Additionally, the statement that “animals can smell fear” is often heard, and can even be found in current popular literature. Taken literally, this statement seems absurd. Fear is an emotion, an internal response which is generated by an animal’s nervous system when the animal encounters a situation it perceives to be dangerous. It is not a volatile substance. One definition of “smell” given by the American Heritage Dictionary is, “A distinctive quality enveloping or characterizing something.” This gives some validity to such phrases as the “smell of success,” and the “smell of victory.” Likewise, the existence of a “smell of fear” seems slightly less ridiculous when it is considered in the context of such a broad (and non-olfactory) definition of smell. Therefore, it is likely that the statement “animals can smell fear” developed from the idea that animals can sense that a nearby animal is acting strangely, perhaps as a result of an alarm response.
It is well-documented that the fear response in most animals includes a series of visual, acoustic, and possibly olfactory stimuli which may alert a potential predator or attacker (Pruitt and Burghardt, 1977; Lorenz and Leyhausen, 1973). Therefore, while it is not possible for an animal to literally smell fear, an individual’s response to fear may include the release of certain odoriferous substances which can be detected by a nearby animal.
For example, in humans, the autonomic nervous system is responsible for enacting a stress response. This response is thought to function in preparing the body for immediate protective action. Characteristic physiological changes comprising the fear response in humans include (1) an increased hear rate, (2) an increase in blood supply to the muscles and to the brain, (3) release of glucose into the bloodstream, (4) dilation of the pupils, (5) increased breathing rate, and (6) an increase in sweat gland activity (Janis et al., 1983). In particular, the increase in sweat gland activity which accompanies a stress response in humans may contribute to an olfactory “fear” signal.
The human body has two types of sweat glands. The first is the eccrine, or sudoriferous, gland, which is a coiled structure responsible for the regulation of body temperature through evaporation of sweat from the body surface. The second type of sweat gland is called the apocrine gland. Apocrine glands are localized to specific areas of the body with the highest concentrations occurring in the region of the axillary organs (armpits) and the groin area (Stoddart, 1990).
In response to emotional stress and sexual stimulation, apocrine glands release secretions through hair follicles. The secretions contain cellular material that is broken down by bacteria, and thus body odor is produced (Memmler et al., 1992). The human stress response may therefore result in the production or intensification of one’s natural body odor which is detectable to a nearby animal. 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.
Certain non-human animals may also produce a scent as a result of fear or emotional stress. Fox and Cohen (1977) suggest that, in mammals, the production of odors may render the rank and the emotional state of an animal identifiable. Many mammalian species deposit various odors when they are alarmed. For animals which typically scent mark objects, such as cats and dogs, odors produced as a result of emotional stress are not usually deposited at a specific marking spot. However, the composition of the secretions which are deposited on a traditional or specific marking spot may actually be changed as a result of alarm reactions (Fox and Cohen, 1977). Studies have demonstrated that fear and emotional stress are conditions which may be communicated by odors in mice and rats, as well (Valenta and Rigby, 1968). One study also suggests that the genital scent glands of two prosimian primates are involved in producing a fear scent (Manley, 1974). Chemical signals produced under stress, however, are thought to function in alerting conspecifics of danger, rather than in eliciting aggression in an intruder. Therefore, taken literally, “the smell of fear” might be more appropriately termed “the smell of danger,” and may function as a warning to others after danger has been encountered, rather than being the cause of a dangerous encounter.
The use of pheromones to alert conspecifics or members of a social group to the presence of an intruder or a potential attacker is common in many animal species. For example, in the presence of an intruder, several species of social hymenoptera secrete pheromones which cause defensive behavior among conspecifics (Maschwitz, 1966). This “alarm pheromone” is thought to have two effects: (1) it alerts conspecifics to the threat of danger, and (2) it acts as a chemical repellent to the intruder. In this manner, the “fear scent” produced by a honeybee worker does not provoke aggression by a predator, but instead functions in hive protection. If an individual worker detects an intruder to the hive, the bee will release an alarm pheromone. Detection of a foreign odor by a honeybee worker may even be enough to elicit an alarm secretion. This alarm pheromone excites workers in or near the hive, and attracts them to the source, and thus to the intruder. The secretion itself does not cause an attack--the attack-producing stimuli come from the intruder, whose behavior usually guides the attack (Maschwitz, 1966). Therefore, if a person approaches a honeybee hive, his body odor (because it is foreign to the hive) may be sufficient to excite and attract the bees. If the person becomes afraid, and moves erratically, he is likely to be attacked by the bees. So, while smell does play a role in hive defense, the odor which the bees detect is not “the smell of fear,” but more likely is “the smell of something foreign.” And, ultimately, it is visual cues which drive the bees to attack the intruder.
Another example of the role that olfactory cues play in communicating alarm among animals is found in carnivorous mammals of the family Mustelidae. Mustelids are distinctive among mammals in their defensive use of anal scent glands to produce olfactory warning signals. All mustelids are characterized as having well developed anal scent glands. In particular, weasels, wolverines, and skunks have been documented to release a repellent odor from these glands when alarmed (Pruitt and Burghardt, 1977). Additionally, during traumatic experiences, the domestic cat and the sand cat are known to void their anal scent glands. The result is a pungent, unpleasant odor, which probably functions in defense as well (Pruitt and Burghardt, 1977). Thus, olfactory signals play a role in the alarm response of these mammals. These olfactory cues, however, appear to function in repelling an opponent rather than in inciting aggression in that opponent. However, if a stressed animal produces a scent which actually causes predators to attack, it is likely that animal will not survive for very long. Thus, it seems that natural selection would quickly act to eliminate animals which produce a “smell of fear” that does not function in some defensive or protective capacity as well.
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, 1976l). For predatory cats, seeing a small object running away is considered to be the stimulus for chasing (Lorenz and Leyhausen, 1973). Therefore, a cat can quickly detect a panic-stricken mouse through visual stimuli alone.
Many police departments and search-and rescue squads use trained dogs to track both suspects and victims through olfactory cues. Usually, the dog is presented with an item on which the object of a search’s odor may be detected, and the dog then follows a scent trail which will presumably lead to the person. The role of a possible “fear scent” in directing a trained dog to a suspect is not well documented. One contributor to the K-9 Academy for Law Enforcement Trainer’s Resource Centers Web page suggests that, “If you want to train your canine to pick out all the people who are afraid, you can do that . . . a guilty persons odor is coming out like a smoke bomb and smells of ‘fear’.” However, evidence which supports this claim remains anecdotal1,2, as systematic study in this area is difficult. Nonetheless, Sigma Pseudochemicals does offer vials of human “fear scent” to those purchasers who are interested.
In the strictest sense, it is doubtful that animals can smell fear. Some animals have been documented to release a scent in response to stress. Such a “fear scent” may play some role in alerting a potential attacker. However, it is more likely that an animal will become aggressive towards a fearful individual as a result of a combination of multiple sensory and behavioral cues. While odor may play a significant role in attack behaviors among animals, olfactory cues probably do not function alone to incite aggression in mammals.
My dogs are trained for cadaver search using both pseudo corpse and the real thing. There was a pseudo scent called ‘fear and trauma’ introduced to them at a training session. The very first alert on this scent was very interesting to me. Each dog reacted slightly different but both alerts were different for a normal cadaver find.
You could tell when the dog came into the scent cone. One dog stopped smelled the wind, and took a more timid ‘sneak up’ approach on where this scent material was buried. He was very tentative in his approach and scratching at the area. He approached the area with body lower to the ground and did a very tentative initial investigation. I guess once he discovered nothing was going to jump out and attack - he did finally give a few barks.
The second dog reacted differently. When she came into the scent cone - she immediately turned toward the scent. Body seemed to swell up and she became more stiff legged on her approach. Her approach was more straight to the scent. When she got to the area of the buried scent, she looked all around, sniffed the air, then went into her sniffing, barking alert.
This only happened the first time they were exposed to the ‘fear and trauma’ scent material. Now they alert to it just like they do for all cadaver work.
2 Heather E. Houlahan related an anecdote:
The one time my older dog, Lilly, was searching for (and found) a person who was alive and terrified, she alerted to him from an extraordinary distance -- almost a mile by air scent -- and was highly agitated while working out the scent.
That said, there are two alternative explanations for what happened (1) the lost person had been in place overnight -- longer than a “practice victim” will stay out for us, and thus his scent had been dispersing far and wide and (2) members of the search party, including me, were afraid that this person had committed suicide -- we were undoubtedly telegraphing anxiety by our voices, body language, etc.
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Janis, I. P. Defares, and P. Grossman. 1983. Seyle’s Guide to Stress Research. (H. Seyle, ed.) Vol 3, pp. 1-42. New York: Scientific and Academic Editions.
Lorenz, K. and Leyhausen, P. 1973. Motivation of Human and Animal Behavior: An Ethological View. New York: D. Van Norstrand.
Manley, G. H. Functions of the external genital glands of Perodicticus and Arctocebus. In Doty, R. L. 1976. Mammalian Olfaction, Reproductive Processes, and Behavior. New York: Academic Press.
Maschwitz, U. W. 1966. Alarm substances and alarm behavior in social insects. Vitamins Hormones. 24: 267-290.
Memmler, R. L., B. J. Cohen, and D. L. Wood. 1992. The Skin, in Structure and Function of the Human Body. Edition 5. Philadelphia: J. B. Lippincott Company.
Pruitt, C. H. and G. M. Burghardt. Communication in Terrestrial Carnivores: Mustelidae, Procyonidae, and Ursidae, in Sebeok. 1977. How Animals Communicate. Bloomington: Indiana University Press.
Stoddart, D. M. 1990. The Scented Ape: The Biology and Culture of Human Odor. New York: Cambridge University Press.
Thompson, J. G. 1988. The Psychobiology of Emotions. New York: Plenum Press.
Valenta, J. G. and M. K. Rigby. Discrimination of the odor of stressed rats. In Doty, R. L. 1976. Mammalian Olfaction, Reproductive Processes, and Behavior. New York: Academic Press.