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.

  • 6
    Because Scientology. – John O Jun 5 '13 at 13:38
  • 2
    Xenu can sense your fear – Monty129 Jun 5 '13 at 18:13
  • I haven't seen the film, and this makes me want to even less. There's no reason to fear it because it can't even see you! – PointlessSpike Apr 8 '15 at 13:45
  • 1
    Because terrible writing. – Valorum Jan 24 '16 at 0:02
  • @Richard I wish I could accept that as an answer.... It's certainly the most accurate description. – Beofett Jan 24 '16 at 0:04

Well, at first glance the idea of a predator that can only track by fear pheromones certainly seems stupid. And second and third glance as well. So, it's probably just stupid.

But, here is one idea where it just might make sense: if you were going to biologically engineer a really horrific monster to attack an alien species, maybe you wouldn't want it to be too good or too versatile, because it might be hard to control - it might even turn on you! Deliberately crippling the design so that they could only attack humans using the narrowest possible set of cues might make them less than perfect predators, but guarantee that they would never threaten anything else.

Here's another idea: by making the humans invest massive amounts of training and resources to counter the Ursa, the aliens could be distracting the humans and force them into adapting into a state that makes them more vulnerable to other sorts of attacks. So the humans spend all their time 'ghosting' and using spears, and next time the aliens attack with machine guns and the humans are caught off guard. The whole Ursa thing could just be a feint...

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    +1... restricting its ability to sense targets as a means of safeguarding the creators is a really good point! – Beofett Jun 5 '13 at 18:07

Terrified humans are easy to track, catch and kill. If a human cannot control its fear response, and we are lead to believe this is a skill possessed by few humans and not easily trained, it makes for a very thorough and efficient hunter.

BUT: This breaks down under closer scrutiny, requiring someone to consider the behavior of the Ursa, its design and its limitations. The Ursa were a genetically-designed biological mechanism depending on its ability to cause fear to track and attack its prey.

  • The only reason the creature worked so well is the creature's ability to generate fear by its horrific appearance and superhuman ferocity. In light of these abilities, the human response is to scream, run, increases our heart rate and perspiration.

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  • This would seem like a decent idea until you consider that most animals which hunt by scent have prodigious tracking abilities. On Earth, scent tracking animals like bloodhounds can track a scent from a target that has passed an area days before, using as little as two cells as a reference to track.

The Bloodhound's physical characteristics account for its ability to follow a scent trail left several days in the past. Under optimal conditions, a Bloodhound can detect as few as one or two cells. The Bloodhound's nasal chambers (where scents are identified) are larger than those of most other breeds. The number of olfactory receptor cells are 4 billion in a bloodhound, compared to just 5 million in a human and 100 million in a rabbit. The surface area of bloodhound olfactory epithelium is 59 sq.in. compared to human's 1.55 sq.in. (10 sq.cm.) Wikipedia > Bloodhound

So why would a standing human who is not moving suddenly stop having a scent?

  • This implies the Ursa's sense of smell isn't all that accurate or sensitive, or is sensitive to a particular singular chemical element at a specific density or threshold. Like the mosquito which hunts by carbon dioxide density near living creatures.

  • It implies their tracking ability may be keyed to a particular secreted hormone which is only present when humans sweat in fear.

  • Some of the hormones involved during the state of fight-or-flight include epinephrine and norepinephrine and cortisol. Epinephrine regulates heart rate and metabolism as well as dilating blood vessels and air passages. Norepinephrine increases heart rate, blood flow to skeletal muscles and the release of glucose from energy stores. Cortisol increases blood sugar and helps with metabolism.

  • Of this list, only the release of glucose should have a sufficient scent profile to be significant in the atmosphere, likely increasing the acidity of carbon dioxide exuded by terrified humans.

As a counter:

  • There are some scientists who believe sweat has the ability to be an information resource used to track terrorists due to its complex mixture of proteins and byproducts of our metabolism emitted from our pores.

  • It also contains volatile compounds, only some of which our noses pick up. They are investigating those compounds with heavy molecular weights, like proteins and peptides, which stick around long after a person and his or her smell have vanished.

With this information at hand, we have to assume the creators of the Ursa:

  • Created an organism with a targeted sense of smell, counting on their (probably limited) understanding of human physiology to track a specific hormone or chemical signature they recognize and could program their creature to track specifically.

  • They never considered humans would have the ability to master their fear response sufficiently to NOT exude whatever signature chemical they have designed their creation to track, rendering them completely blind.

  • Whatever the source creature the creators of the Ursa used it was not an animal which hunted by scent normally. Such a modification would be able to track the cellular material of humans even if they were NOT afraid in a fashion similar to bloodhounds on Earth.

  • There is one other possibility: A creature not of Earth, may not have the ability to easily distinguish creatures from Earth, chemical differences may make it more difficult to perceive or track us. Hence the Ursa may have been created with a specific chemical trigger the aliens recognized to give them the ability to lock-on to a target. See: Chirality

  • 1
    +1 Good info. But it still brings up the question: why (presumably) remove other senses? – Beofett Jun 5 '13 at 16:01
  • I don't believe the creature HAD other senses. I suspect this construct was exactly that, a construct, created with a particular purpose in mind, having no existence before it was DESIGNED... – Thaddeus Howze Jun 5 '13 at 16:02
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    @Beofett They were made by an alien species, the presumption of other senses is not a safe assumption. – user1027 Jun 5 '13 at 18:36
  • @Keen yeah, that's why I didn't include it in the actual question. The second to last bullet point in this answer does imply that if an existing creature was modified to create the Ursa, it would have had some other form of sense, but that's a big if, and Thaddeus seems to be of the opinion that most likely there was no source creature (which does seem plausible). – Beofett Jun 5 '13 at 19:06
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    Downvoting? Why? I have made the strongest case for a rational development, on a case by case basis. Perhaps what this really means is it is time to consider retiring from the Stack. – Thaddeus Howze Nov 27 '13 at 9:11

Id like to think about the fear based tracking system as a technological problem rather than a benefit. As it has been mentioned earlier, the creatures were designed to only hunt humans. They did not want the creatures to be able to go rogue and turn on thier creators, hence the first waves lack of breading organs and the use of a one track sensory-identification system. However they could just as easily have used base human scent as a means for tracking. It is likely that human fear pheromone was the only scent the aliens were able to pick out. They may have even picked the scent thinking that it was human aroma, and did not realize that thier sample was tainted.

This explains the somewhat random redesigns of the ursa body plan. People kept on winning against the ursa due to ghosting, yet the aliens never got feed back on the creatures performance, so they simply kept upgrading the creatures hardware. Very similar to cracking Inigma durring war Two only with BO.

More than anything though this is a narrative exploit more than anything, to let the hero win by litteraly overcoming his fear, and to justify its creepy no eyes face. Really a person just needs to close in their scent by wearing a body suite, or sitting inside an air conditiond tank to ghost an ursa. See also the anime Blue Gender and the movie Mimic for variations on this theme. Also the movies Nausica and of course, Pacific Rim on the technological perspective.


The benefits of tracking by fear is simple. In a real life study that was done at the University of Toronto they did a running test. When you are running normally for no reason you are not motivated and you don’t sweat as much and there is no evidence of someone even running there before. The second one was alot faster and his odour was alot stronger and most of all he had a high chance of having a heart attack and there was evidence of someone being there. So for me i think the reason the Ursa's where given the power to track by smelling fear is because when you are scared you are more likely to be found because of your increased odour and heart rate. The Ursa's can hear that your heart is pounding out of your chest and when you are that scared you are more likely to have a heart attack. So even if a Ursa just looked you in the eyes when you’re in that state you could die. Also when someone sweat's animals can smell that especially since the Ursa's have evolved to kill humans. I could be wrong but that is only my personal opinion on why i think they made the Ursa's track by smelling fear.


I think that the creators had created the Ursa for a type of entertainment: they pit the Ursa against (un)willing competitors in a game of survival - the challenge being to not be detected (not showing fear), thus not getting killed. When they learned of the humans occupying Nova Prime, they presumably looked upon this as an affront - so they sent what they had available to deal with the problem. If all they sent were the Ursa, we must not have appeared as enough of a threat to deal with personally.


I think humans have a very good motivation to create it not that it would be wise in the long run. In terms of warfare, we constantly are trying to make it so that people are not put at risk unnecessarily. If this creature is design to track human fear, and when in battle naturally how can you not get afraid at some point? This would be an inhumane use of a creature and a gross form of combat but just saying hypothetically to argue that there are reasons for the Ursa to exist. It would support the fact that the Ursa seems to kill humans but leave them skewered and uneaten on tree branches.

To understand exactly why the Ursa is around read this site. http://io9.com/the-alien-backstory-you-didnt-see-in-the-after-earth-t-509618063

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