Nope, it's a mob characteristic.

They tried to give mobs a bit of flavor, which is nice.

They also had spawn cycles based on day/night, maybe even the month but that has gone by the wayside from what I can tell for now.

I do hope that makes it in for the revamp of the world zones, but they have to make sure mob distribution is OK so the economy can function.

Finding the mobs and understanding their cycles was pretty fun for me as its a semi-sandbox characteristic.