The Sense of Smell—Dogs or Us?
By Max Sherman
“Smell is a potent wizard that transports us across thousands of miles and all of the years
we have lived. The odors of fruits waft me to my southern home, to my childhood frolics in the
peach orchard. Other odors, instantaneously and fleeting, cause my heart to dilate joyously or
contract with remembered grief. Even as I think of smells, my nose is full of scents that start
awake sweet memories of summers gone and ripening fields far way.” -- Helen Keller.
There was a recent report that researchers in France are working on a sensor to detect
explosives carried by individuals who hope to smuggle them through airport security systems. The hope is to create a device that could supplement or even supplant the best
mobile bomb detector—a sniffer dog.1 Dogs have a scent sensitivity many thousands of
times greater than humans. In fact, this ability is one of the more curious facts in nature.
Humans and other primates, however, have relatively good senses of smell.2
This is true even though human evolution has been characterized by a gradual
increase in vision and a reduction in the sense of smell. The change likely is due to a
progressive diminution of the nose over many eons as the eyes gradually moved to the
middle of the face to enhance the depth of vision. There is little doubt that over many
years this facial change has resulted in a progressive reduction in the proportion of functional olfactory receptor genes.
Mice, for example, have approximately 1,300 olfactory receptor genes, of which some
1,100 are functional, compared to humans, who have only some 350 functional genes
(smell receptors) out of approximately 1,000.3,4 Scientists still cannot explain how the
350 smell receptors are able to account for the human ability to detect thousands of different odors.5
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Humans, however, perform as well as, or better than other mammals. Many humans
actually make a living with their noses. Think of oenologists (wine experts), perfumers and
food tasters. This article will briefly explain the sense of smell, human nasal anatomy,
body odors of different ages and human olfactory abilities to detect odors and close with
a discussion about a dog’s unique capacity to smell. The subject matter may appeal to
those of us with a nose for news about the nose.
The Sense of Smell
Smell has been called the mute sense, one without words. According to Diane Ackerman,
“Lacking a vocabulary, we are left tongue tied, groping for words in a sea of inarticulate
pleasure and exaltation. We see only when there is light enough, taste only when we put
things into our mouths, touch only when we make contact with someone or something,
and hear only sounds that are loud enough. But we smell always and with every breath.“6
Humans may view smell as an aesthetic sense, but for most animals, smell is the
primal sense, one they rely on to identify predators, food and mates.7 Smell is the most
direct of all senses. Only substances volatile enough to spray microscopic particles into
the air have an odor.8 This ability allows us to enjoy the taste of food.
The nose, where the ability to smell begins, is composed of two nasal cavities
separated by a middle wall, the nasal septum. Initial detection of odors takes place at
the posterior of the nose, in the small region known as the olfactory epithelium. Odor
molecules flow back into the nasal cavity behind the bridge of the nose where they are
absorbed by the mucosa containing receptor cells bearing microscopic hairs (cilia).
Specialized proteins, known as receptor proteins, extend from the cilia. There are approximately five million of these cells, which then send impulses to the brain’s olfactory bulb or
smell center via fibers known as axons.
The olfactory epithelium also contains neuronal stem cells that generate olfactory
neurons throughout their life spans. Each of these neurons expresses only one odorant
receptor. The receptor is activated by a specific odor, which in turn activates a specific
region in the brain. Unlike most neurons, which die and are never replaced, the olfactory
sensory neurons are continually regenerated.9 When the olfactory bulb detects an odor,
it signals the cerebral cortex and sends a message straight into the limbic system. The
brain’s limbic system is an intermediary between the brain stem and the cerebrum, and is
involved in central control over instincts, motivations and feelings.10
Somewhere in this arrangement lies an intricate logic the brain uses to identify the
odor detected in the nose, distinguish it from others and trigger an emotional or behavioral response.11
Body Odors of Different Ages
Our natural body odor goes through several age-dependent stages in chemical composition as we grow older.12 In human and non-human animals alike, signals hidden within
body odor chemistry have been posited to aid in mate selection, kin detection, individual
recognition and sex differentiation.13,14
In humans, dermal body odors originate from a complex interaction between skin
gland secretions and bacterial activity,15 and skin gland composition and secretion change
in an age-dependent manner throughout development. Sebaceous glands are found over
much of the skin’s surface and they secrete a complex mixture of lipids and fatty acids,
both important precursors to human dermal body odor. The sebaceous gland is less active
in young age and reaches peak activity in adulthood. The gland sharply returns to low
activity in the mid-to-late portion of the seventh decade of life.16
To date, two chemically related compounds have been confirmed to vary with age:
nonenal and nonanal. Both compounds increase with age, particularly in older individuals,
who exhibit a sharp increase in concentration.17
Sniffer Dogs
Scientists have known since the 1950s that dogs and other keen-scented mammals such
as rats and rabbits have a specialized anatomical structure in their nasal cavities. Called
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the olfactory process, it is a large maze of highly convoluted airways that humans and
other primates lack. In dogs, the recess lies right behind the eyes and takes up almost
half of the interior of the nose.18
Using a computer model of the canine nose, scientists have discovered that when a
dog sniffs, each nostril pulls in a separate odor sample. The dog can tell which nostril is
pulling in the scent, so it knows which direction to go when tracking. The dog’s nose has
a unique nasal airflow pattern, which helps transport odor molecules quickly via a single
airway to the olfactory recess.19
There are several other differences between the human and dog olfactory systems,
which explains the dissimilarity in olfactory acuity or the sensitivity of the sense of smell.20
The first is the size of the nasal cavity and the amount of air that can be inhaled. German
shepherds, for example, can breathe in five times as much air as a human.21 Dogs have more
olfactory sensory cells than humans; estimates of 5 million in a human compared to 125
million in a dachshund and 300 million in a bloodhound.22 The dog’s olfactory bulb is about
40 times larger than the human’s.23 These differences result in a human being able to smell
a mixture of odors compared to a dog that can smell a range of distinct and different scents.
The dog also has something else a human does not; the vomeronasal gland. The
gland is actually a pair of long, fluid filled sacs that open into either the mouth or the
nasal cavity; the gland is located in the area above the roof the mouth. It appears to be an
accessory olfactory organ that allows dogs to identify scent and possibly pheromones.24
In addition to dogs’ uncanny ability to detect odors, they also have extremely keen
night vision. Dogs can see in light five times dimmer than that in which a human can see.
This is due in part to the tapetum, a mirror like structure in the back of a dog’s eye that
reflects light, giving the retina a second chance to register light that enters the eye. The
tapetum also causes dog eyes to glow at night.25
Final Thoughts
The olfactory bulb that relays smell information from the nose to the human brain contains
one of the densest collections of insulin receptors outside the pancreas. In one study,
researchers found that mice made obese on a high-fat diet exhibited dampened responses
in their olfactory bulbs. It is possible that olfactory dysfunction can cause or contribute to
obesity, and targeting smell may be an approach to treatment with drugs or devices.26
There also has been a recent report that scientists have identified a large multi-protein
complex in olfactory neurons in the fruit fly that plays a major role in selecting the carbon
dioxide receptors to be expressed in appropriate neurons. This could lead to other research
with mosquitoes to lower their ability to sense carbon dioxide and thus their ability to
locate human hosts.27 Obviously, there is still much to be learned about the sense of smell.
References
1. Fountain H. “Devices go nose to nose with bomb-sniffer dogs.” New York Times. 16 October 2012.
2. Shepherd GM. “The human sense of smell: Are we better than we think?” PLOS Biology. 2004; May 2(5):e146.
3. Young JM et al. “Different evolutionary processes shaped the mouse and human olfactory receptor gene families.” Human
Molecular Genetics. 2002; 11:535-46.
4. Zhang X, Firestein S. “The olfactory receptor gene super family of the mouse.” Nature Neuroscience. 2002; 5:124-33.
5. Rinaldi A. “The scent of life. The exquisite complexity of the sense of smell in animals and humans.” EMBO Reports. 2007;
July 8(7):629-33.
6. Ackerman D. “A Natural History of the Senses.” Vintage Books, New York, 1993.
7. Axel R. “The molecular logic of smell.” Scientific American. 1995; 273(4):154-9.
8. Op cit 6.
9. Op cit 7.
10. Nuland S. “The Wisdom of the Body.” Alfred A. Knopf. New York, 1997.
11. Op cit 7.
12. Mitro S et al. “The smell of age: perception and discrimination of body odors of different ages.” PLOS One. 2012;Vol
7(5):e38110.
13. Baum MJ et al. “Prenatal and neonatal testosterone exposure interact to affect differentiation of sexual behavior and partner preference in female ferrets.” Behavioral Neuroscience. 1990; 104:183-198.
14. Sergeant MJ et al. “Women’s hedonic ratings of body odor of heterosexual and homosexual men.” Archives of Sexual
Behavior. 2007; 36:395-401.
15. Wysocki CJ, Preti G. “Human body odors and their perception.” Japanese Journal of Taste and Smell. 2000; 7:19-42.
16. Pochi PE et al. “Age related changes in sebaceous gland activity.” Journal of Investigative Dermatology. 1979: 73:108-111.
17. Op cit 12.
18. Morell V. “The secret of a dog’s sniffer.” Science Now Daily News. 9 December 2009.
19. Ibid.
20. Stejskal SM. “Death, Decomposition, and Detector Dogs.” 2013; Boca Raton CRC Press.
21. Pearsall MD, Verbruggen H. “Scent : Training to Track, Search and Rescue.” 1982; Loveland CO, Alpine Publications.
22. Op cit 20.
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23. Correa JE. “The dog’s sense of smell.” Alabama Cooperative Extension System www.aces.edu/pubs/docs/U/UNP-0066.
Accessed 31 October 2012.
24. Snovak JE. “Barron’s Guide to Search and Rescue Dogs.” 2003; Hauppauge NY, Barron’s Educational Series.
25. Science Daily. “How well do dogs see at night? sciencedaily.com/releases/2007/11/071108140336.htm. Accessed 20
October 2012.
26. Wang SS. “Revealing the unexpected dangers of obesity.” Wall Street Journal. 29 October 2012.
27. Science Daily. “How cells in the nose detect odors: braking mechanism in olfactory neurons helps generate amazing diversity of sensors.” www.sciencedaily.com/releases/2012/11/121114172941.htm. Accessed 14 November 2012.
Author
Max Sherman is president of Sherman Consulting Services in Warsaw, IN. RAPS recently published a collection of Sherman’s
work, From Alzheimer’s to Zebrafish: Eclectic Science and Regulatory Stories. He can be reached by email by contacting
[email protected].
© 2013 by the Regulatory Affairs Professionals Society. All rights reserved.
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