I have two guinea pigs and I've noticed how rarely they drink, almost to the point where there's either barely or no water gone from their water bottle each day. They do drink eventually, but it takes a very long time. This isn't out of the usual for them, it's been like this since I've had them. They are behaving perfectly normal and eating regularly. Despite this, I still worry that something may be wrong. Should I be concerned? If so, what can I do?