The Dog's Nose and Scent
Written by Tom Osterkamp   

This article appeared in the May-June 2020 issue of Evidence Technology Magazine.
You can view that issue here.

Note: This excerpt from Detector Dogs and Scent Movement 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).

THE SENSE 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.

Physiology And Function
Olfaction 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.

Figure 1 — A schematic diagram of the olfactory system in mammals which shows how dogs detect scent molecules. (From Conover 2007. Used with permission of the Nobel Committee for Physiology and Medicine.)

An odorant 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 sniffing.

There is 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.

The 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).

Most 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.

Synthetic 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).

The above 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).

When 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.

Figure 2 — The internal aerodynamics of canine olfaction for the left nostril. Top: The olfactory recess (yellowish-brown) on the right. Flow paths during inhalation are distinct for respiration and olfaction. Bottom: Velocities for the same flow paths show high velocity (red) in the front of the nose and low velocity (blue) in the olfactory recess. (From Craven 2008. Courtesy of Dr. B.A. Craven.)

Part of 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 nasal cavity.

On 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.

Figure 3 — Surface dust particles disturbed by exhalation jets from the dog's nose. Flow directions for sources on the surface show that these air jets blow particles in a cloud to both sides of the nose and back in the photo. Some of the airborne particles can then be inhaled. (Courtesy of Dr. G.S. Settles 2002a.)

Observations 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).

This 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.

Figure 4 — Surface of the skin (magnified 2410 times) which shows several partly detached skin flakes and small fragments of detached skin. (From Clark 1974. Used with permission of Cambridge University Press.)

McLean and 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.

It appears 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.

There is 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.

Dogs 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.

Studies of 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.

The sense 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 others).

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 Author
Tom 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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