Guinea pigs bite as a means of communication or as a response to fear and pain. They may also nip accidentally when seeking to get a treat, and they bite each other as part of dominance behavior.
Guinea pigs often nibble on their owners' fingers or clothing to communicate stress.The guinea pig may be tired of handling, seeking privacy for urination, or protesting skin irritation from a mite infection or from having its fur stroked the wrong way. Guinea pigs also nibble as part of social grooming. Guinea pigs are more likely to nip when carried improperly or when stressed by noise.