The Chihuahua is the smallest breed of dog in the world and is named for the Chihuahua State in Mexico.

Appearance
Chihuahuas are best known for their small size and large erect ears.

The AKC (American Kennel Club) recognizes two varieties of Chihuahua: the long-coat and the smooth-coat. Many long-coat Chihuahuas have very thin hair, but other long coats have a very dense, thick coat.

Breed standards for this dog do not generally specify a height, only a weight and a description of their overall proportions. As a result, height varies more than within many other breeds. Generally, the height ranges between six and ten inches at the withers. However, some dogs grow as tall as twelve to fifteen inches. AKC show dogs must weigh no more than six lb (2.7 kg); the FCI standard calls for dogs ideally between 1.5 and three kg (3.3 to 6.6 lb), although smaller ones are acceptable in the show ring. However, pet-quality Chihuahuas (that is, those bred or purchased as companions rather than show dogs) can, and do, range above these weights, to ten pounds or even more if they have large bone structures or are allowed to become overweight. This does not mean they are not purebred Chihuahuas, it only means that they do not meet the requirements to enter a conformation show. Oversize Chihuahuas are seen in some of the best, and worst, bloodlines.
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Chihuahuas come in many colors, from solid blacks to solid whites, spotted, or a variety of other colors like fawn (tan), chocolate, blue (gray), silver, tricolored (chocolate, blue, or black with tan and white markings), brindle, and merle; and, each of these colors varies in shades and tones, as fawn can be a term to describe a tan dog from a very pale cream to a deep (almost red) tan, or any shade in between, and the chocolate coloration can range from a milky light shade of brown, to a deep mahogony brown, to a dark brown that is almost black.

Temperament
Chihuahuas are prized for their devotion and personality. Their alertness, intelligence and size make them easily adaptable to a variety of environments, including the city and small apartments. While Chihuahuas are often stereotyped as high-strung, correct training and socialization can result in an outstanding companion animal.

Chihuahuas are not well-suited as small children's pets because of their size and physical fragility. However, many Chihuahuas focus their devotion on one person, becoming overly jealous of that person's human relationships. This can be mitigated through socialization. Chihuahuas also tend to have a "clannish" nature, often preferring the companionship of other Chihuahuas over other dogs. Also, Chihuahuas seem to have no concept of their own size, and may fearlessly confront larger animals, which can result in injury.Chihuahuas are sensitive to the cold due to their small body size. Chihuahua owners often dress their dogs in sweaters or coats in cold weather. However longer-haired chihuahuas may be fine without additional protection and in the summer may still be known to pant.

Health
This breed requires expert veterinary attention in areas such as birthing and dental care. Chihuahuas are also prone to some genetic anomalies, often neurological ones, such as epilepsy and seizure disorders.

Chihuahuas, and other toy breeds, are also prone to the sometimes painful condition known as patella luxation.

Another genetic abnormality in Chihuahuas and toy breeds is hydrocephalus, or water on the brain. This condition surfaces in young puppies and often results in the death of a puppy by the time that it reaches six months of age. It is thought that this disease is often diagnosed by the pup having an abnormally large head during the first several months of life, but other symptoms are more noticeable (since "a large head" is such a broad description). Chihuahua puppies exhibiting hydrocephalus usually have patchy skull platelets rather than a solid bone, and typically are lethargic and do not grow at the same pace as their siblings. A true case of Hydrocephalus can be diagnosed by a veterinarian, though the prognosis is grim.

Chihuahuas are also known for their moleras, a soft spot in their skulls. Chihuahuas are the only breed of dog to be born with an incomplete skull. The molera does fill in with age, but great care needs to be taken during the first six months until the skull is fully formed. Many veterinarians are not familiar with Chihuahuas as a breed, and mistakenly confuse a molera with hydrocephalus. The Chihuahua Club of America has issued a statement regarding this often deadly misdiagnosis [1].

Chihuahuas are also prone to eye infections due to their large, round, protruding eyes and their relatively low ground clearance.

Chihuahuas exhibiting the Merle coloration , or out of merle parents, are prone to myriad additional health complications. The Merle coat pattern is a carrier of possible severe eye conditions and blindness, deafness, hemophilia, sterility, and numerous other health concerns. Buyers owning or wishing to purchase a merle Chihuahua should do extensive research on the possible health concerns of this coloration.

Although figures often vary, as with any breed, the average lifespan for a Chihuahua is approximately 8 to 14 years of age.