Favorite pets in the U.S. break down as follows:

1. cats

2. dogs

3. fish

4. birds

5. ferrets, rabbits

6. other small animals (rats, mice, gerbils, hamsters, etc.)

7. reptiles

More households in the U.S. have dogs than cats, but there are more cats per household for those households that have cats.  This is probably because some households may have, for example, one dog but two cats.

Information provided by Drs. Foster & Smith (drsfostersmith.com).
http://www.peteducation.com/article.cfm?cls=0&cat=1275&articleid=1324

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