In After Earth, the Ursa are apparently bio-engineered to kill humans (for unspecified reasons).
Part of this bio-engineering is that apparently their makers (again, for unspecified reasons) decided that the Ursa should hunt by tracking the chemical secretions associated with a fear response.
If someone can completely banish fear, they become utterly invisible to the Ursa (called "ghosting").
What I can't figure out is why something would be engineered to track solely by this method.
Rather than focus on whether this is a plot hole, I'm interested in what benefits, if any, does tracking by "smelling fear" provide?
Was "fear smell" the only sense available to them? From Will Smith's character's description of his first time "ghosting", it almost seemed to imply that the Ursa couldn't even respond to tactile stimulation.