Cats beat out dogs... in sheer numbers, anyway. According to the American Veterinary Medical Association, there were about 81,721,000 cats in U.S. households as of 2007; that's compared with 72,114,000 dogs. Just over 32 percent of households owned a cat, and the average cat owner had at least two felines.
By the way, cats and dogs can live together without causing mass hysteria. A 2008 study published in the journal Applied Animal Behavior Science found that if cats and dogs are introduced when the cat is under 6 months old and the dog under a year, the two species can coexist in peace. Interspecies clashes may be nothing more than a breakdown in communication, the study found. For example, averted eyes signal aggression in a cat and submission in a dog. The animals introduced early on seemed to understand each other's signals, the researchers found — almost as if they were bilingual.