I have limited experience with Great Pyrenees being used in a livestock guardian capacity. They WILL investigate a human intruder. They will remain vigilant. They may be friendly or they may be standoffish. They aren't generally aggressive with passive humans. That would be an undesirable trait since they are free roamers. As free roamers they aren't quite sure where property lines are. Being aggressive could lead to their demise. I have a lot of trust in the breed.