Ahead by a nose

19 April 2013 by Malcolm Campbell, posted in Biology

I wish we could see perfumes as well as smell them. I'm sure they would be very beautiful.” From Anne of the Island (1915) by Lucy Maud Montgomery

This week, the dogs had their blindfolds removed. Not real blindfolds of course, but metaphorical ones. For the past three months, one major component of the dogs’ outdoor world had been masked by the cold weather. Scent.

For dogs, as for many other fellow mammals, the world is shaped as much by scent as it is by sight and sound. During the winter months, many of the odours that help create a dog’s perception of the outdoor world are locked away - quite literally stored in a deep freeze until the spring thaw.  For dogs, winter creates a metaphorical blindfold for the sense of smell in this outdoor environment. With the warmth of spring, outdoor odours are unlocked again, unleashing a whole other dimension to a dog’s view of the world in which they live.

It’s easy to understand why a dog’s world is so shaped by scent. Dogs are said to be “macrosmatic”. That is, they have a strong, broad sense of smell. Contrast this with humans, who are said to be “microsmatic”, the very opposite of dogs, with a poor sense of smell. One need look further than the noses on our respective faces to see why this difference is so pronounced.

The canine nose is a marvel of nature.  As with other animals, the canine nose is a multitasker. It functions to inhale and exhale for breathing, while also collecting odours for the purpose of smelling. Notably, the canine nose separates these two tasks. During inhalation, the air is split between two passages, one for breathing and the other for the sense of smell, known as olfaction. For we humans, olfaction occurs through a small region in our nasal cavity, and we detect odours as they flow in and out as we breathe. Contrast this with dogs. In dogs, inhaled breath is split. While the majority makes its way to the lungs, about 12% of inhaled air is routed to recessed regions inside the nose dedicated to olfaction for each nostril.  For each nostril, these recessed regions contain three astonishing mazes, called turbinates – labyrinths shaped by bone and cartilage, which provide a scaffold for specialised olfactory cells. These cells are characterised by the presence of hair-like fringes, cilia, which are ideally suited to capture odour molecules as they flow past. So as not to be confounded in their purpose, exhaled air bypasses the turbinates, and exits on the sides of the nose. The manner in which spent air exits the nose not only avoids interfering with current olfaction, but also simultaneously directs new scents into the nose. In the dog’s nose, evolution has crafted a device that makes exquisite use of fluid dynamics.

As if optimised architecture were not enough to set the canine nose apart, it also contains some astonishing cellular and molecular innovations. For example, the ciliated cells of the turbinates create a huge surface area for the detection of scent, up to 100 cm2 per nostril. Of this surface area, up to 80% is populated by neurons, which will transit the signal derived from the odour to the brain. There are thought to be hundred of millions of these neurons in a dog’s nose. By comparison, the human nose possesses 5-10 million. Odours are perceived by these neurons through the action of olfactory receptors. Olfactory receptors capture odorant molecules. This chemical signal is then converted to an electrical signal that is transmitted to the brain by the neurons.

Olfactory receptors are thought to operate according to a lock-and-key model. Odorant molecules have a particular shape. Olfactory receptors have a three dimensional pocket into which these molecules can fit. The entry of the odorant molecule into the three-dimensional pocket of the olfactory receptor “unlocks” the activity of the receptor, resulting in the transmission of a signal. The lock-and-key metaphor is somewhat misleading, as the rigidity of a lock and a key do not adequately capture some of the inbuilt flexibility in olfaction. Nevertheless, the lock-and-key model provides a useful framework to consider how diversity in the number of locks (olfactory receptors) can lead to a greater capacity to detect odorants (keys).  And this is precisely the case for canine olfaction. Dogs have more than two times the number of different active genes that encode olfactory receptors in comparison to humans. Given that olfactory receptors are likely to report on odorants by working in concert, even the two-fold difference in olfactory receptor diversity creates vastly greater potential to discriminate complex odours.

Remarkably, the canine olfactory system is not limited to perception through the turbinates. There is also an accessory olfactory system. The accessory olfactory system makes use of a specialised structure, originally called the Jacobson’s Organ, the vomeronasal organ. The vomeronasal organ is separated from the turbinates, and is located at the base of the nostrils and the roof of the mouth.  The vomeronasal organ is not only physically separated from the turbinates, but it performs a different function in olfaction, detecting molecules that are imperceptible to us, but which elicit specific behaviours in dogs, pheromones. Neurons in the vomeronasal organ detect pheromones, and transit signals to the brain that shape the perception of other odours, and ultimately change canine behaviours.

Working together, the primary and accessory olfactory systems provide dogs with a wealth of information about their world. This information is not merely limited to the difference in the range of odours present in the environment. Dogs’ ability to gather information from one nostril versus the other suggests that they perceive odours in three dimensions. We humans understand a visual three-dimensional world. It is difficult for us to perceive other senses in three dimensions. What does a three-dimensional smell “look” like? Imagine your world populated by clouds of odour, with the potential for clouds to overlap. Each cloud contains information about its source, and may contain information about the sources’ relations to each other.

The suggestion that dogs “visualise” scent in three dimensions is not far fetched. The role that olfaction plays as a spatial cue has been beautifully tested using moles. Moles prove to be a useful experimental model to test this role of olfaction, as their vision is limited, and as they must negotiate below-ground three-dimensional environment in the dark. By manipulating the nostril through which scent is perceived, the role that olfaction plays in the moles’ perception of its physical place in world can be tested.  It turns out that moles rely on the directionality of odour cues, as perceived by one nostril or the other, to navigate their world. It is not a stretch to imagine that dogs could use odour cues in a similar manner, albeit in a manner that also makes use of visual cues.

Beyond the three-dimensional information provided by olfaction, the intensity of scent, and its movement, also provides some indication of the passage of time. Recently deposited scents will be more intense; those that are fainter may have been deposited some time ago. Thus, it may be that dogs’ not only gather spatial information from scents, but scents might also provide dogs with temporal cues – how long ago was something here? It is tempting to speculate that scent may also provide dogs with a sense of history. Who was here? How long ago?

All told, our canine companions have an olfactory capacity that far exceeds ours. It’s been estimated that canine olfaction is somewhere between 10000 and 100000 times better than ours. Unsurprisingly, a large portion of the canine brain is given over to interpreting this information. It’s been hypothesised that as much as a third of the canine brain processes olfactory signals.

In the world of odour, we are the ones who are blindfolded. Apart from lacking the ability to perceive faint scents, or odour in three or four dimensions, we are undone by complex mixtures of odorants. For us, mixtures of 30 odorants creates the scent equivalent of “white”. Just as the mixture of colours appears as though it has no colour, the mixture of scents gives the impression of no scent. Contrast this with the truly astonishing ability of canines to discern subtle changes in our body chemistry as revealed by odour.  No small wonder that we make use of their incredible noses.

And yet, those odours that we can perceive mean so much to us. Scent is an incredible evoker of past experience, we have strong “olfactory memory”. Perfume and fragrance sales show that we value scent as a means to shape how others perceive us. We add fragrances to mask unpleasant smells. This is not without consequence for our fellow creatures who are not as insensitive to odour. Imagine the way we have bombarded their senses, with our penchant for odorants and perfumes. How have we confounded their capacity to perceive the world? With our desire to make the world a better smelling place, is it also possible that we have undermined our own capacity to use scent to inform our own perceptions?

Our ability to perceive odours appears to shape our relationships with each other. Recently, a small study found that people born with anosmia, the absence of a sense of smell, were affected in their relationships with sexual partners. Men who were born with anosmia had a markedly reduced number of intimate interpersonal relationships. Women born with anosmia felt less secure in such relationships.  We may lack the finely honed apparatus that dogs possess to perceive the world of scents, but it seems that scents still shape the way we move through life. For us, perhaps scent has become less about where and when, and more about who. Something to bear in mind as we doctor the fragrances of our lives.

References:

Catania KC (2013) Stereo and serial sniffing guide navigation to an odour source in a mammal. Nature communications 4: 1441.

Craven BA., Paterson EG, & Settles GS (2010) The fluid dynamics of canine olfaction: unique nasal airflow patterns as an explanation of macrosmia. Journal of The Royal Society Interface 7: 933-943.

Croy I, Bojanowski V, & Hummel T (2013) Men without a sense of smell exhibit a strongly reduced number of sexual relationships, women exhibit reduced pa rtnership security–a reanalysis of previously published data. Biological Psychology 92:556–558

Lawson MJ et al. (2012) A computational study of odorant transport and deposition in the canine nasal cavity: Implications for olfaction. Chemical Senses 37: 553-566

Quignon P et al. (2012) Genetics of canine olfaction and receptor diversity. Mammalian Genome 23: 132-143

Weiss T et al. (2012) Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white. Proceedings of the National Academy of Sciences 109: 19959-19964

Images: All photographs by Malcolm M. Campbell.

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3 Responses to “Ahead by a nose”

  1. Peter Apps Reply | Permalink

    To make an "olfactory white" from a mixture of 30 compounds requires some very careful formulation that adjusts the concentration of each component so that they all have approximately the same odour intensity. Since this never happens outside the laboratory, the existence of "olfactory white" really does not throw any light on how good the human sense of smell is. Hot off the press is the finding that humans can distinguish over one trillion odour mixtures; http://www.sciencemag.org/content/343/6177/1370.abstract

    • Malcolm Campbell Reply | Permalink

      Thanks for your comment, Peter! You are correct that construction of "olfactory white" does require careful formulation under laboratory conditions. While such mixtures may not arise in nature (although it is a formal possibility), the usefulness of such experiments is that they provide information about how mixtures of compounds work together to create olfactory sensation. The new study published in Science that you cite is remarkable, and worthy of further consideration. [In fact, you'll see it cited in today's #SixIncreibleThingsBeforeBreakfast in my Twitter feed.] Please note that the post above was written almost a year prior to the work published in Science. To see such remarkable new results come to light just a year after the post above points to the rapid progress that is being made in the science of olfaction! Very exciting times indeed. It will be the subject of a post in the future. Stay tuned!

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