Advanced AI abilities are always touted by games, but usually the effect translates into enemy soldiers playing peek-a-boo with you from behind a crate. So when we fired wildly in the direction of a charging mutant dog, we were pleased to see it turn tail and run, even though we'd only winged it. But we were even more impressed when, after chasing it over a hill, the dog returned to its pack and brought them galloping in our direction. Impressed, and dead.
Above: The beasts aren't stupid - neither are the humans
We even watched it happen to AI characters, as a rival merc took down one beast with a shotgun, persuading the second ugly mother to flee for cover. In the interests of research, we then shot him in the back a few times - in his injured state he was no match for us, so he began retreating back to his camp and reinforcements, covering his escape by shooting back at us. We're not talking Mensa candidates, but Stalker's inhabitants are shrewd survivors.