They are not very durable and as such will try to stay as far back from their enemies as possible, shooting at them with assault rifles and ducking and weaving away from enemy fire. In the case that an enemy gets too close, they will either leap at them or hit them with a claw swipe to knock them away. If there is any loot on the field, they will run over to it and pick it up. If left alone long enough after picking up loot, they will stash it and the loot will become unrecoverable.
Thief Rat Tree: Make you slightly stronger,but mostly focuses on swiping loot off enemies.
Lab Rat: Makes you slightly slower, in exchange for very powerful attacks.Plague Rat Tree: Enhances corrosive damage and speed.