because of the pressure on its heart to move around.[16]

Dogs are often called "man's best friend" because they fit in with human life. Dogs can serve people in many ways. For example, there are guard dogs, hunting dogs, herding dogs, guide dogs for blind people, and police dogs. There are also dogs that are trained to smell for diseases in the human body or to find bombs or illegal drugs. These dogs sometimes help police in airports or other areas. Sniffer dogs (usually beagles) are sometimes trained for this job. Dogs have even been sent by Russians into outer space, a few years before any human being. The first dog sent up was named Laika, but she died within a few hours.[17]

There is much more variety in dogs than in cats. That is mainly because of the way humans have selected and bred dogs for specific jobs and functions. It may also have something to do with the fact that dogs are pack animals, whereas cats are not.[18] 