How Animals Smell: A Whiff of Scent Science

In this and the blogs which follow, I’ll be looking at what science has to say about smell and its implications for animals and humans.

As a Herbal Choices consultant, I am particularly interested in an animal’s sense of smell and its complex relationship with emotions, memory, experience and behaviour.

Olfaction (the sense of smell) provides an animal with a myriad of information from the presence of food and toxins to the proximity and suitability of a mate.

But the most recent research points to further dimensions of this intriguing sense, and even today, there are still controversies and disagreements surrounding the science of both human and animal olfaction.

An animal’s sense of smell has complex relationship with emotions, memory, experience and behaviour.

We all get that our noses have the starring role in olfaction and that the ability to taste hinges largely on our sense of smell.

From the nose, all the information about smell is sent to the Main Olfactory Bulb located just below the forebrain where it is dealt with before accessing higher olfactory processes in different parts of the brain. But is that it?

Take the vomeronasal organ (VNO). It’s an organ capable of absorbing far larger molecules than the cells of the nose and connects to an Accessory Olfactory Bulb. Depending what you read, it’s argued that only animals have this additional organ (situated between the back of the throat and the nasal cavity). But anatomical research from centuries ago suggests otherwise.

According to these ancient anatomical endeavours the VNO was found in human cadavers, particularly those of young children. It was found to be much reduced in size and difficult to locate in human adults.

Reading this had me wondering if an increased sense of olfaction facilitated by a VNO could explain why kids are such picky eaters. My son’s riposte to my grumbles one dinnertime was: “I’m not fussy, my taste is more sensitive.” Perhaps he’s right!

The vomeronasal organ (VNO) is an organ capable of absorbing far larger molecules than the nose

The VNO is primarily associated with detecting pheromones - another reason why the young of a species may benefit since lactating mothers produce special pheromones which enhance bonding. This is the principle behind Adaptil calming collars.

When an animal is using their VNO, you can actually see it because they pull a funny face. It’s known as the Flehman’s reflex. Really clear with a horse, a little more subtle with a dog (it can resemble a grin, their teeth may chatter).

I frequently see this response during Herbal Choices’ sessions. It’s a great sign that the animal is actively engaged and drawing odour particles over their VNO.

It also demonstrates how odours can be directed to specific areas of the brain via the receptor neurons (nerve cells) of olfactory organs. Neuroscientists have found that even within the VNO there is a dual system for detecting odours which then connect to different areas within the Accessory Olfactory Bulb itself.

In addition to progress in neuroscience, genetics gives us far more insight into how the sense of smell serves animal’ species, and how this is influenced by evolution.

It comes as no surprise to learn that many animals possess olfactory neurons that detect specific components of urine. This enables the use of urine for communicating social cues via pheromones.

Animals possess olfactory neurons that detect specific components of urine

Another factor that is important to many species, but we have lost, is the ability to detect the presence of C02.

Scientists in China discovered that mouse olfactory sensors are able to detect CO2 at levels about twice the normal level found in the air we breathe. Interestingly, compared to their wild counterparts, the specially-bred lab mice’s ability to do this was diminished. Sensing high levels of CO2, mice will respond by seeking air with lower levels.

The gene that enables mice to detect CO2 is a ‘pseudogene’ or damaged copy in many primates, including humans and so for us, CO2 is odourless.

Studying mice has also revealed the role of specialised olfactory neurons (Grueneberg ganglion cells/GG neurons) that are particularly significant during the newborn stage, as they are activated by suckling behaviour. Once activated, these neurons respond particularly to two cues which put the rodent pups in jeopardy: the absence of their mother and cooler temperatures.

The gene that enables mice to detect CO2 is a damaged copy in humans and so for us, CO2 is odourless.

NOT being able to smell mum combined with a decrease in temperature sets off strong neural activity in these GG cells: an alarm response which is a life-saving mechanism as seeking warmth and finding mum will switch off this response.

While a mouse nose can sniff out a decrease in temperature, canine noses’ ability to sense quite the opposite has hit the headlines.

Research in Sweden and Hungary focused on investigating the full function of dogs’ cold, wet noses (it’s a cooling mechanism for one thing) found that they were able to detect weak radiating heat even better than human hands. This skill adds a further dimension to a dog’s immense powers of olfaction, one that could well help in their ability to track prey. But in the same way that baby mice are able to detect a drop in temperature, for blind newborn puppies, their ability to detect heat would enable them to keep tabs on mum.

Dogs’ cold, wet noses are able to detect weak radiating heat even better than human hands

But back to the GG neurons. These were tested in adult mice with several substances to observe which other stimuli activated them. There was no response until alarm pheromones (sadly harvested from dying mice) were used. The resultant activity in these olfactory neurons suggests that mice are able to smell death.

And if mice can smell death, why shouldn’t other animals also be able to do this?

Nala, a teacup-poodle living in Minnesota, USA, belongs to a hospice manager who decided it would be an enriching experience for the residents to interact with his perky little dog.

Nala was a natural, and became an immediate favourite. However, after a while, it was observed that she seemed to be especially drawn to those who were dying and would often stay glued to their side until they had taken their last breath.

This mysterious ‘angelic’ behaviour was valued for the comfort it offered, but what directed her behaviour if not knowledge provided by senses we simply don’t have?

There are many dogs sensitive to noises, certain people or other dogs and the immediate escalation in the sympathetic or ‘stress’ response manifests in body language and behaviour. Those of us who have encountered traumatised dogs will be familiar with how rapidly they turn to ‘the four Fs’ when dealing with stress: flight; fight; flirt; freeze. These are behaviours driven by the response of the central nervous system and the consequent release of hormones.

What we can’t yet know for sure is the extent to which the clamour of sensory input includes olfactory cues. With reference to those stressed out mice in China, we find that the same GG neurons sensitive to alarm pheromones also elicit a behavioural response to these pheromones by means of the animal freezing. This could well be true of other species.

I hope this short scientific sojourn demonstrates just how powerful multi-faceted smell-sensing systems are for non-human species and that by using more than a single olfactory organ involving several different parts of the brain an animal can respond to smell on several levels: hormonally, emotionally and behaviourally.

References:

https://www.ncbi.nlm.nih.gov/books/NBK55967/#ch15_sec4

https://eu.usatoday.com/story/news/2015/04/13/inspiration-nation-teacup-poodle-nala-angel/25701151/

https://www.sciencefocus.com/news/dogs-cold-noses-are-ultra-sensitive-heat-detectors/