Grease gland
on the "Rump" of a male guinea pig (just above there bottom), if you part the hair you should find a greasy spot. This is called the "greased gland" and secrets a sticky smelly grease used for scenting. This needs extra special cleaning so boys dont get smelly. I dont have much trouble with my boys i find piggy shampoo is enough, but someone else on here might be able to give extra tips.