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Ever since childhood I've been told that if you encounter an unfamiliar dog, you should not show fear, because if you do, the dog is more likely to become aggressive.

I have been told that this is because "dogs can smell fear"

I'm sure there are many other factors which could potentially affect this interaction, but for the purposes of this question, I want to focus mainly on the ability to "smell fear."

Given the acuity of the canine sense of smell, it has always seemed plausible that the dog may smell slight persperation or even potentially some subtle shift in body chemistry associated with the "fight or flight" response, but has it been scientifically proven that dogs can actually smell fear?

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    I think you (and others) may be attaching too much significance to the word "smell." The phrase "I smell a rat" means that you suspect a spy or double-crosser is about but not that you literally smell a difference on someone. A more charitable phrase would be "dogs sense fear" in much the same way people do. – MrHen Jun 1 '11 at 21:25
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    I agree with MrHen. Dogs (and other animals as well) are extremely good in reading our body language. They may be able to recognize the fear from your posture change, walk change or something like that. – Suma Jun 1 '11 at 22:19
  • I think that this question should be re-named in order to specify whether it is about dogs smelling fear in humans or in other dogs. I did notice my dog having a very foul odour coming out of his anus when I put my hand on him while he was asleep (he was very scared, probably was having a bad dream). Even I could smell it. I think that's a common thing to happen with dogs. Humans probably don't produce this odour but I don't know. – Felipe Almeida Apr 21 '13 at 0:16
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    @FelipeAlmeida This is a very late answer, but dogs have an organ called anal gland that produces a foul-smelling liquid. Usually the gland is regularily emptied when the dog defecates, but it can also get blocked and later empty at once, emitting a very strong odour. It is almost certain that this is what happened to your dog. In fact the cause-and-effect is most likely the other way around: the blocked anal gland caused him some mild discomfort, which in turn caused uneasy sleep. – Moyli Nov 23 '15 at 23:31
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From http://zebra.sc.edu/smell/ann/myth5.html:

Thompson (1988) suggests that human thermoregulatory responses to stress or fear may provide a valid measure of the intensity of emotion, but do not provide much specific information about which emotion an individual is experiencing. Thus, if a person becomes fearful in the presence of an animal, that animal may be able to detect a new odor, and may therefore be alerted to the person's presence. it is, however, unlikely that such an olfactory cue provides the animal with enough information to detect that the person is fearful. Additionally, it is unlikely that such an olfactory cue acts alone to incite aggression in the animal (Thompson, J. G. 1988. The Psychobiology of Emotions. New York: Plenum Press. ).

Fear is motivating, and the fear response in most animals includes behavior modification. Therefore, in addition to the production of a "fear scent" by certain mammals (for which evidence does not abound), the fear response in animals includes a series of behaviors which can incite an aggressive response in a potential attacker. Even slight behavioral signs of fear which result from an animal's detection of a nearby predator might make it more vulnerable to attack. For example, the unusual or irrational movements which result from a fear response can immediately catch a predator's attention, and result in instantaneous attack (Curio, E. 1976. Zoophysiology and Ecology: The Ethology of Predation. Vol 7. New York: Springer-Verlag. )

An additional tidbit (not research backed up but to the best of my prior knowledge true):

People who are afraid of dogs often stare at them, which the dog interprets as being confrontational (source: http://canidaepetfood.blogspot.com/2009/11/do-dogs-sense-fear.html)

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It's not really proven, but there is circumstantial evidence to back this up. For example, when you are scared, you give off stress hormones. And they have a smell

The flood of epinephrine, norepinephrine and dozens of other hormones do release a very distinctive smell. This smell may not be obvious to most humans but there is no doubt a dog can smell it simply because their senses are greater than a human.

The dog may also be smelling the increase of sweat from you

The impact of excessive sweating grows worst under stress which may be psychological or emotional.

and here:

there is ample medical evidence that points to a link between sweating excessively and anxiety and stress

Also, a dog may not just smell your new odour, but detect a change in your heartbeat. It can sense your heartbeat from five feet away, and so if it was close enough, can detect your heart going faster and faster, which is caused by smell hormones.

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    -1: This answer is poorly referenced. Please provide proper (i.e. peer reviewed papers) references for: 1) hormones have smell 2) dogs can smell hormones in sweat (I assume we are not saying they smell them in blood) 3) the amount of hormones in sweat is sufficiently high that a dog can smell it 4) a dog can sense your heartbeat from five feet away – nico Oct 18 '12 at 21:31

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