An Olfactory World

During the past two weeks, we’ve discussed the evolution from wolf to dog, and the powerful communication that can exist between human and canine. Today, we’re going to dive deeper into the inner world of dogs by examining their olfactory system. While humans primarily depend on visual input, dogs receive their most relevant information through their noses, snoots, and wiggly sniffers.

Most people have heard that dogs have more sensitive noses than humans, but that is just the start of the story. Here are some impressive facts:

1. Dogs possess up to 300 million olfactory receptors in their noses – humans have around 6 million.
2. The olfactory processing center in their brain is more than 40 times larger than the human olfactory processing center, proportionally speaking.
3. Dogs can independently control each nostril.
4. Dogs have separate pathways for incoming air – one used for respiration, and one used for olfaction.
5. Dogs have a whole other sensory organ for smelling! (Tyson, 2012).

Even before a scent particle enters the dogs’ nose, these animals are already hard at work. Some dogs, like blood hounds, have evolved physical traits like long ears which help guide more odorants into the nasal passages (Gadbois & Reeve, 2014). Most dogs have very wet noses. This is no accident. The mucus glands in the nasal cavity help capture and dissolve odorants in the air even before they hit the olfactory tracts (Quignon et al., 2003). Even the small slits at the edges of each canine nostril play an important role – outgoing air is pushed into these outgoing channels and then swirls as it exists, pushing more fresh air in. This allows the dog to have an almost continuous stream of new olfactory information, even without actively inhaling or sniffing (Eiting, Smith, Perot, & Dumont, 2014). Since each nostril can be controlled individually, it is possible for the dog to judge concentration and even direction of the stimulus with incredible speed and accuracy. When a dog sniffs, he increases the efficiency of this by almost three times (Rygg, Van Valkenburgh, & Craven, 2017).

From there, the odorant is sucked into the nasal passage. In humans and many other animals which do not rely on olfaction as a primary sense, the odorants are processed as the air is inhaled for respiration (Quignon et al., 2003). The olfactory epithelium is the inner nasal cavity which is equipped with odor receptor neurons, and it is on the top of our nasal cavity. In dogs and some other mammals, however, some of the inhaled air (between 10 and 15%) flows to a separate back recess of the nasal cavity which is entirely dedicated to olfaction (Craven, Paterson, & Settles, 2010). In this section, there are structures called turbinates which filter each odor molecule based on its chemical property. The receptors within the turbinates then send electrical signals to the olfactory bulb based on this analysis. The olfactory bulb then communicates with the rest of the dog’s brain (Issel-Tamer & Rine, 1996). In a groundbreaking fMRI study preformed on conscious dogs, researchers were able to see exactly where these activations tend to take place. Specifically, there is strong activation in the piriform cortex, the periamgygdaloid cotex, and the entorhinal cortex (all which tends to process smells), as well as within the cerebellum (which is often implicated in movement and cognition) and the frontal cortex (which is generally thought to control higher-order cognitive processes) (Jia et al., 2014). While the exact reasons for this connectivity remains unclear, these robust connections indicate that dogs filter these odorants and use the information gathered in the olfactory bulb to make decisions – should they eat an object? Should they run away or approach?

Finally, Jacobson’s organ, or the vomeronasal organ, serves as the final part of the puzzle from odor molecule to behavioral response. It processes volatile odorant particles which are not even usually detectable to the human nose – primarily pheromones (Tyson, 2012). Dogs can decode these chemical signals to learn information about other animals’ sex, mood, and more. This is one reason dogs love to sniff the urine of other animals! These odorants are identified and coded using a different pathway than the primary olfactory system, but which run in parallel (Craven, Paterson, & Settles, 2010). These connect to similar parts of the brain as the primary system, but they also show strong connectivity to the pituitary gland, which serves to regulate the dog’s own hormone production (Jia et al., 2014).

While this is hardly the whole story, research into canine olfaction is increasing, and so are its uses. Now that we’ve examined a bit about how dogs have such an incredible sense of smell, stay tuned next week as we dive deeper into the ways they use this power to sniff out everything from missing persons, cancer, and bomb threats.

References

Craven, B. A., Paterson, E. G., & Settles, G. S. (2010). The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia. Journal of The Royal Society Interface, 7(47), 933–943. https://doi.org/10.1098/rsif.2009.0490.

Eiting, T. P., Smith, T. D., Perot, J. B., & Dumont, E. R. (2014). The role of the olfactory recess in olfactory airflow. Journal of Experimental Biology, 217(10), 1799–1803. https://doi.org/10.1242/jeb.097402.

Gadbois, S., & Reeve, C. (2014). Canine Olfaction: Scent, Sign, and Situation. In A. Horowitz (Ed.), Domestic Dog Cognition and Behavior (pp. 3–29). https://doi.org/10.1007/978-3-642-53994-7_1.

Issel-Tamer, L., & Rine, J. (1996). Organization and expression of canine olfactory receptor genes. Proceedings of the National Academy of Sciences, 93(20), 10897–10902. https://doi.org/10.1073/pnas.93.20.10897.

Jia, H., Pustovyy, O. M., Waggoner, P., Beyers, R. J., Schumacher, J., Wildey, C., Deshpande, G. (2014). Functional MRI of the Olfactory System in Conscious Dogs. PLoS ONE, 9(1), 1–21. https://doi.org/10.1371/journal.pone.0086362.

Quignon, P., Kirkness, E., Cadieu, E., Touleimat, N., Guyon, R., Renier, C., Galibert, F. (2003). Comparison of the canine and human olfactory receptor gene repertoires. Genome Biology, 4(12), R80. https://doi.org/10.1186/gb-2003-4-12-r80.

Rygg, A. D., Van Valkenburgh, B., & Craven, B. A. (2017). The Influence of Sniffing on Airflow and Odorant Deposition in the Canine Nasal Cavity. Chemical Senses, 42(8), 683–698. https://doi.org/10.1093/chemse/bjx053.

Tyson, P. (2012). Dogs’ Dazzling Sense of Smell. Nova. https://www.pbs.org/wgbh/nova/article/dogs-sense-of-smell/.