do rats stand guard

I have 4 female rats the head rat has a tumor and thats about when the other 3 take turns sleeping and watching and few hours later other rat comes out and the one that was out goes in, expect the one with the tumor does not take a turn. the one thats been with the one with the tumor the longest protects her i have seen that before but Ive never seen them stand guard over their house before. I do know other rodents will but Ive never heard of rats doing it. I was just wondering if anybody else has heard of it or seen it?