A young girl contacted us to ask why her guinea pig is always nibbling or biting the hands of whoever holds her. It's very unusual for guinea pigs to bite, especially so persistently, but it does happen. It's even more unusual for them to bite so hard that it draws blood.
Beyond the #1 reason ("Put me down...I gotta pee!"), the most common reasons for nibbling or biting (in no particular order) are listed below.
Your hands smell like their food pellets, hay, veggies, fruit, fresh grass, or something else that just smells yummy. A guinea pig will quickly stop nibbling when s/he realizes it's your finger s/he is chewing on.
Noise or other commotion in the immediate area is frightening them. This commotion could be anything -- a thunderstorm, a vacuum cleaner, a loud TV or stereo, folks getting rambunctious with the family Wii. If something is frightening a guinea pig and it doesn't feel secure in your arms, it will...ahem...persuade you to put it back in its cage so it can burrow into a hideout house (where it does feel safe).
The guinea pig feels like it's being mishandled. Guinea pigs want to feel secure and well-supported. If a guinea pig feels like its feet are hanging in mid-air, or like your shoulder is too high of a perch, or that it's being jostled too much, it will let you know. Some guinea pigs don't like to be carried while you walk around the house or go up and down stairs, and will demonstrate their objections by biting you. No guinea pig likes to get caught in a "tug of war" between two people who want to hold him/her. And they have no tolerance for getting passed from person to person to person in a short stretch of time.
They're afraid of the person holding them. Guinea pigs will become permanently fearful of a person who has been angry or abusive toward them, or a person who has consistently mishandled them. If a pig's history is unknown to you (e.g., you got them from a shelter or found them abandoned outdoors), you may have to deal with the consequences wrought by whatever human(s) preceded you. A guinea pig that was abused by, say, little boys may always have a generalized fear of little boys. Love and patience from you may help lessen the fear, but may never fully overcome it.
Pain & Discomfort
A guinea pig that's feeling sore in one spot, or in pain all over, won't want to be held. Sources of pain or discomfort can include: bruising, strains, or sprains caused by rambunctious play; getting nipped by a cage mate; skin irritation that's painful to the touch; and arthritis. It should also be noted that sick pigs don't want to be handled much either.
As social and cuddly as they are, sometimes guinea pigs just want to be left alone. Maybe it's feeding time and they want to focus on the lettuce leaf you just gave them. Maybe it's their usual nap time. Maybe their cage mate has been crowding them with too much attention and they just want some space from everyone. Maybe they were in the middle of a good game with a cage mate. And maybe they just don't feel like being social, for no particular reason. Humans have days like that...so do guinea pigs (though a lot less often).
Bottom line: listen to and respect what your guinea pig is telling you. He's hoping you'll hear him sooner rather than later.