how is their behavior?
highly affectionate, playful, extremely patient and eager to please.
Cavalier King Charles Spaniel
The breed is highly affectionate, playful, extremely patient and eager to please. 
how big does a king charles cavalier get Historically the Cavalier King Charles Spaniel was a lap dog and is small for a spaniel, with fully grown adults comparable in size to adolescents of other larger spaniel breeds. did this species diverge from some other group? Yes - spaniel can you please describe this species? A type of gun dog with a long silky coat and drooping ears, broad muzzle. The prevailing color is liver and white, sometimes red and white or black and white, and sometimes deep brown, or black on the face and breast, with a tan spot over each eye. are they popular? Yes - The Cavalier King Charles Spaniel has grown in popularity in the United States and ranks as the 19th most popular pure-breed in the United States.
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Cavalier King Charles Spaniel [SEP] Temperament
The breed is highly affectionate, playful, extremely patient and eager to please. As such, dogs of the breed are good with children and other dogs. Cavaliers are not shy about socializing with much larger dogs. They will adapt quickly to almost any environment, family, and location and suit city and country life. Their ability to bond with larger and smaller dogs makes them ideal in houses with more than one breed of dog as long as the other dog is trained. Cavaliers rank 44th in Stanley Coren's "The Intelligence of Dogs", being of average intelligence in working or obedience. Cavaliers are naturally curious and playful, but also enjoy simply cuddling up on a cushion or lap, making them excellent companion or lap dogs for medical patients and the elderly.