An excerpt from: Detector
Dogs and Scent Movement
Written by Tom Osterkamp
is concerned with the internal and external aerodynamics of the nose involved
in the sense of smell (olfaction) as they relate to training and searching with
search dogs (SDs).
OF SMELL involves detection and perception of chemicals (odorants or
scent molecules) inhaled by the dog. These scent molecules in the environment
enter the dog’s nose in the gas phase, in the solid phase as particulates from
a source, and attached to particulate matter such as dust or skin flakes. In
the warm and humid environment of the nose, scent molecules in the gas phase
and those detached from particulates contact sensors (receptors) that generate
electrical signals which are sent to the brain. Signal processing and learning
by the brain result in the “perception” of an odor or scent.
links the dog’s brain to their external environment. Figure 1 is a schematic
diagram of the “wiring” system for olfaction in mammals, including dogs.
There is a
cavity (olfactory recess) at the rear of the nose behind and below the eyes
with a lining of tissue (epithelial layer) that contains the receptor cells (neurons)
(Figure 1). The ends of the receptor cells have 10 to 30 cilia, tiny hairlike
structures immersed in mucous, that are in contact with inhaled air containing
odorants (Marples 1969). The binding sites for odorants in the air are located
on the cilia. Each receptor binds to a single type of odorant. When this
occurs, the reaction produces a tiny electrical signal that is transmitted to a
kind of junction box (glomerulus) in the olfactory bulb of the brain. Several thousand
receptors for a specific odorant are randomly distributed in the nasal lining
and are connected to the same glomerulus. Mitral cells in the glomerulus send
the signal to the olfactory cortex of the brain where information from several
types of scent receptors is combined into a pattern (an odor object) that
characterizes the perception of a scent. The brain can do this because each
odorant activates a unique combination of receptors.
also possesses properties that can contribute to scent discrimination (Schoenfeld
and Cleland 2005). These include volatility and water solubility that influence
movement of molecules through the nose during inhalation. The random distribution
of receptor cells makes it possible for dogs to modify scent patterns by
influencing the number of molecules reaching different receptors during a
sniff. Dogs can then modify and analyze scent patterns by regulating their
another olfactory system for detecting scent that uses the vomeronasal organ in
the roof of a dog’s mouth. It is thought that this system is used primarily for
detecting pheromones relating to sexual activity. However, retrievers used in
duck hunting, trailing dogs following a trail through water, and water search
dogs have been observed tasting the water. The process is different from
drinking in that the mouth is partially opened and water flows through it and
out the sides of the mouth. This suggests that the dogs may be using the
vomeronasal organ to detect scent in or on the water or it may be possible for
the dogs to “taste” scent.
foregoing description of the olfactory system of mammals suggests that the
physical and chemical characteristics of odorants that allow them to bind to
receptors form the basis for the dog’s perception of scent. While binding to
receptors is an initial and necessary step in olfaction, it appears that memory
and perceptual learning play a fundamental role in scent discrimination and
perception (Wilson and Stevenson 2003; Lovitz et al. 2012). The process of
perceptual learning involves continued experience (training) with a scent which
leads to improved detection, recognition, discrimination, and perception of
that scent. It is important to train regularly with all the scent variations
that SDs will be tasked to find (Goldblatt et al. 2009).
scents are mixtures of many odorants. A mixture of only two odorants can result
in a new scent that is weaker or stronger than either odorant, or one odorant
can mask the other partially or completely. This makes it impossible to predict
how a mixture will be perceived by SDs. The results of studies of mixtures of
four or more types of scent molecules (Wilson and Stevenson 2003; Lovitz et al.
2012; Goldblatt et al. 2009) indicate that people, dogs, and other animals do
not perceive all the individual scent molecules in a source but perceive the
mixture as a whole; it is synthesized. The idea that dogs can smell every
component in a stew is probably not correct.
processing based on experience uses memory and pattern recognition to form an
odor object for the mixture. The process is similar to the sense of sight in
that the brain does not perceive each pixel of a tree but forms a visual object
that it perceives as a tree. Formation of odor objects by the brain is a
relatively simple, efficient, and flexible way of perceiving odors in the
environment that gives an almost unlimited ability to smell new odors. Thus,
the search object that SDs seek is likely an odor object (Gazit et al. 2005a).
description of the sense of smell is primarily concerned with the details of
sensing and perception that are internal processes. However, considerable
practical information can be obtained by considering the fluid dynamics (air
and scent movement) interior and exterior to the dog’s nose. Dogs smell by
sniffing and Figure 2 is an accurate, three-dimensional model of the
left canine nasal airway, reconstructed from high-resolution MRI scans (Craven
2008; Craven et al. 2010).
inhaling, air is drawn into the nostril from a distance up to about 4 inches.
Dogs tend to reduce this distance to zero (Settles and Kester 2001), which
provides them with the highest scent concentration, allows them to sample the
source independently with each nostril, and to discern the spatial distribution
of the source. Flow paths during exhalation bypass the olfactory recess,
leaving scent-laden air there for more time, which enhances absorption of scent
molecules. Sniffing frequency for free-air sniffing is about 4 to 7 times per
second in short bouts. Sniffing can be observed in the movement of the thin
skin on the side of my Beagle’s nose but is difficult to see on the side of my
Lab’s nose due to the thicker skin. If the source is inaccessible, the bouts
are longer with a frequency as low as of one sniff every 2 or 3 seconds. Notice
the long deep sniffs a dog takes when sniffing at the crack of a closed door to
an inaccessible room.
the inhaled air is used for respiration, and part is directed into the
olfactory recess (Figure 2) where hundreds of millions of receptor cells are
located (I would like to know who counted them). Thus, olfactory and respiratory
airflows are separated, each with a distinct flow path and velocity through the
exhalation, an interior flap of skin at the front of the nose closes, and slits
on the side of the nose cause air jets to be directed to the sides of the dog’s
nose and downward (Settles et al. 2002). This nearly eliminates mixing of the
exhaled air with the inhaled air so that fresh, uncontaminated scent is introduced
into a dog’s nose during sniffing. Laboratory experiments have shown that dust
particles smaller than about 100 μm can be made airborne and subsequently
inhaled (Figure 3). This indicates that these jets can also dislodge
particulates, skin flakes, and, probably, adsorbed volatile organic compounds
(VOCs) from surfaces.
of cadaver dogs (CDs) show that dogs cast about, typically cross wind, when
searching for a buried source or a hidden one on the ground surface. On
detecting the source, their movements become more directed as they follow the
scent plume. As they approach the source, their noses are close to the ground
surface and they typically pass over the source while sniffing, and then back
up slightly until their noses are directly over the source or downwind of it.
The dogs attempt to put their noses as close as possible to the source before
giving their alert, a desirable behavior. This method maximizes the ability of
the dogs to detect scent since they can detect scent molecules in the air,
cause particulates to become airborne and then inhaled, and actively dislodge
scent molecules from surfaces which can be inhaled. Not all dogs use this
method exactly. There are differences between dogs and between different breeds
of dogs (e.g. hounds and some dogs tend to work with their noses always close
to the ground).
abbreviated description of the scenting process of dogs indicates that dogs
smell VOCs in the gas phase and from particulates (since these may emit VOCs)
that contact the receptors in their nose. They cannot smell particulate materials
such as skin flakes and dust particles directly. Skin flakes (Figure 4) contain
volatile glandular secretions (skin oils) and bacteria, which produce VOCs that
dogs can smell (Syrotuck 1972). Dust particles may contain VOCs from
explosives, drugs, and cadavers since, under dry conditions, dust particles
adsorb VOCs on their surfaces. With the dog’s head nearly vertical (Figure 3),
the slits on the sides of the nose direct the exhaled air jets downward and
backward along the surface, which causes particulates to become airborne and
allows the dog to inhale some of them. On inhalation, the particulates are
brought into the warm and humid environment of the nose. For dust particles
with adsorbed VOCs on their surfaces, the humid environment provides water
molecules that replace the adsorbed VOCs and allows their transfer to the
receptors where they can be detected.
Sargisson (2005) proposed a variation of the effects of the air jets observed
by Settles et al. (2002). When sniffing dry soil surfaces, the dog’s moist, exhaled
air jets may cause VOCs adsorbed on the dry dust particles to be replaced by
water molecules, and be released into the air where the dog can inhale and
detect them directly. While this hypothesis is plausible, Phelan and Webb
(2002) suggest that it may not be necessary because of the robust scenting
ability of dogs.
that the scenting methods used by SDs to detect sources involve direct sniffing
of source VOCs and airborne particulates. Dogs can also use exhalation air jets
to remove VOCs and particulates from surfaces which can then be inhaled. VOCs
adsorbed on the particulates can then be desorbed in the humid and warm air in
their noses, which frees them for detection. These methods are part of the
sniffing process which dogs use to detect scent. In addition, conditions
dictate which methods are most efficient, and dogs appear to learn to select
the appropriate methods for those conditions.
It is of
interest to know whether dogs use their eyes in addition to their noses to
locate sources. Explosive dogs (EDs) have been tested under both virtually dark
(very low light intensity) and full light conditions in controlled (indoor) and
uncontrolled (field) environments (Gazit and Terkel 2003b). The main sense used
by these dogs for detection was their scenting ability, not only when vision
was difficult (in very low light intensity) but also in full light. Neither the
presence nor the absence of light influenced the dogs’ detection ability. However,
in addition to scent, search and rescue (SAR) dogs use their eyes to locate
visible sources or subjects. Some hounds that trail with their noses to the
ground learn to look for subjects. This difference between EDs and SAR dogs
appears to be due to their training. EDs are trained almost entirely with hidden
sources that can only be found using their scenting ability. SAR dog training
generally includes subjects and sources that are sometimes visible, especially
at short distances, so that the dogs learn to look for them.
little information on limits to the ability of CDs to detect VOCs from buried
decomposing bodies, but experiments with EDs have shown that their sensing
thresholds for explosives can be lower than those of laboratory instrumentation
(Phelan and Webb 2002 or 2003). There were differences in the sensitivities of
the dogs. Not all dogs could reach these low levels and some dogs could not
sense even high levels. There were differences in the sensitivity of an individual
dog on sequential days and in the reliability of an individual dog at a given
level of VOCs. Variations in training history and methods also resulted in
different capabilities between dogs. This suggests it would be desirable to
test the ability of candidate SDs to detect low scent levels and to select a
dog with a suitable nose before investing the time, money, and effort in
training and to define and use optimal training methods.
usually start to pant at temperatures in the 80s°F, but this depends on age,
physical conditioning, level of work being performed, and genetic factors.
Panting is thought to reduce a dog’s scenting ability because, with their
mouths open, more air is drawn directly into their lungs through their mouth
rather than through their nose. Experience with hardworking airscenting hunting
dogs indicates that they appear to work better at air temperatures in the 60s°F
or less. However, search dogs and hunting dogs appear to pant while running and
are still able to scent the target source, indicating that some of the inhaled
scent that enters through the nostrils moves into the olfactory recess.
racing sled dogs, hunting dogs, and other working dogs (Grandjean and Clero
2011; Reynolds 2017) discuss diet (nutrition) and exercise (physical training)
requirements for optimum physical performance. A study by Altom et al. (2003)
investigated the effects of diet and exercise on olfactory acuity (scenting
ability). A group of dogs was exercised 30 min/day, 3 days per week on a
treadmill, while another group was exercised 10 min/day, 1 day per week. After
12 weeks, the two groups were subjected to a stress test (treadmill) for 1
hour, followed by measurements of their olfactory acuity (ability to detect an
odorant). The exercise group did not show a reduction in their olfactory acuity
from their pretest baseline. However, the non-exercise group showed a 64%
reduction in olfactory acuity following the physical stress test. This data
shows that a moderate physical conditioning program can help SDs maintain their
scenting ability during periods of intense work. Gazit and Terkel (2003a) found
that increased panting resulted in a significant decrease in explosives
detection, but that the dogs were eventually able to adjust to working in
extreme conditions. Grandjean and Clero (2011) point out that nutrition and
training are interrelated and provide extensive recommendations for various
types of working dogs.
of smell in dogs and other animals has evolved by natural selection for the
ability to use chemical cues (scent molecules) in activities associated with
finding food, reproduction, and survival. Consequently, they are extremely
effective at detecting and discriminating odors associated with these activities.
This implies that the threshold for detecting some odorants may depend on their
importance for survival to the dog. Some sources (e.g. explosives) that dogs
are required to detect do not meet this requirement. However, dogs quickly
learn that finding these sources enhances activities that are a normal part of
their lives (obtaining food and bonding with pack members, handlers, and
The above description of the olfactory system for dogs
suggests that the size of the system and the number of receptors would indicate
the ability of the system to detect and discriminate scent. Rats, mice, and
dogs have about the same number of functional olfactory genes and their
olfactory systems are comparable in their ability to detect and discriminate
odorants. However, dog olfactory recesses vary widely in size (compare
Bloodhounds and Rottweilers to Beagles and Terriers) and all are much larger
than those of rats and mice, a seeming contradiction. This suggests that other
factors may be involved in the sense of smell in mammals.
About the AuthorTom Osterkamp holds a PhD in physics from St. Louis
University. He taught classes in physics and geophysics—and conducted research
on things frozen (primarily floating ice covers and permafrost)—for 30 years at
the University of Alaska and is currently Professor Emeritus. He is a founding
member, former board member, and former training officer for Gateway Search
Dogs. He is a former member of the Board of Directors of the North American
Search Dog Network (NASDN). He is a former vice president and president of the
SAR Council of Missouri as well as a founding member of the Canine Search and
Rescue Association. Osterkamp has been active in K9 SAR for 23 years, including
more than 1,500 hours formal instruction in NIMS, SAR, ICS, Arctic survival,
and dog training specifically in: scent theory, air scenting, disaster, first
responder, trailing, cadaver, water search, and evidence. His dogs have passed
more than 40 national level certifications including NAPWDA, IPWDA, NNDDA,
NASAR, NSDA, and US Mantrailing Association. His articles have been published
in the Journal of Forensic Sciences, Advanced Rescue
Technology, and in newsletters such as SAR Dog Alert and SAR
Dog News. He has taught classes and seminars on such topics as scent
theory, area search, trailing, water search, and cadaver dog training locally
and nationally including Alaska and Canada.
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