Trail Age: More Than Just a Number
Earlier this week I was contemplating the age of a trail and its ramifications. It was a deep rabbit hole, People! So I will try to keep it brief.
The catalyst for these ruminations was that I needed to put together a training plan for Rudy. We have been Lost Dog Tracking together and in the next phase of our training we need to be able to demonstrate successful completion of tracks up to 24 hours old. The training progression for aged trails is pretty standard and involves incrementally increasing their chronological age from 'fresh', to 2 hours old, to 4 hours old, to 8 hours old…and so on. This got me thinking…what is an aged trail anyway?
To be honest, I expected the answer to be more straightforward, but it wasn't.
The first thing that surprised me was that a citable definition was actually hard to come by. At first I thought that I had lost my researching edge… but then I was relieved to discover that Osterkamp had reached the same conclusion [1]. Perhaps it is so obvious as a concept that it doesn't need one, yet the scientist in me needs this formal stake in the ground!
Then in May 2021, the American Academy of Forensic Sciences entered the chat. They defined an aged trail as "a human scent pathway that has been present for some period of time, typically expressed with a timeframe associated with the track/trail" [2]. In their view, a trail becomes aged at 24 hours, yet other sources suggest a trail is aged after 30 minutes [3, 4] or be older still [1]. This isn't about being right or wrong about when a trail becomes 'aged'. It is about perspective and purpose. In some cases the age threshold forms part of a competency standard [2], in others it is used to define a progression milestone [5]. It can also come from an understanding that is distilled from 'walking the miles' [1], as a result of logistical practicality or as the earliest moment where significant changes could occur within the scent picture [6].
Although the timepoint at which a trail becomes 'aged' appears to be context-dependent, there is a unifying theme: they are all based on elapsed time. Anyone who has also explored this topic may have come across the term 'Set Time', which is "the length of time between the target placement and when the dog is deployed to detect the target odour" [7, 8]. The underlying concept is closely related to trail age and is a useful model for thinking about how elapsed time is quantified in canine scent work. When applied to trailing, the analogous calculation would be the time interval between the Trail Layer beginning to deposit their scent and the dog team starting to trail:
Set Time or Trail Age = T(Dog start) - T(Trail Layer Start)
From an operational and practical standpoint, this makes a lot of sense because the Last Known Position (LKP) is typically treated as the origin of the trail. Essentially, the trail begins where the target person was last known to be. As the science of canine olfaction and practice of canine trailing have developed, terms such as trail age and set time have emerged as ways of describing, quantifying and standardising what handlers and trainers were already doing in the field.
It is worth mentioning here, that not all tracking disciplines calculate trail age in the same way. Whilst operational tracking and sport trailing commonly measure age from the Trail Layer's first footstep, other disciplines can use a different rationale depending on their training objectives and requirements. In reality, over short distances the differences between these calculated times are negligible and are unlikely to have a significant influence on training or performance. However, it is good practice for handlers who are recording data to know how those data are defined and calculated within their training programme, especially if these values are reported in tracking apps. Yes…they may be calculating trail age differently too! And no…it isn’t obvious exactly what is being calculated until you check!
So now we have it: a value for trail age! Deliciously simple to calculate, capture and compare…and even play trail-age-top-trumps with your session-mates. Not that any of us would ever do that. But therein lies a problem. As a variable, trail age is very easy to measure. But to comprehend the significance of this ageing on the trail itself is anything but simple.
Broadly speaking, human scent trails can be considered a by-product of bacterial processes, albeit highly individualised by each person's unique microbiome and bodily secretions [9]. Exposure to environmental factors, such as weather conditions, surface type, UV strength and contamination, alter the chemical composition of the trail. As a result, the trail as an entity is changing from the moment it is deposited in the environment. It is important to consider that ageing does not just mean that the scent becomes weaker. Yes, overall concentrations of odourants may decrease and noise from environmental contaminants may become increasingly prominent. However, as a trail ages, the relative composition of scent compounds can also change, as some compounds may dissipate, re-emerge or degrade more than others and the uniformity of the scent along the trail may be disrupted [9, 10]. We just don't know exactly how. Every trail has a shelf-life and, eventually, the components of the scent trail will degrade to levels below which even a competent trailing dog cannot reliably detect and follow. Exactly where the limit of detectability lies cannot be easily defined because there are so many unanswered questions about the stability and viability of scent molecules in the environment [10].
Typically, training on aged trails does not begin at the limit of detectability. Rather it begins at the current limits of the dog's understanding of what a 'human scent trail' can smell like. Take Rudy's Lost Dog Tracking, for example. So far his training has involved working 'fresh' trails; therefore, it is reasonable to assume that the limits of his understanding have largely been developed within this olfactory context. As we increase the age of the trails, he is more likely to encounter scent pictures that will not match his understanding of what a trail should smell like. When this occurs, he may need to work harder, both physically and mentally. He may appear to struggle. Sometimes he may be able to solve the problem and other times he may not. From our perspective, we may conclude that increasing a trail's age makes it more difficult. Maybe it does…but we do not know, a priori, how difficult a dog will find a scent problem they have never worked. After all, anything unfamiliar is a challenge until it becomes familiar, at which point it is practiced.
Perhaps it is more helpful to see these moments as our dog bridging the gap between scent pictures they expect and those they have yet to understand are an extension of the same scent problem. Sometimes this gap will be small. Sometimes it will be large. Sometimes it will be intractable. This is where trail age becomes a useful training variable. By progressively and systematically exposing a trail to the influence of the environment, we are also incrementally changing the scent picture encountered by the dog. Through this process, the dog learns that the scent trail can present somewhat differently and still be the correct solution. For example, Rudy learns that the scent picture associated with a 'fresh' trail is no longer the only answer. Viewed in this way, modulating trail age is not simply a progression target, but a framework within which we recalibrate our dog's understanding of what a viable human scent trail is. And it is this capability that will enable success on future aged trails.
This brings us to another important point: trail age is an imperfect proxy. Although, it is possible to change the age of a trail in a systematic way, the corresponding change in the scent picture is not equally as predictable, measurable or obvious. Which is why training progression is often not linear. Let's consider an example of a hypothetical trail aged by two hours. Imagine, if it were possible to attempt this trail twice, with the only difference being where the dog begins: up- or downwind of the from the Trail Layer's hide. If the dog succeeds in one scenario but not the other, what have we learned about the capability of the trailing dog? What I'm trying to highlight here is that even though the trail age is chronologically identical in both scenarios, the olfactory context is not. In one scenario, the dog may get useful scent information originating from much later in the trail or even from the Trail Layer themselves. Therefore, solving the problem without relying on the aged trail at all. In the other, the dog may need to interpret a much greater proportion of the aged trail before finding the Trail Layer's position. Both would be recorded as two-hour aged trails in our log books, but only one would progress the dog's understanding of aged scent trails.
Outdoor training will always be multivariate. Weather, wind, surface, terrain and many other environmental factors come together to shape the scent picture encountered by the dog [1, 9, 10]. We can never expose our dogs to every possible combination of these variables, but through our training we can try to maximise the coverage of the possible scent pictures. Looking back over a week's worth of 'thinking', I realised that I wasn't only designing a training plan for Rudy to complete a 24-hour aged trail. A trail that has been aged for 24 hours is not valuable because it is 24 hours old. It is valuable because of the olfactory experiences it may provide for our dog. Therefore, this is our true goal: to ensure that training provides Rudy with sufficient exposure to different scent pictures, so that each unfamiliar scent picture he encounters is only a small gap for him to bridge.
So, I will leave you with this final thought. Our dogs are not trained on the trails we think we have laid, but on the scent pictures that they actually experience. Perhaps that is why trail age is more than just a number.
References
[1] Osterkamp, Tom. 2021. ‘Aged Trails 1’. https://k9search.net/aged-trails-1/.
[2] AAFS Standards Board. 2021. Standard for Training and Certification of Canine Detection of Humans: An Aged Trail Using Pre-Scented Canines. American Academy of Forensic Sciences. https://www.aafs.org/sites/default/files/media/documents/026_Std_e1.pdf.
[3] Gorenflo, Lisa. 2022. ‘Mantrailing Global Terminology’. https://www.mantrailingglobal.com/post/mantrailing-uk-terminology.
[4] Trailing K9s. 2021. Trailing Terms.
[5] Dobler, Alis, Henny Martinoni, and Jeff Schettler. n.d. GAK9 TRAILING Certification Standards. https://tttk9.com/wp-content/uploads/2022/06/GAK9-Trailing-Certification-Standards-EN.pdf.
[6] Love, Kathryn. 2026. ‘Aged Trails: Significance of 30 Minute Threshold’.
[7] SWGDOG. 2018. SWGDOG - TERMINOLOGY. https://www.nist.gov/system/files/documents/2018/04/25/swgdog_terminology.pdf.
[8] Sloan, Katylynn B., Michele N. Maughan, Caitlin E. Sharpes, Robin R. Greubel, Shawna F. Gallegos, Aleksandr E. Miklos, and Lindsay D. Waldrop. 2025. ‘The Manipulation of Odor Availability of Training Aids Used in Detection Canine Training’. Frontiers in Allergy 5. doi:10.3389/falgy.2024.1445570.
[9] Syrotuck, William G. 2000. Scent and the Scenting Dog. Barkleigh Productions, Incorporated.
[10] Osterkamp, Tom. 2020. Detector Dogs and Scent Movement. CRC Press.
