Introduction

Domestic dogs (Canis lupus familiaris ) have worked alongside humans for thousands of years, primarily used for hunting, guarding, and even forensic work by the ancient Greeks . Even now, dogs support humans by assisting those with disabilities, herding livestock in agriculture, providing protection in law enforcement and military, and utilising their sense of smell to find a vast range of substances . Dogs have searched for numerous targets including accelerants, hazardous chemicals, explosives, illegal drugs, disease in humans like cancers, diabetes and epilepsy, live humans, cadavers, and more . Within canine scent work, one of the most up and coming areas is that of conservation.
Dogs began working in conservation in the 1890’s in New Zealand when they supported efforts to translocate kiwis and kakapos away from areas inhabited by invasive predators . Since then, there has been an almost unlimited scope to their application. Conservation detection dogs (CDD) can perform a variety of tasks like searching for live or dead specimens, nests or burrows, and residual scent from hair or urine . Additionally, scat surveys have been used for indicating animal presence particularly by using DNA analytical techniques like barcoding (i.e., species identification ) and profiling (i.e., identification of an individual organism ) located scats, especially when the scat of different animals is visually indistinguishable . CDD use has been documented in 62 countries across over 480 biological species including terrestrial, avian, and aquatic mammals, birds, reptiles, amphibians, insects, molluscs, fungi, bacteria, and invasive plants . Seemingly, scent detection dogs have “limitless potential” and their application is restricted only by the “human imagination” . They are invaluable, especially during a time when biodiversity is deeply threatened and the risk of extinction faces many species .
Given that most animals have olfactory capabilities for navigation and communication , why are dogs used most frequently for conservation detection work rather than other species? A key factor is the sheer capacity of canine olfaction. Dogs have up to 250 million olfactory receptors, depending on breed, in comparison to five million in humans and can detect odours at concentrations as low as one part per trillion whereas analytical instruments are restricted to parts per billion . This is due to the unique anatomy of canine nasal organs and brain . However, rats, insects, and pigs can also be trained to perform scent discrimination like CDD , so why are these species used less frequently?
For conservation work, trainability and capability in the field are required in addition to olfactory acuity . Canine domestication means that the species has been selected for sociability, motivation, and flexibility of learning ; psychological traits necessary for conducting complex scent work alongside humans. Furthermore, most conservation work takes place outdoors for several hours in varied weather, topographical, and vegetation conditions, meaning CDD must be able to traverse great distances, over extended periods of time, whilst manoeuvring through obstacles. As such, specific physical features are sought when selecting a dog: stamina, agility, and resilience to temperature to name a few . These are characteristics seen in many dogs that are rarely found in smaller or less domesticated species.
CDD have been highly beneficial to conservation outcomes. Their use is non-invasive which protects environmental and wildlife welfare and is preferable to capture-recapture methods . Across many circumstances, CDD are faster, can find more samples, and cover greater distances during a survey than other methods . For example, found that when comparing humans and CDD during searches for bat carcasses at wind turbine sites, CDD took on average 40 minutes to conduct a search versus humans taking 2 hours and 46 minutes and CDD found 75% of targets versus humans finding 20%. Furthermore, using CDD can reduce sampling bias as they do not rely on visual information to find targets the way methods like human surveys and camera-trapping does. Therefore, CDD are more capable of finding obscured samples and those in visually less obvious places . Additionally, CDD can play the role of ambassador for conservation work through people’s affinity towards dogs .
However, like any detection tool, disadvantages must also be considered. CDD teams are expensive both in terms of time and money. It takes months, if not years, to train a CDD and its handler along with the monetary cost of training and maintaining the dog through transport, housing, food, etc. . Acquiring samples for training can be difficult both practically and legally depending on whether the target species is elusive, endangered, or invasive . Moreover, despite generally high efficacy rates, substantial variation occurs which brings CDD reliability into question. Indeed, modern guidelines for conservation methods, such as ‘What Works in Conservation 2021’ by along with governmental protocols for target species searches , do not include CDD despite their widespread use, which may be indicative of the concerns around their efficacy. Given that conservation suffers from underfunding , the tool used for a project must be worth the cost.
Hence this review aims to answer the questions how, why, and to what extent does efficacy vary, as these must be understood to achieve the best results possible when using CDD. To do this, all available CDD studies were searched for (n=67) and analysed in light of these questions. A major difficulty facing CDD work is a lack of standardisation across the field . Although efforts to standardise procedures for the use of scent detection dogs in general have been made , they have not included the specifics of CDD work. At present in CDD literature, terminology for analytical measures is inconsistent , sample sizes are small leading to low statistical power , up to 70% of CDD studies report no training details, and almost 25% are considered poor quality . All these factors together greatly harm the field’s reliability and replicability, which is key to verifying results and improving future research. By assessing efficacy and methodology, issues in the literature can be highlighted, thereby increasing understanding of best practice. In this review, the efficacy of CDD will be investigated across training, testing, and operational searches and when searching for different target species. Once efficacy rates have been established, the factors affecting efficacy will be discussed along with how methodological problems may be contributing.