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Sniffing Science: From Humans to Dogs to Nano-noses

Our five senses - sight, hearing, touch, taste, and smell - are our connection to the outer world. The sense of smell (scent) is evolutionarily our oldest sense, yet it is also the most frequently disregarded. From single-celled bacteria to elephants, the top contenders in the list of species with the finest scent, all living organisms can detect odors, i.e., airborne chemicals in their surroundings. In fact, for most animal species, the primary window into the sensory world is the ability to recognize and adaptively respond to environmental chemical cues. Olfaction is, after all, the vertebrate analog of chemical sensing.

Making Sense of Senses

While most animals (and even our ancestors) rely on their sense of smell to seek food and water, find a partner, and communicate, modern humans navigate the world primarily through sight and hearing. As visually-oriented beings, we fail to comprehend the underlying significance of chemical sensitivity to life, or, in other words, the importance of our sense of smell (Hutmacher, 2019). It is both primitive and intricate, making it challenging to learn and even more difficult to understand how smell translates into our increasingly computerized world (Sarafoleanu et al., 2009). Our understanding of how our sense of smell influences the way we navigate the world lags far behind our grasp of hearing and vision. As we move safely through the world, we recognize friends (and foes) and identify objects through the power of sight, which allows the human brain to schematize the external environment (Hutmacher, 2019). Sound, on the other hand, not only enables people to communicate but also provides hints about the distances of objects (Middlebrooks & Green, 1991). While objects come in distinct shades, sizes, and forms, sounds come with different volumes, pitches, and tones, connecting us to a world in motion.

What about the sense of smell? It only takes a glass of milk and a piece of chocolate chip cookie to bring you back to your childhood. This is because the sense of smell is very deeply embedded in emotional memory (Herz & Engen, 1996). An odor is a chemical particle that enters the nose, attaches to olfactory receptor neurons and then travels to the brain's olfactory bulb, where it is converted into a format that the brain can interpret. Although these cells, like other neurons, extend a thicker fiber called an axon, olfactory axons are unique in that one end directly links to the brain, while the other projects to the outside environment (Rinaldi, 2007). This information is subsequently carried by brain cells to the amygdala, where emotions are decoded, and then to the neighboring hippocampus, where learning and memory formation happen (Kadohisa, 2013). This functional network enables a close relationship between memories, sensations, and odors. Only smells can reach the brain's emotions and memory regions in such a direct way.

Figure 1 -Smell and memory are intimately connected in humans because odors take a direct route to the limbic system of the brain, including the amygdala and hippocampus, which are associated with emotions and memories (Gillespie, 2021).

Coming back to the crunchy cookie, try to pinpoint each scent you experience when eating it, without comparing it to any other smell. While this may seem like an easy task, you will almost surely fall short: there is more to odor than meets the eye. The main difficulty, however, stems from the fact that smells (which play a large part in what we call taste) are in themselves notoriously challenging to explain. This owes to the fact that the human race lacks an appropriate odor lexicon (Jönsson & Stevenson, 2013; Martina, 2022). The sense of smell is one of the most evocative senses, and probably the most difficult one to articulate.

Smelling in Stereo: A Dog’s Wet Nose

While humans rely primarily on sight to perceive their surroundings, spending more time deciphering visual data than olfactory information, dogs are the polar opposite: their remarkable sense of smell far surpasses that of humans. Although dogs use all of their senses to engage with the world, olfaction is their primary form of communication, not only conveying information about the current environment but also providing clues about the past, for example, the presence of a prey in the past (Siniscalchi et al., 2018). Dogs rely on the intensity of smells to keep track of time; while the weakest odors are the oldest, the strongest ones are also the newer. By distinguishing between new and old smells, dogs actually perceive events and substances across time intervals (Horowitz, 2016).

Dogs use this intricate web of distinct odors to create a three-dimensional picture of the environment over time, which is crucial when searching for food, identifying danger, or identifying suitable mates.(Kokocińska-Kusiak et al., 2021). Dog noses have up to 300 million olfactory receptors, compared to roughly 6 million in humans, and have a 40 times larger brain area devoted to scent analysis. Furthermore, dogs have neophilia, which makes them drawn to new and exciting odors (Phoenix Veterinary Center, 2023). In fact, the dog's odor detection capability has been claimed to be 10,000-100,000 times more than that of an ordinary human, with the canine's bottom limit of detection for volatile organic compounds being one part per trillion (Walker et al., 2006); that equates to half a teaspoon of salt dissolved in an Olympic-sized swimming pool.