Olfactory Repellents in Road Ecology: What We Know and What to Focus on in the Future

Abstract

Road transport systems kill millions of animals on every inhabited continent each year, and thousands of human lives are lost. Odour repellents (ORE) are a WVC mitigation measure which have been extensively applied across central Europe to prevent or minimise the number of ungulate–vehicle collisions (UVCs). OREs aim to increase the vigilance of ungulates near roads and therefore change their behaviour in areas where vehicle collisions may occur. Despite many scientific papers on the topic of odour repellent effectiveness, a lack of behavioural studies means there is still little understanding of the mechanism of ORE functionality. OREs are applied as an area repellent, so their effectiveness is influenced by multiple factors, and constantly discussed by both academics and the lay public. This paper summarises the state of knowledge about application and effectiveness of odour repellents in road ecology, and suggests research questions to fill information gaps.

1. Introduction

As road transport systems have grown, their impact on wildlife has become more significant and an increasing problem worldwide [1]. Road transport systems now kill millions of animals on every inhabited continent each year, and thousands of human lives are also lost as a result of these collisions [2,3,4]. Wildlife–vehicle collisions (WVCs), especially with wild ungulates, pose both traffic safety [3,5] and wildlife management issues [3,6]. Patterns of WVCs are continentally or regionally specific, differing in (i) technical aspects of transport infrastructure; (ii) traffic intensity and types of vehicles; (iii) type and size of animals; (iv) life cycles and animal behaviour patterns; (v) environmental conditions; and (vi) seasonality, weather events, and climate change. These aspects determine local approaches to preventing, minimising, or compensating for WVCs.

Odour repellents are a WVC mitigation measure primarily focused on ungulates [7,8]. They have been extensively applied in central Europe to prevent or minimise the number of ungulate casualties on roads due to collisions with motor vehicles on roads with lower traffic intensity [7], but there is still little understanding of exactly how they work [9]. OREs aim to increase the vigilance of ungulates near roads and therefore change their behaviour in areas where collisions with vehicles may occur [10,11,12,13]. However, due to the nature of their application as an area repellent, their effectiveness is influenced by a considerable number of factors and thus is constantly discussed both by the scientific community and the lay public.

The aim of this article is to summarise the state of knowledge about application and effectiveness of odour repellents in road ecology and, based on the synthesis of presented knowledge, provide basic suggestions regarding research questions to fill information gaps.

Conceptual Framework of the Work

Generally, we focused on an issue of proven knowledge about the application of odour repellents as a preventive or mitigation measure for wildlife–vehicle collisions. At the end of the year 2023, Bíl et al. [7] finished probably the largest and most robust study in Europe focused on odour-repellent effectiveness as a preventive or mitigation measure for WVCs which compares data from 134 road segments with complete records using a Before-After-Control-Impact approach. From the results point of view, it was a schizophrenic situation because we knew much less about the background of the results than we expected. These findings were the main motivation for writing this manuscript. We wanted to clearly name the topics to which it is necessary to pay intensive attention in future research, to understand how OREs can effectively work and thus be applied.

An intensive literature search was performed using search engines such as Scopus, Web of Science, and Sciencedirect. The search focused on the most common words in connection to OREs, such as odor, odour, olfactory, olfaction, repellent, odour repellent, odor repellent, scent, scent fence, deer repellents, mammal repellents, predator odours, predator odors, predator urines, predator faecal odours, predator fecal odor, chemical barriers, chemical deterrents, and habituation and their interaction and implication within road ecology. At the same time, we studied the websites of the producers and the safety data sheets of the products most frequently used as preventive or mitigation measures for WVCs.

2. Basic Synthesis of What We Know

Chemical repellents have been used since the 1970s to protect agricultural crops, areas of conifer seedlings, and orchards from game damage and over time they have been also used within road ecology with the main goal of preventing or minimising numbers of WVCs. Within road ecology, chemical or natural substances are the most commonly used.

2.1. Mechanism and Main Active Substances Used

A specific constraint on the use of OREs within road ecology is the need to apply them as an area repellent; direct application to wildlife food resources is not practical. This limits how and to what degree they may be perceived by animals. Within road ecological management to prevent or minimise ungulate–vehicle collisions (UVCs), OREs are most often applied in the form of liquids, aerosols, or within polyurethane foams [14,15]. The key element of their functionality is the volatile active ingredient. The most frequently used active substances in OREs include (a) isovaleric acid, (b) nonane acid, (c) oil acid, (d) low fat acid, (e) animal fats and oils, and (f) 2-Undecanone, either singly or in combination (isovaleric acid—nonane acid, isovaleric acid—2-Undecanone—nonane acid, or isovaleric acid—oil acid) [16,17].

2.2. ORE Effectiveness in Linkages to the Target Species

Odour repellents are a technical solution to managing WVCs with a chemical concentrate that mimics the odours of large carnivores (bear, wolf, lynx) and humans [18,19]. OREs should both deter wildlife from crossing the road and increase their vigilance [15,20]. Target species for OREs are mainly ungulates in the European context (roe deer, red deer, moose, fallow deer, and wild boar). Considerable inconsistency in demonstrating the efficacy of OREs has been published de facto for all target species cited, with no major differences [9,10,12].

2.3. ORE Effectiveness versus Habituation

Many studies have already addressed the topic of the effectiveness of OREs [21], and have provided very different results both regarding the effectiveness of individual OREs, and the time of habituation to them [22]. The possibility of habituation and the time frame within which it can potentially appear in wildlife behaviour is one of the most debated topics in relation to the application of OREs. Although Curtis and Eshenaur [23] confirmed the long-term effect of the Trico repellent, other studies point to habituation after only a few days or weeks [7,22,24,25]. A better understanding of the factors involved in habituation could help prolong the effectiveness of odour repellents [26]. However, previous experience with a predator whose odour is contained in the ORE [27] or the individual’s personality [28] can also play a role.

2.4. International Experiences of Odour Repellent Effectiveness

Widely variable data are available on the effectiveness of OREs. Product manufacturers report high rates of wildlife accident reduction, for example up to 90% [29], and an average of 76% and in some individual cases 100% [17]. In Europe, OREs are most often used in the Czech Republic, Slovakia, Italy, Slovenia, Lithuania, Poland, Portugal, Austria, Romania, Switzerland, Spain, and Germany [19]. For comparison, ORE application in practice shows the following results.

• Germany

Kerzel and Kirchberger [30] recorded a decrease in traffic accidents involving roe deer from 22 cases to 2 cases on the monitored road section within one year. Staines et al. [31] researched ORE effectiveness on six test road sections in Bavaria and northern Westphalia. They found that 60% of the animals encountering the areas treated with OREs crossed the road beyond the “scent fence” at an untreated section, and 20% crossed the road despite the ORE treatment but moved very rapidly, without delay. The remaining 20% of animals were unaffected. Trothe et al. [32] reported a reduction in WVCs of about 57%, but their conclusions were not supported by statistical analysis. Lutz and Walburga [33] evaluated the use of OREs and found that fallow deer were not repelled at all and red deer were only repelled from the road for a short time, but roe deer approached the treated area very carefully. This study did not find any reduction in wildlife mortality.

• Austria

Lebersorger [34] reported a reduction in WVCs of between 30 and 80% in a length of road treated with ORE, but pointed out an increased number of collisions with wildlife outside the treated section. Steiner [35] found a relatively high efficiency (70% reduction in collisions with roe deer) when using a combination of an odour repellent with optical and acoustic measures. Ruzicka [36] reported a decrease in collisions with roe deer at an ORE-treated road section by up to half or more, and recommended not using an odour repellent all year round in order to prevent wildlife habituation.

• Czech Republic

Mrtka et al. [37] monitored WVCs on a 46.7 km section of the D1 motorway in 2008 and 2009. Based on the collected data, the two most risky sections for WVCs were identified, with a total length of 3 km; these were treated with ORE. In 2010, the reduction in deer–vehicle collisions along the treated sections ranged from 50 to 69% compared to the pre-treatment average from 2008 to 2009. Kušta et al. [12] studied a stretch of second-class road and parallel regional railway, spaced some 200–400 m apart. As a control, animal mortality on the road and railway was monitored in 2011. In 2012 and 2013 ORE was applied to the same sections. After treatment, a reduction of 37% in WVCs was found. Bíl et al. [8] concluded that the effectiveness of OREs in reducing UVCs was in the range of 26–43%. The largest and most robust study of ORE effectiveness in Europe is Bíl et al. [7]: this compares data from 134 lengths of road with complete records using a Before-After-Control-Impact approach. The results, based on the Cochran–Mantel–Haenszel statistical approach, suggest that application of OREs should reduce deer mortality. A higher efficiency was found for the first seven weeks after application.

• Lithuania

Balčiauskas and Jasiulionis [38] tested odour repellents in the gaps between motorway fencing at wildlife crossing points. The results showed a 42% reduction in the number of mammal crossings and a reduction in mortality as well.

• Norway

A before-and-after site comparison study in 1985–2003 along a railway through forest in Hedmark County, Norway [39] found that application of ORE did not reduce moose collisions with trains. In another study, in ORE-treated areas, there was an average of 0.3 collisions/km/year compared to 1.8 collisions/km/year before. However, there was large variation in effectiveness between sections and the reduction was not statistically significant [9].

• Denmark

A replicated, controlled, before-and-after study in a conifer plantation in Denmark (in 2006) found that OREs did not reduce visits by deer in individual research plots [40]. Littlewood et al. [9] reported that roe deer visited a similar number of ORE-treated plots before and even after application.

• Canada

Brown et al. [41] tested OREs on arctic reindeer and found an insignificant impact on the incidence of collisions. A replicated controlled study between 1996 and 1998 in Ontario, Canada found that 18 different OREs (trialled for potential to deter animals from roads) did not deter white-tailed deer, elk or moose. Animals used a similar proportion of trails with repellents applied (63–80%) and trails without repellents (62–74%) [9,42].

• USA

Nolte et al. [43] reported that ORE application had no effect on black-tailed deer behaviour. In a meta-study, Huijser et al. [20] found evidence of the effectiveness of OREs to be unclear or questionable.

• Slovenia

In Slovenia, the effect of chemical repellents was tested on 11 problematic road sections (total length of 13.4 km). The number of deer–vehicle collisions on these sections decreased by 44% in comparison with figures from the previous year, or 37% in comparison with the average from four years before the trial. However, in the results, they indicated that the number of definite DVCs at adjacent road sections dramatically increased at the same time, resulting in a total decrease rate of only 15% (which, however, did not differ from the changes on control road sections). Therefore, the positive effect of chemical repellents as a solution to reduce DVCs was not confirmed [6,44].

• Colombia

Three types of ORE were tested to deter didelphis marsupialis from populated areas. Significant results were demonstrated when Bait + Ammonia and Bait + Creolin were applied [45].

• Uganda

Oniba and Robertson [46] conducted tests of the “smelly elephant repellent” method, which was invented in 2013 by a group of students as a competition innovation. The ORE was developed for elephants, with which the local population had serious problems, especially during the harvest season. Testing showed success in 24 out of 30 cases, with the failure in 6 cases being due to incorrect or even no application by farmers.

• Australia

Of the five repellents tested on goat behaviour, tiger dung and worm extract showed the highest effectiveness, but the effectiveness decreased with each day of use. Behavioural responses of goats to types of repellents also varied. With the tiger droppings, they behaved very greedily and mistrustfully, while, with the worm extract, they showed signs of offense (sniffing, shaking their heads, and moving away quickly) [24].

• Brazil

Changes in the capybara’s behaviour were demonstrated after the application of the jaguar dung repellent; however, after 5 days, it was clear that the capybaras had become accustomed to the smell and their reluctance to enter the marked area was decreasing [25].

2.5. Application

OREs are usually applied by hunting associations, road managers, and local government organisations. A common problem is very weak or no coordination between these interest groups. Other problematic factors associated with ORE application include the choice of location (extent of the treated area) and the duration of the active application. OREs are often not applied only in the riskiest places at the riskiest times but are applied all year round over very long road segments. All these factors are essential for the possibility of habituation of wildlife to OREs (see Section 2.3) and thus strongly influence the efficiency of OREs.

The most common mechanism of ORE application is using polyurethane foams as carriers of active substances (e.g., Hagopur Duftzaun scent barrier foam). The foam itself continues to contain the active odour for first several weeks after application. After this, rejuvenation of the odour active substances has to be provided (e.g., Hagopur Duftzaun barrier concentrate) [7]. Foam carriers are most often mounted on wooden battens, branches or tree trunks. An important factor of ORE efficiency is adherence to the regular and maximum-permitted spacing between the application points of the foam carriers; therefore, application on wooden battens is prevalent (Figure 1).

Wooden battens to which OREs have been applied can be damaged or destroyed during roadside maintenance (grass cutting) or because of agricultural activities in neighbouring fields [10,11,12,47]. Another possible way of applying OREs is spreading the carriers on the ground surface. An example of a carrier is wood sawdust rolls [10]. No scientific evidence is available on the efficiency of using wood sawdust rolls as ORE carriers placed on the ground surface, only statements from producers.

Comparison of the effectiveness of active ingredients in deer repellents has historically been provided mainly for the purpose of protecting agricultural areas [48,49], areas of conifer seedlings [50], or areas of orchards [51]; however, similar comparisons in road ecology are missing.

2.6. Wildlife Reaction to Odour Signals of Predators

Predation represents a selective force in the evolution of wildlife behavioural characteristics [27]. According to Apfelbach et al. [27], predator odours have three distinct effects on the behaviour of potential prey: (i) reducing activity; (ii) reducing foraging and feeding; and (iii) causing the animal to move to a safe location where predator odour is not present. Ginkel et al. [52] investigated whether returning large carnivores will modify ungulate behaviour and found that placing wolf urine did not significantly affect deer behaviour. Individuals may react differently to the presence of a predator’s scent depending on the lengths of time they are exposed to it. Eccard et al. [53] found that roe deer react with increased vigilance when faced with an immediate risk of predation by lynx, but are less responsive to danger when exposed to odour signals repeatedly and for longer periods. At the same time, Benhaiem et al. [54] and Jayakody et al. [55] suggest that roe deer or red deer respond with increased vigilance in areas of increased human disturbance.

2.7. Summary of What We Know

• The use of OREs is based on the assumption that wildlife will react to the presence of a predator or human scent.
• Use of OREs is recommended for lower-class roads with relatively low vehicle intensity, rather than for major roads and expressways or motorways with high vehicle intensity.
• Application of OREs is usually performed using a carrier (polyurethane foam or a biodegradable carrier) coated or injected with the repellent substance.
• Typically, carriers are attached to purpose-installed wooden or plastic poles, or to the trunks and branches of trees or bushes, or road guardrails, at regular intervals.
• Wooden or plastic poles may conflict with the maintenance requirements of roadside vegetation (grass cutting) or the agricultural management of neighbouring sites: wood or plastic poles with active OREs are often damaged or completely destroyed.
• Applied OREs are also subject to theft (both the wooden poles and any other type of carrier or installation).
• Depending on the specific ORE product, the active ingredients should affect animal behaviour in the treated area for several weeks.
• Some studies of the effectiveness of OREs in reducing ungulate–vehicle collisions show a statistically significant result but in the practice application around the roads, no information is available on ORE type; (a comparison of active ingredients in deer repellents was provided for agricultural crop protection, protection of area of conifer seedlings, or areas of orchards).

3. What to Focus on in the Future

More than 30 years have passed since the first application of odour repellents as a solution to prevent or minimise WVCs, and despite considerable attention from the scientific community, their use is still subject to more questions than facts. Some of the most important questions where further research is needed to provide reliable information are the following:

• What are the best criteria for selecting suitable locations for ORE application (collision risk, traffic intensity, surrounding biotopes, length of road section, target species)?
• Are there any significant differences in spreading (evaporation) of active substances of OREs in treated areas over time (i.e., how does the concentration of active substances decrease after one week, two weeks, three weeks, etc.)?
• Are we able to measure concentrations of active substances of OREs in the air at the treated area?
• If so, are we able to measure how the concentration of active substances in the air decreases with increasing distance from the road? Is there any significant difference between types of ORE (e.g., does one evaporate faster than another)?
• Do low temperatures, rain, or other weather events significantly affect ORE evaporation rates, and therefore their effectiveness?
• What is the dependence of ORE functionality on high air humidity or wind (both of which may affect carrier to animal ORE transmission)?
• Does dense vegetation (grasses and bushes) influence the effectiveness of ORE when it is applied to the ground surface?
• Are there any differences in ORE effectiveness between ground surface application and above-ground application (at approximately one metre height on wooden poles)?
• Where ORE application on wooden poles has been found to be effective, are these results an effect of the smell itself or are they an effect of visual change in the local environment (in the form of the newly installed poles)? It would be desirable to perform a blind test with installed wooden poles without ORE application.
• Can wildlife become habituated to the active substances in OREs?
• If so, how long does it take for wildlife to stop responding to odour signals?
• Are OREs cost-effective?
• What are the environmental consequences of ORE application (e.g., foam holder degradation and the generation of waste, microplastics, and water contamination)?

4. Conclusions

Olfactory or odour signals have a wide range of applications. They mark the home ranges and pathways of mammals, control sexual behaviour, determine each individual’s individual and social status, and are used for communication.

An essential condition for achieving a higher level of understanding of the real effectiveness of OREs is a change of approach in its interpretation. It is necessary to abandon the approach of using changes in WVCs to interpret the effectiveness of OREs, even in the case of the use of control sections within BACI approaches (Before-After-Control-Impact). The focus should be rather on behavioural studies that will be able to iterate on potential changes in wildlife behaviour following ORE application. Interpretation of movement activity using direct camera footage or GPS position records may seem to be the right direction of research.

Another essential condition is to develop procedures and methods to quantify the presence of active substances in the air (or the speed of their release), and how their concentration is affected by both biotic and abiotic factors.