The ability to detect and follow an odour trail is of importanceto many animal species, enabling them to locate food, prey,mates, home or avoid predators. Animals may come across an odourtrail by chance or as a result of a search pattern and willneed to determine the direction of the trail to move eithertowards the odour source (e.g. if prey), or away (e.g. if predator).Domestic dogs have the ability to follow an odour trail butlittle is known about the underlying mechanisms of this behaviour.
The olfactory abilities of dogs are utilized by many organizationsto assist in their work, e.g. search and rescue (Fenton, 1992
),identification of individuals (Schoon and De Bruin, 1994), locationof cadavers (Komar, 1999), detection of drugs (Lorenzo et al.,2003), explosives (Furton and Myers, 2001) and fire accelerants(Kurz et al., 1994), and in conservation work to assess populationsize (Smith et al., 2003). Their successful use in these tasksrests on two key factors: (i) dogs possess a more sensitiveolfactory system than humans and thus are able to detect odoursat lower concentrations; and (ii) the dog is trainable. Furthermore,following an odour trail is an essential part of the wild dog'sbehavioural repertoire, enabling it to locate food, and thususing this ability for human purpose (e.g. search and rescue),is building on a finely honed natural ability.
Early, more anecdotal, reports (e.g. Romanes, 1887) that dogscan follow an odour trail have been superseded by experimentalstudies (e.g. Steen and Wilsson, 1990; Wells and Hepper, 2003)which have confirmed that dogs are able to track individualsby their odour. However what odour cue(s) dogs use remains opento speculation.
Although terminology may differ between authors (Syrotuck, 1972;Lowe, 1981; Pearsall and Verbruggen, 1982; Johnson, 1997), inbroad terms there are two general types of odours that the dogmay detect and use to follow an odour trail. First, the individualodour of the person being followed. This comprises their ownpersonal smell arising from their skin, sebaceous, apocrineand eccrine secretions and the smell of their clothes, perfume,washing powder, etc. Second, is what may be termed contact ordisturbance odour. As the individual walks over the ground theirfootsteps impact on the surface, crushing vegetation, insects,etc., releasing an odour based on this contact. Thus, as theindividual moves through an environment their individual odourand that arising from contact/disturbance leaves a distinctiveodour trail. The dog may perceive odour from this trail by airbornecurrents or from its deposition on a surface. Thus, an individualodour may be detected in the air that blows over the individualand carries airborne scent rafts away from the individual, orfrom where the individual has deposited their odour either whencontacting the ground or surrounding vegetation or when skincells, for example, fall off the individual's body to the groundaround where the individual has walked. Contact/disturbanceodour may be detected directly from the surface affected orpotentially in the air if the odour is volatile. Many factorsmay influence the salience of the odour that forms the trail,e.g. temperature, humidity, etc. (Syrotuck, 1972). It is notintended to review this aspect more fully here other than tonote that many factors will influence the odour trail left behindby the individual which will affect the ability of the dog todetect and follow the trail.
Dogs that follow an odour trail may be broadly divided intothree types, determined by their behaviour on the trail (Bryson,1984; Johnson, 1997). Air scenting dogs follow the odour trailwith their head up in the air and are considered to be followingthe airborne scent rafts emanating directly from the individualproviding the odour and being carried away by air currents.It would follow that these dogs can only follow a trail up-wind.Trailing dogs follow the trail with their head up when movinginto the wind and head down when following the trail in thesame direction as the wind. They often do not follow directlyon the path trodden by the individual laying the odour and atbends may overrun before turning. It is considered that thesedogs are following the individual scent deposited by contactwith the ground surface. Tracking dogs follow the trail withtheir head down and noses on the path and follow very closelythe footsteps of the individual. It is assumed that trackingdogs are following the odour deposited on the ground and maybe detecting contact or disturbance odour. It should be noted,however, that these three characterizations are based on thebehaviour observed in the dog and as yet there is little experimentalevidence to confirm or deny these observations; in particularthere are few studies to determine what cues dogs actually employ.
Kalmus (1955) suggested that the dog detects and follows cuesof individuality and found that dogs remain on the trail ofa specific individual even when it is crossed by other individuals.Furthermore, when individual odours were prevented from beingdeposited through covering of the shoe (Romanes, 1887) or bythe wearing of a whole body suit (Pearsall and Verbruggen, 1982)dogs were unable to follow the trail. However, others have observedthat dogs were unable to track a trail of body odour cues, andcould only follow a trail when there had been ground contact(Budgett, 1933). Indeed the odour cues caused by ground contactseem important for tracking on older odour trails (Johnson,1977). Thus what cues dogs use in tracking odour is unclear.The fact that dogs can track on hard surfaces (Steen and Wilsson,1990) where there may be little ground disturbance suggeststhat individual odour cues are sufficient to enable a dog tofollow a trail.
Once it has detected an odour trail, the dog has to make a decisionabout which direction to follow the trail. There has been noexperimental study to elucidate the mechanisms used by dogsin following an odour trail. However, the ability to followan odour trail and locate its source is essential for survivalin many animal species and much progress has been made in identifyingthe mechanisms used to navigate to an odour source in some species,particularly certain crustaceans and insects.
Considerable work has been undertaken examining the dynamicsof odour plumes, i.e. the path and spread of odours as theymove away from their source (e.g. Weissburg, 2000). It is nowrecognized that odours do not disperse in a linear continuousgradient, but rather their dispersal is subject to turbulence,which creates a much more dynamic and complex odour stimulus.The odour plume is comprised of filaments and patches of odour,of varying concentrations, separated by areas of clean air orwater where no odour is present (Murlis et al., 2000; Weissburg,2000). The stimulus the animal must decode in such conditionsis one of varying temporal and spatial characteristics, in particulara stimulus that is erratically distributed (Moore and Atema,1991). Thus animals are faced with a variable intermittent odoursignal from which to determine direction. Under such circumstancessampling to obtain information on the odour gradient is verydifficult and to be successful would have to be averaged overlong time scales (Vickers, 2000): longer than the time observedfor orientation responses in crustaceans and insects. This indicatesthat animals have evolved strategies and mechanisms to obtaininformation on direction and enable orientation to an odoursource in this environment (Vickers, 2000).
A key problem presented by an odour plume is that of signalintermittency. One strategy employed by individuals to overcomethis is odour-gated rheotaxis. The animal, upon detecting anodour it wishes to respond to (e.g. prey), moves upstream orupwind. Thus animals need to detect both flow and the odour.As the source is intermittent, the animal will experience persistentloss with the signal and therefore needs to have a strategyfor either maintaining or regaining contact with the odour signal.Detailed study of crabs, lobsters and moths has revealed someof the complexity of mechanisms used to locate an odour source.
The blue crab (Callinectes sapidus) moves upstream upon detectionof an appropriate odour (e.g. prey) and maintains its directionto the source by comparing odour stimulation received by receptorson its left and right legs (Weissburg and Zimmer-Faust, 1993,1994; Weissburg and Dusenbery, 2002; Keller et al., 2003). Lobsters(Homarus americanus) similarly move upstream upon contact withan appropriate odour but may also use the internal chemicaland fluid dynamical structure of the odour plume (eddy chemo-rheotaxis;Atema, 1996) to maintain direction and locate odour source (Mooreet al., 1991; Atema, 1996). Moths fly upwind upon detectionof a relevant odour using optomotor anemotaxis (i.e. visualfeedback during flight to assess direction of flow) coupledwith a counterturning motor pattern generated within the centralnervous system to jump from odour patch to odour patch withinthe plume (Mafra-Neto and Cardé, 1994, 1996; Baker andVickers, 1997; Cardé and Mafra-Neto, 1997). The finescale structure of the odour plume also influences navigationalbehaviour (Moore and Atema, 1991; Kozlowski et al., 2001). Themore complex the structure of an odour plume, the more efficientis the orientation of the crayfish (Orconectes virilis) to theodour source (Keller et al., 2001). The intermittency of theodour signal — the time between patches of odour —is an important determinant of upwind progress in moths, whofly straighter and quicker to sources of higher pulse rate frequencyand slower and adopt a more zigzag course to lower frequencies(Mafra-Neto and Cardé, 1994, 1998; Justus and Cardé,2002). Moreover, examination of the receptors used to respondto odour reveals they have become adapted to use this intermittencyinformation to determine direction, e.g. receptor cells of lobstersact as temporal filters and enable spatial information to beextracted from the odour plume (Gomez and Atema, 1996).
A comprehensive review of the navigation abilities in odourplume is beyond the scope of this paper but this brief introductionserves to illustrate two key points: (i) the information providedby the odour as it disperses from it source is complex; and(ii) animals have evolved behavioural and receptor mechanismsto enable orientation to the odour source from this information.
Little is known about the mechanisms underlying the abilityof the dog to determine direction from an odour trail. Indeedthere was (is) some debate over whether dogs can actually determinedirection. Budgett (1933) claims dogs are unable to determinethe direction of a trail, and anecdotal (Morrison, 1980; Schwartz,1980) and some experimental studies (Mackenzie and Schultz,1987) support this. Recent work has found, however, that dogsare able to determine the direction of a trail (Steen and Wilsson,1990; Wells and Hepper, 2003). Moreover, they do this usingolfactory cues. For example, Wells and Hepper (2003) opposedthe visual and olfactory cues present in a trail, i.e. the olfactorycues indicated the trail went to the right but the visual cuesindicated the trail went to the left, and observed that dogsfollowed the olfactory cue.
How dogs determine the direction of an odour trail is unknown.Given that dogs can determine the direction of an odour trail,it can be assumed information is present within the trail toenable this. In this paper we assess the dog's ability to useinformation available from the footsteps of an odour trail todetermine direction. Specifically do footsteps provide sufficientinformation to determine direction (experiments 1–3) andhow many footsteps are required to enable a dog to determinethe direction of an odour trail (experiments 4 and 5).
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