Guinea pigs can sometimes be territorial and will fight with each other. This usually happens between unaltered male guinea pigs who don’t know one another very well. They fight to defend their territory and to determine who will be the dominant of the two.
Guinea pigs of both genders that have been spayed or neutered may fight as well when they are first introduced. In most cases, the guinea pigs need to work out the hierarchy between them before they will get along.
Guinea pigs sometimes nip when they are being held because they have to go to the bathroom or are tired of being handled. If the nip was gentle, it was just a message to let you know a guinea pig needs to go back to its cage.
If a guinea pig bites you hard, however, it is probably afraid. You need to take more time bonding to win its trust. Give treats and spend time near the cage, talking to it. Don’t rush into handling a guinea pig. Wait until it seems comfortable with you just petting it while he’s in his cage before you attempt to pick it up.
Whenever a guinea pig bites you or someone else while it is being handled, do not put it down. If you do, you are teaching it that biting ends the handling session. This will encourage a guinea pig to bite more. Instead, spend more time bonding while it’s in the cage. Next time you handle it, try wearing a light gardening glove so if it bites, it won’t hurt you.