1 Introduction
1.1 Trail Scent
This chapter is concerned with scent from a moving person, its characteristics, how it gets to the ground, how the local environment influences this “ground” or trail scent, and how this scent may be used by dogs to follow the path (trail) of the person. No distinction will be made between tracking and trailing since the dogs follow the same scent, and the differences appear to be a result of their natural tendencies and training methods.
Early ideas about the ability of dogs to trail a person involved shoe odor, crushed vegetation (footprints), and scurf. The scurf hypothesis is similar to Syrotuck’s (1972), which identified the scurf as rafts (skin fakes). Syrotuck (1972) thought that the ground disturbance (footprints) and gases produced by bacteria acting on the skin fakes were used by dogs to follow the trail of a person. Footprints produce scent from bacteria acting on crushed vegetation and in disturbed soil. Since this scent is not specific to an individual, it will not be considered here because TDs are scent specific.
Scent from the human body appears to consist of gases produced by bacteria acting on the skin and skin fakes (including skin fakes from breath), volatilization of secretions on skin and skin fakes, and VOCs from breath. This scent is specific to an individual and has two components: particulates (skin fakes and bacteria) and gaseous VOCs from all the sources. Dogs can trail and discriminate scent on hard surfaces, which suggests they are using scent from skin fakes and/or VOCs. Human scent may be modified by race, diet, disease, toiletries, and contaminants in the environment.
Lidwell et al. (1959) showed that airborne particles found in hospitals carried an average of about four microbes on particles with a median diameter of 13 µm. These particles were identified as fragments of human skin (Davies and Noble 1962). Particle sizes ranged from 12 to 22 µm (Noble and Davies 1965) and weighed about 2.5E-8 oz.
Syrotuck (1972) assumed a fake size of 14 µm and two billion cells on the skin surface with 1 in 30 shed daily to calculate that about 40,000 full size cells were shed by the body each minute or roughly 700/sec. However, Clark (1974) showed that when a person entered a small room and walked slowly around it for 28 min, the number of particles smaller than 10 µm in the air increased about the same as the number of the much larger skin cells that would have been shed. Many of the recovered particles appeared to be fragments of skin. These particles would have been coated with skin secretions and some would have carried bacteria to contribute to the human scent picture. It is not known if these small skin fakes were part of the larger shed skin fakes or were in addition to them.
1.1.1 Human Thermal Plume
Understanding trail scent requires information on how scent from a person reaches the ground. Skin is normally warmer than the adjacent air so that the skin heats the air which produces thermal convection from the body to the surrounding air (Figure 1). This results in a human boundary layer next to the skin that flows upward from the ankles to the top of the head. It is about 8 in thick around the upper body, moves at a speed of about 1 ft/sec, leaves the body in a plume at the shoulders and top of the head, and may extend up to about 6 ft above the head (Settles 2005). Every location on the body contributes scent to the human boundary layer so that all types of scent released by the body or on the body are in the upward moving layer and plume. This includes skin fakes, VOCs, explosives, drugs, disease carrying bacteria, and scent from disease. Clothing is not an impediment to the shedding of skin fakes because the pores in the weave of almost all fabrics are much larger than the average fake. However, the bellows action of pant legs and shoes while walking may be responsible for some of the skin fakes and VOCs deposited on the trail as shown in Figure 2 (Clark and de Calcina-Goff 2009).
Syrotuck (1972) suggested that this body air current was responsible for projecting scent into the air above the head of a person, which would fall to the ground along the path of the person. However, settling velocities are so low for skin fakes (0.04 to 0.4 in/sec) and scent molecules that they would be carried far downwind (hundreds of yards) before contacting the ground. Since TDs generally follow much closer to the trail, some other process must bring the scent to the ground.
If a person is standing still in calm air, the human thermal plume carrying scent over the person’s head would slowly settle around the person and deposit on the ground. In wind, the plume that is not deposited would be carried downwind and would be detectable by SDs. If a person is walking in calm air, the plume would be incorporated into a turbulent wake with eddies behind the person (Figure 3, Edge et al. 2005). If a person is walking in wind, Figure 3 indicates that the location of the wake relative to the person will be determined by the wind speed and direction. The pattern of the deposited scent is unknown but probably covers much more area on the ground than footprints. These considerations indicate that trail scent is a kind of long, more or less continuous scent print.
Figure 3 Sketch of the wake produced by the thermal plume behind a walking person shows the wake contacts the ground where skin fakes and VOCs would be deposited. This creates the trail of the person. (Based on photography produced by Settles 2020b, courtesy of Dr. G.S. Settles.)
Figure 3 shows that the eddies in the wake contact the ground and would be expected to deposit scent there. This indicates that trail scent on the ground in the immediate vicinity of the path of the person is a result of the thermal scent plume and eddies in the turbulent wake associated with the moving person and wind. Scent that is not deposited on the ground would be carried downwind. A trail that is in the same direction as the wind or opposite to it would result in maximum scent on the ground and one that is perpendicular to the wind would result in minimum scent.
Since most of the trail scent appears to originate in the human boundary layer, it is derived from every location on the body, contains all types of human scent, modifications to that scent, and all non-human scent carried by the body. The presence of a wake associated with a moving person that contains non-human scent (explosives, drugs, other contraband and hazardous materials) is the basis for wake trailing dogs.
The surfaces where fakes and VOCs are deposited on the trail and downwind of it include soil, water, vegetation (blades of grass, stalks of plants, leaves), and hard surfaces (rock, asphalt and concrete). It is thought that vegetation plays the most significant role (collects the most scent). Deposition is determined by the properties of the VOCs, fakes, surfaces, temperature, humidity, and other factors (Simonich and Hites 1994). Deposition of VOCs (partitioning between the gas and solid surface) depends on temperature, but deposition of fakes does not. Once deposited, sunlight may degrade fakes and some VOCs.
It is known that VOCs move to the earth’s surface at lower ambient temperatures and to the atmosphere at higher ambient temperatures. This suggests that there is a daily cycle in the concentrations of VOCs deposited on surfaces and that concentrations should be higher during the night and for surfaces in shade, compared to those surfaces in sunlight (Simonich, personal communication).
Hot and dry air and surfaces can dehydrate the skin fakes to the point where bacterial activity almost ceases (Syrotuck 1972). Surfaces cool when they go into shade and at night. This increases humidity and dew, which causes them to rehydrate and again produce VOCs.
Observations of TD handlers are consistent with this behavior of VOCs. Generally, it is easier for dogs to trail when the air is moist and cool, in shaded areas, at night, and when handlers cast their dogs into shade to recover or start a trail.
When fakes are deposited on surfaces, they continue to emit VOCs from the bacteria and secretions. Each fake would produce a tiny discrete plume of VOCs that would move downwind (Figure 4), where some VOCs would attach to surfaces.
Figure 4 Schematic drawing of a skin fake attached to a surface. It continues to emit VOCs (dots), which may also attach to surfaces. Changes in wind direction and turbulence would produce multiple plumes and, in a short time, a cloud of VOCs around each deposited fake.
Changes in wind direction and turbulence near the surface would produce multiple plumes for each fake resulting in a cloud of VOCs in the area of each deposited fake. Movement of the fakes would increase the size and concentration of the scent cloud.
After a short time, the trail would consist of clouds of multiple overlapping plumes of VOCs from each fake in the immediate vicinity of the path of the person. This could produce a continuous distribution of scent on the ground provided the fakes and VOCs could attach to the available surfaces. Variations in surface properties would be expected to result in patches of variable scent concentration. VOCs and skin fakes in the wake that are not deposited would be carried some distance from the path.
This explanation of trail scent is supported by the available evidence and provides a process by which trail scent can be deposited on the ground along the path of a person (Figure 3) and a process that can increase the coverage of scent along that path (Figure 4), perhaps like Figure 5.
This behavior of sources that emit VOCs has been described in field studies of pheromones that emit VOCs (Karg et al. 1994). It is hypothesized herein that it would also apply to the emission of VOCs by skin fakes although with some modifications.
1.2 Effects of Environmental Conditions
The effects of surfaces, sun or clouds, weather (temperature, moisture, wind), terrain, vegetation, and turbulence on scent and scent plume movement have been discussed in The Dog’s Nose and Scent, Scent and Wind, and Above-Ground Searches. Trail scent consists of gases produced primarily by bacteria acting on skin and skin fakes, volatilization of secretions from skin and skin fakes, and VOCs from breath. These bacterial gases and VOCs are the components of trail scent. Bacterial gases are produced while the bacteria remain active. Bacterial activity requires favorable conditions which include food, acidity, time, temperature, oxygen, and moisture. Assuming other conditions are present and favorable, temperature and moisture are the primary conditions which influence activity with warm and moist the most favorable.
Figure 5 Schematic of trail scent deposited along the path (trail) of a person based on Figures 3 and 4.
1.2.1 Temperatures and Moisture
Environmental conditions can interact individually and in concert to create a myriad of different settings for trails. For example, the temperature of a surface depends on slope, orientation, sun or clouds, air temperatures, vegetation, moisture, wind, and other factors. We are concerned with the temperature range where trailing dogs usually work. Generally, this range is from freezing to about 100°F. When considering the effects of temperature on trail scent, it is the temperature of the surface where the trail exists that is more important than the air temperature. Surface temperatures in shade and at night are generally cooler than the air temperature and, in sunlight, are generally warmer than air temperature.
Extremely high or low temperatures influence the ability of bacteria to produce gases. The availability of bacterial gases and VOCs would be expected to increase with increasing temperatures. Although the amount of scent available increases when temperatures increase into the 90s°F and higher, the dogs usually find it difficult to continue working. If high temperatures are associated with low humidity, skin fakes would be expected to dry which would reduce availability of bacterial gases.
This situation can also develop at relatively low air temperatures (70s°F) since dark surfaces (e.g. asphalt) can be 50°F above air temperatures and dry. If the surface is dry bare soil, VOCs may adhere to the soil particles, which leaves less trail scent. Flakes may also adhere to concrete and asphalt or be trapped in tiny holes in the surface, but there is no information on this.
At temperatures like those in a refrigerator (typically <40°F), bacterial activity slows and almost ceases below freezing. At temperatures like those in a freezer (<0°F), bacterial action ceases. This does not mean that TDs cannot trail, since VOCs from the trail scent are still present and can also continue to volatilize from secretions on the fakes although at much reduced rates. Also, skin fakes can be inhaled and produce detectable VOCs in the warm and humid environment of the nose. Experience with TDs in Interior Alaska has shown that they can successfully trail at temperatures of −30°F on frozen surfaces, ice, and hard packed snow.
The type of snow and its thickness determines whether dogs can trail when snow has fallen on the trail. Experience suggests that dogs can still trail with ½ ft of new powder snow on the trail, but a few inches of wet snow that has frozen
makes trailing difficult to impossible. Most dogs learn to follow visible footprints in the snow, but others seem to trail as if the footprints were not there.
1.2.2 Wind, Terrain, and Vegetation
Wind speed and direction determine where the scent in the wake will contact the ground, but there is no information available. For wind speeds much greater than the speed of the subject, the scent would contact the ground near their footprints and just downwind of the subject. TDs usually trail on the downwind side of the track. Movement of the scent that does not contact the ground will be determined by the interaction of the wind with terrain, vegetation, obstacles, and by thermal turbulence in the same way as a scent plume (Scent and Wind and Above-Ground Searches). Vegetation typically has 6 to 14 times more surface area than the land on which it is growing.
This provides a lot of surface area to intercept fakes and VOCs and may enhance trail scent as noted. Observations of TDs trailing in deciduous forests suggest that the dead leaf surfaces appear to hold scent well, possibly as the combined result of shade, increased moisture, and the presence of cavities formed by the leaves.
2 Training and Deployment
2.1 Scent Articles
Scent articles can be anything the subject has touched or worn. Since trail scent originates from all locations on a body, a scent article from any location would be useful, but areas that produce more scent would be preferable. Bacterial populations are large in areas on the body where the skin is moist, primarily the armpits, genitals and groin, head, soles of the feet, and palms of the hand. Underwear, used baby diapers, hats, pillows, shoes, socks, and gloves that have been in contact with the subject for extended periods of time are desirable scent articles. The handler should personally collect the scent article. If this is not possible, everyone who may have been in contact with it must be present when the dog is scented. Use of the scent inventory method (Mark Holmes, trailing seminars) is desirable if the scent article is contaminated.
Objects that the subject has touched may include burglary tools, bomb fragments, rocks, letters, notes, coffee cups, and blood, among others. Wiping surfaces that the subject has touched can also be used to obtain a scent article. These include steering wheels, door handles, windowsills, and other items.
Footprints are commonly used scent articles. A scent article from the driver’s seat of a vehicle can be obtained by placing an absorbent paper or cloth on it for 10 to 15 min. Some handlers recommend covering the article while it is absorbing scent.
Scent articles can be obtained using noncontact methods such as the STU-100 unit (Eckenrode et al. 2006), which is used to vacuum scent from the subject or from objects that the subject has touched. Since the human scent plume contains scent from every part of the body, a more complete scent profile of a subject may be obtained by vacuuming at the top of the head. Another method is to use the human scent plume that exits the body above the head and shoulders in places that the subject frequents. An example is the headliner in an automobile above the driver’s seat, which can be wiped with an absorbent paper or cloth that becomes the scent article of the driver (Burt Crawford, personal communication).
Scent articles can be obtained from firearms, bows, knives, and other weapons and from projectiles such as bullets, pellets, arrows, and rocks. Fired shell casings can also be used. The body of an assault victim has been used as a scent article. Another possibility would be to wipe the skin of a victim that has been assaulted, although this does not appear to have been done.
Scent articles stored in airtight containers appear to lose some scent initially, but then level of after the first 2 weeks and are still usable months later (Prada et al. 2015). Fired shell casings stored in a freezer bag for over a year have been successfully used by dogs to trail. These results and others have verified that human scent is stable and reproducible over long times.
2.2 Turns and Wind
TD handlers should be constantly aware of the wind direction on trails. Whatever channels the wind, channels scent and may be a barrier to the dog. If the channeled wind and scent are in the same direction and the trail turns out of the channel, the dog tends to significantly overshoot the turn.
Typically, wind direction is at some angle to the trail other than 90°, but considering these turns (Figure 6) can help to understand what to expect. TDs usually trail somewhat downwind of the trail, depending on their inclination and training. For a crosswind leg with an upwind turn, the dog encounters a lot of scent at the turn from the trail upwind and should immediately turn with the trail. For a crosswind leg with a downwind turn, the scent decreases quickly as the dog passes the turn and it should give an indication of it (e.g. head turn).
A downwind leg with a crosswind turn carries a lot of scent downwind past the turn. The tendency of the dog is to continue past the turn in decreasing amounts of scent until they realize they are getting out of scent. If the dog has been trained using a line (leash) check, apply it about a leash length past the turn in training and periodically on a blind trail. A turn of a downwind leg is especially difficult in a channel. On an upwind leg, the dogs seem much more animated. If the wind is slightly variable, the dogs tends to quarter the trail, or they carry their heads high which causes them to scent the turn and cut of the corner in the trail. If they pass the turn, the scent decreases sharply, and they usually give an indication. If not, apply leash tension.
Figure 6 Schematic examples of right-angle turns showing the effects of wind direction on the dog’s behavior.
For the scent collectors (grass, curbs, barriers), dogs usually trail on the grass, along curbs or along a barrier some distance from the trail if the wind is significant.
Trails that are laid primarily upwind are likely to result in the dog air scenting the subject. Trail patterns that begin and end near the same location (letter C or U shape) should not be used regularly. Some dogs quickly learn that the trail always goes back to the area of the start and a variable wind may allow the dog to scent the end of the trail near the start. Different legs of a trail should be far enough apart so that the dog cannot scent an upwind leg, or the trail should be laid so that the upwind leg is run before the downwind leg.
2.3 Buildings and Wind
The effects of buildings on wind are to cause the wind to flow over and around them, creating 3 dimensional eddies on the ends and sides that are difficult to predict. For the special case of wind nearly perpendicular to the sides of buildings, the behavior is like that of ridges and islands. Away from the ends of the buildings, vertical two-dimensional eddies occur (Figure 7) which produce ground level circulation away from the building on the upwind side and toward the building on the downwind side. For a trail along the building on the upwind side, scent will be carried away from the building and dogs will trail there, especially when scent collectors are present within a distance about the height of the building (e.g. curb, vegetation). For a trail on the downwind side and less than the height of the building downwind, scent will be carried to the building and the dogs will trail close to it.
Figure 7 Vertical profile of air passing over a building away from the ends. Scent from the trail (*) on the upwind side collects against the barrier and on the downwind side, against the building.
2.4 Air-Scenting Trails
TDs that carry their heads high tend to air scent trails when the wind from a leg that has not been run blows across or toward the leg being run. On fresh trails, the distance between the legs can be a city block which indicates that the scent is likely an airborne scent plume. However, air scenting has been observed on trails aged 24 hrs with legs 200 yds apart. This suggests that the airborne scent may be coming from the bacteria on skin fakes, secretions on the fakes, and/or VOCs from surfaces, although there are no observations to confirm this hypothesis.
2.5 Direction of Travel
The behavior of TDs when encountering a track with an unknown direction is of interest to all handlers. Tree phases of behavior by TDs have been identified (Thessen et al. 1993): searching, deciding, and tracking. Frequency of sniffing was about 6/sec in all three phases. Once the track was found, the deciding phase began. This was clear because the dogs usually halted for a moment. The dogs moved more slowly, had longer sniffing periods, and often slowed or stopped when they passed a footprint. The deciding phase lasted 3 to 5 sec while the dogs sniffed with their noses close to the ground. They needed 2 to 5 footprints to determine the track direction, and once the decision was made the tracking phase began.
Since dogs can determine the direction of an odor trail left by a human, it is of interest to determine the length of the trail needed for them to do it. Six dogs, able to determine direction of travel, were tested on a 21-footstep trail laid on 21 individual carpet squares (Hepper and Wells 2005). The dogs correctly determined direction 1 hr after the trail was made but were unable to do so when the order of the footsteps was randomized by rearranging the order of the carpet squares. Sealing the feet of the trail layer prevented any foot or shoe odor from being deposited but retained any scent made by the carpet disturbance. The dogs could not reliably determine direction of the trail which indicates that they used the individual’s odor deposited in or near a footstep rather than any effects related to compressing the wool carpet squares. When the carpet squares were removed, it was shown that the dogs were able to determine direction from 5 footsteps which required about 2 sec to make. At normal walking speed, these dogs could determine the difference in scent between two positions on the trail separated by about 6 ft.
These studies did not investigate the influence of the age of the trail. It does not appear that the dogs in these studies were trained using scent articles, which may have influenced the results. TDs that are trained with scent articles can usually determine direction of travel and, with some testing standards, are required to do this for trails 24 hours old.
2.6 Car Trails
There is often doubt expressed about the ability of dogs to follow a person in a moving car. However, skin fakes and VOCs are exhausted from the car vents so that trailing should still be possible. The following examples, which are only a few among many, show that car trails are possible. The question is not whether dogs can do them, but “Under what conditions can car trails be done?”
2.6.1 Missing Child
In 1993, a child was reported missing near Denver, Colorado. Tree days later a 4 yr-old bloodhound named Yogi was scented with a pair of the child’s underpants by his handler, police officer Jerry Nichols, and began a trail near the child’s home. The team trailed from W. Grand Ave., near Belleview and South Broadway, continued along Broadway to C-470, exited at Kipling Street, and then west to the mouth of Deer Creek Canyon (Figure 8). The length of the trail was about 14 mi and it was done in about 7 hrs. This trail was exceptional because of its age, length, extensive hard surfaces, and vehicle and personnel distractions. The next day, searchers picked up where the team left of, and while searching Deer Creek Canyon, discovered the child’s body. Later, Yogi led investigators from Deer Creek Canyon back to the apartments where the murderer lived near W. Grand Ave.
Figure 8 Approximate trail of Officer Nichols and Yogi from near Belleview and Broadway to Deer Creek Canyon, a distance of about 14 miles.
Figure 9 Car trail of a kidnapped boy. Scent collected on weeds, grass, and leaves on the side of the trail which made the trail easier for the dog and she clearly preferred trailing on them.
2.6.2 Missing Boy
A car trail involving scent collectors (weeds, grass and leaves) occurred when a boy was kidnapped in a rural area between his house and school bus stop (Figure 9). The trail was started at his house at 10 p.m., about 6 hrs after he was abducted, and went up the gravel lane to the bus stop where he caught the bus to school in the morning. It turned left on a gravel road, and the dog trailed on the right side of the road along and in weeds to a paved highway, crossed the highway, and trailed west on grass in a shallow ditch along the north side of the highway. After more than 100 yds, she left the ditch and moved farther away from the road along a fence where the leaves were piled a foot or so deep next to a tree line. After another 100 yds, the handler called a halt to the trail since it was obviously a car trail implying that the boy had been abducted.
The trail was about 14 hrs old from the house, 6 hrs old from the bus stop, with the wind from the southwest at about 20 mph, and with a total length about ½ mile. It is not likely that the car exceeded 45 mph on the highway. Tree days later the abductor was apprehended and the boy rescued. He verified that the trail the dog followed was the direction he was taken by vehicle.
2.7 Aged Trails
The stability of human scent has been widely studied but there does not appear to be any information on the stability of trail scent. Handlers have observed that trail scent is stable over time frames of several weeks when conditions are good (warm and moist), like those that occur near sea coasts. However, scenting conditions can vary with time of day and weather. For example, it is known that trails made in the morning on an asphalt surface can be extremely difficult for dogs to run during the afternoon on a sunny, hot summer day. However, when the surface goes into shade and at night, it cools, and the trail becomes much easier for the dogs. Syrotuck (1972) believed that the difficulty was a result of drying skin fakes and reduction of bacterial activity on them but did not consider VOCs from other body sources. There does not appear to any further analyses of trail scent, especially of the relative roles of VOCs produced by bacteria acting on skin fakes and those produced by other sources.
There are many reliable reports by handlers of TDs working aged trails up to a month old but it is not usually possible to obtain an independent verification. A verified trail involved a man thought to be suicidal who was trailed 16 days after he had disappeared in a shipyard in the Pacific Northwest. The age of the trail was verified by police report and a surveillance camera. Daytime temperatures were in the 60s and 70s°F. The trail started at a railroad station, was about ¼ mile long, and led to a locked gate at a shipyard which he apparently crawled under or over it (Figure 10). Trail surfaces consisted of cut grass, railroad tracks, covered walk, and paved parking lot. In the shipyard, the surface was gravel and only parts of the trail could be found there, but it appeared that the man was moving toward the dock after leaving his belongings in a shed.
Figure 10 A trail aged 16 days near the Pacific Northwest seacoast verified by police report and security camera. The trail was heavily contaminated by cars and pedestrian traffic.
This trail was heavily contaminated since it crossed the entrance to a busy ferry terminal. It is estimated that hundreds of cars, several hundred trains, and more than a thousand people crossed the trail before it was run.
There does not appear to be any information on the characteristics and behavior of aged trail scent. Does the scent remain on the ground, linger in the air, move daily between the ground and air? Is scent from the feet made by the bellows action of walking in pants and shoes important? What is the relative importance of VOCs from bacteria and from other sources? Can bacteria survive weeks or a month and, if so, under what conditions? What scents do dogs trail on asphalt surfaces weeks after the trail was made? What are the effects of various surfaces and environmental conditions?
3 Summary
Scent from the human body appears to consist of gases produced by bacteria acting on the skin and skin fakes (including skin fakes from breath), volatilization of secretions on skin and skin fakes, and VOCs from breath. This scent is specific to an individual and consists of particulates (skin fakes) and gaseous VOCs from all the sources.
Skin is normally warmer than the adjacent air and heats the air, which produces thermal convection from the body to the surrounding air (Figure 1). This results in a human boundary layer next to the skin that flows upward from the ankles to the top of the head. Every location on the body contributes scent to the human boundary layer so that all types of scent released by the body or on the body are in the upward moving layer. This includes scent from skin fakes, VOCs, explosives, drugs, disease carrying bacteria, and disease. The boundary layer exits the body at the top of the shoulders and head which produces a plume (Figure 2).
A walking person produces a turbulent wake with eddies behind them, and the plume is incorporated into this wake. Eddies in the wake cause part of the plume to contact the ground and deposit scent there (Figure 3). Scent from a walking person may also be deposited by the bellows action of the pant legs and shoes (Figure 2). Consequently, trail scent consists of the skin fakes and VOCs deposited in the path of the person and downwind. Scent not deposited would be carried away by the wind.
Deposition of VOCs depends on temperature, but deposition of fakes does not. Once deposited, sunlight may degrade fakes and some VOCs. Trail scent can be enhanced by changes in wind direction and turbulence on the surfaces where the fakes are deposited, which would produce multiple mini plumes for all the fakes (Figure 4) and result in a cloud of VOCs in the area of each deposited fake (Figure 5). There is a daily cycle in the concentrations of VOCs deposited on surfaces, and concentrations there should be higher during the night and for surfaces in shade compared to these surfaces in sunlight. This may be the reason that dogs can trail better on surfaces in shade and at night.
When considering the effects of temperature on trail scent, the temperature of the surface where the trail exists is more important than the air temperature. The availability of bacterial gases and VOCs would be expected to increase with increasing temperatures. Although the amount of scent available increases as temperatures increase into the 90s°F and higher, the dogs usually find it more difficult to continue working. If high temperatures are associated with low humidity, skin fakes would be expected to dry, which would reduce availability of bacterial gases.
At temperatures like those in a refrigerator (typically <40°F), bacterial activity slows and below freezing it almost ceases. At temperatures like those in a deep freeze (<0°F), bacterial action ceases. Experience with TDs in Interior Alaska has shown that dogs can still successfully trail at temperatures of −30°F on frozen surfaces, ice, and snow. This may be a result of inhalation of skin fakes and VOCs. Experience suggests that dogs can still trail with ½ ft of new powder snow on the trail, but a few inches of wet snow that has frozen makes trailing difficult to impossible. Some dogs learn to follow visible footprints in the snow, but others seem to trail as if the footprints were not there.
Vegetation typically has 6 to 14 times more surface area than the land on which it is growing. This provides a lot of surface area to intercept fakes and VOCs and may help enhance trail scent. In deciduous forests, dead leaf surfaces appear to hold scent well, possibly as the combined result of shade, increased moisture, and the presence of cavities formed by the leaves.
Scent articles can be anything the subject has touched or worn. Since trail scent originates from all locations on a body, a scent article from any location would be useful, but areas that produce more scent would be preferable. Underwear, used baby diapers, hats, pillows, shoes, socks, and gloves are desirable scent articles.
Wiping surfaces that the subject has touched can also be used to obtain a scent article. These include steering wheels, door handles, and windowsills among others. Footprints are commonly used. A scent article from the driver’s seat of a vehicle can be obtained by placing an absorbent paper or cloth on it for 10 to 15 min. The headliner in an automobile above the driver’s seat can be wiped with an absorbent paper or cloth which becomes the scent article.
Scent articles can be obtained from firearms, bows, knives, and other weapons and from the projectiles such as bullets, pellets, arrows, and rocks. Fired shell casings can also be used. The body of an assault victim has been used as the scent article.
The effects of wind direction on trail scent (Figure 6) and on turns in the trail, trails near buildings, channeling, and scent collectors are discussed (Figure 6 and 7). TDs that carry their heads high tend to air scent trails when the wind from a leg that has not been run blows across or toward the leg being run. The distance between the legs can be a city block and air scenting has been observed on trails aged 24 hrs with legs 200 yds apart.
Trails that are laid primarily upwind are likely to result in the dog air scenting the subject. Trail patterns that begin and end near the same location (letter C or U shape) should not be used.
Dogs can determine the direction of a trail left by a human and they have a distinctive behavior when acquiring it. When an hr old trail was detected, it required up to 5 sec and 5 footprints to decide on the direction. It was shown that the dogs used trail scent rather than ground disturbance to determine direction of travel.
Dogs can do car trails. Two car trails are described including one 3 days old and 14 mi long (Figures 8 and 9). There are reliable reports of TDs working aged trails up to a month old (Figure 10). It is not known what components of trail scent are used nor how the dogs can follow these very old trails.
There does not appear to be any information on the characteristics and behavior of aged trail scent. Does the scent remain on the ground, linger in the air, or move daily between the ground and the air? Is the scent from the feet and legs made by the bellows action of walking in shoes and pants important? What is the relative importance of VOCs from bacteria and from other sources? Can bacteria survive weeks or a month, and, if so, under what conditions? What scents do dogs trail on asphalt surfaces weeks after the trail was made? What are the effects of various surfaces and environmental conditions?
Tom Osterkamp
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