Dogs breeds commonly used in scent detection work include Labrador Retrievers, Bloodhounds, German Shepherds, Malinois, Beagles and several other hounds and spaniels. These breeds have traditionally been used as hunting dogs, search and rescue dogs, police dogs and military dogs. There is a common assumption (in part supported by their large number of scent receptors) that these breeds have a superior scenting ability. On the other hand, short-nosed (brachycephalic) breeds like pugs, boxers, and bull dogs are often assumed to have a poorer sense of smell due to their squished faces and the reduced size of their rostrum.
Hall et al. (2015) set out to test the assumption that German Shepherds (a common scent detection dog) would outperform Pugs (a short-nosed breed) and Greyhounds (a sight hound) in a scent discrimination test.
Hall et al. (2015) recruited pet dogs for the study that were in good health and had no prior scent training. The subjects originally included 10 German Shepherds, 11 Pugs, and 10 Greyhounds. However, 1 Pug and 9 of the Greyhounds refused to participate in the experiment. The dogs were trained to alert to a cotton ball with anise oil buried in a bin of pine shavings. In discrimination tests, the dog had to select the target odor that was in one of two bins. They also ran the test using lower scent concentrations (10% and 1% dilution) of the anise extract. For comparison to the scent tests, they ran a visual discrimination test using different size Styrofoam cups.
Results of the Study
Pugs performed significantly better German Shepherds in proportion correct (i.e. correct selection of the bin containing the target odor). By the end of the testing, 9 out of 10 pugs had an 83% or greater accuracy compared to only 3 out of 10 German Shepherds. (See Figure 2 from Hall et al. 2015)
Pugs also outperformed German Shepherds on the dilution tests. The Pugs maintained a high level of accuracy at the 10% dilution, and the Pugs’ average performance at the 1% dilution was greater than the German Shepherds performance at the 10% dilution. (See Figure 2 from Hall et al. 2015)
In the visual discrimination tests, Pugs and German Shepherds had similar accuracy. (See Figure 4 from Hall et al. 2015)
Greyhounds were excluded from analyses due to the small sample size.
The results of this study did not support the hypothesis that a traditional scent dog (the German Shepherd) would outperform a short-nosed breed (the Pug) in a simple scent discrimination test. In fact, Pugs outperformed German Shepherds even at low scent concentrations. The researchers conclude that the differences “cannot easily be explained as general learning differences, general trainability, or simple motivational differences” because the two breeds performed similarly at the visual discrimination test. Unfortunately, the Greyhounds could not be motivated to perform the test, so they were excluded.
Why might Pugs outperform German Shepherds?
Previous researchers have assumed that Pugs’ physical characteristics such as shrunken rostrums, crowding of the ethnoturbinate bones (see photos at the end of this article), and rotation of the olfactory lobe lead to reduced olfactory (scenting) ability (Roberts et al. 2010). The results of this study do not support this assumption and further research is clearly needed. Hall et al. (2016) suggest that Pugs may have a higher density of olfactory receptors which might counteract the effect of their shrunken rostrum. German Shepherds have 225 million scent receptor cells, but I was unable to find any research relating to pugs (citation needed). The placement of the olfactory recess may also play a role, but hasn’t been studied in Pugs. Hall et al. (2016) also postulate that the difference in performance may be due to behavioral rather than physical differences. For example, sniffing is thought to play a large role in olfactory perception and “Pugs may engage in a superior sniff pattern.”
Does this mean that Pugs should be the new breed of choice for scent detection work?
Despite their performance in this particular test, in most cases Pugs would not make superior scent detection dogs because they are more prone to overheating and are not be able to perform the physical tasks of larger breeds. Hall et al. (2015) are careful to point out that the results of this study should not be generalized to the larger population. The German Shepherds in this study were pet dogs with no prior scent detection training while most German Shepherds used in serious scent work such as search & rescue, police and military are carefully selected from working lines of German Shepherds. If Pugs were tested against these dogs, it is quite likely that the German Shepherds would outperform the Pugs. Another study by Polgar et al. (2016) found that scent breeds performed better than non-scent breeds and short-nosed breeds in a natural detection task, but the difference was only statistically significant at the lowest level of concentration.
Hall NJ, Glenn K, Smith DW, Wynne CDL. 2015. Performance of Pugs, German Shepherds, and Greyhounds on an Odor-Discrimination Task. Journal of Comparative Psychology 129: 237-246.
Polgar, Z, Kinnunem M, Ujvary D, Miklosi A, Gacsi M. 2016. A Test of Canine Olfactory Capacity: Comparing Various Dog Breeds and Wolves in a Natural Detection Task. PLoS ONE 11(5): e0154087.
Roberts T, McGreevy P, Valenzuela M. 2010. Human Induced Rotation and Reorganization of the Brain of Domestic Dogs. PLoS ONE 5(7): e11946.
Learn more about dog scenting ability from the Canine Olfaction Lab at Texas Tech University.