guniea pigs

Question
What does it mean when my boar guinea pig bites the butt of my sows?

Answer
depends, is he just nipping it or does it look more like he is gnawing it?

if it is a nip it could be as innocent as "oi shift" or "thats mine" if she is by food or a water bottle.
if it is gnawing then it is more a grooming/checking if she is in season, if the female pees on him then she isn't.

rabbits do it too and whichever it is it is a perfectly normal behaviour.
all the best.