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Why's a Dog's Nose so Special ?

The canine nose, apart from it having prints that are as unique as human fingerprints, it plays a critical role in a dog's behaviour, health, and well-being. This remarkable olfactory system is a testament to the evolutionary adaptations that have allowed dogs to excel in various roles, from hunting and tracking to search-and-rescue operations.

The dog's nose showing the nostrils and the philtrum.
The dog's nose showing the nostrils and the philtrum.

The anatomy of a dog's nose is a remarkable and highly specialized organ, a marvel of anatomical and physiological adaptation that sets it apart from other animals. The smell abilities of dogs are amazing and considerably beyond those of humans. Dogs' sense of smell appears to be their primary sense, enabling them to locate the source of an odor, which is essential for detecting food, danger, or potential mates for reproduction, as well as to absorb both current and historical information about their immediate surroundings.

The vertical groove between a dog's nose and upper lip border is called the philtrum, and it serves a functional purpose. A small amount of saliva is retained in this groove after each time a dog licks his lips and is pulled upward by capillary action, directing odorants from mouth to smell receptors of the nose.. This helps to keep the dog's nose damp, if not wet, which makes it easier for the dog to capture scent particles.  from the air


Humans may teach dogs to use their olfactory skills in a range of disciplines, with a detection limit that is frequently considerably lower than that of high-tech laboratory equipment. Dogs' unique anatomical and physiological characteristics make them exceptionally good at sniffing out drugs, explosives, changes in emotions as well as changes in human cell metabolism during various illnesses, including COVID-19 infection and other diseases like cancer, diabetes, and other infectious conditions. A dog's nose has several unique features that contribute to its exceptional olfactory abilities, a keen sense of smell and communication.

A schematic illustration showing the traditional structure of the chemical substances detection system.
A schematic illustration showing the traditional structure of the chemical substances detection system.

Here are some details about what makes a dog's nose special:

1. Olfactory Bulb and Receptors

Dogs have an exceptionally developed olfactory system, possessing an extraordinarily high number of olfactory receptors in their noses compared to humans. While humans have around 5 to 6 million olfactory receptors, dogs have an estimate of between 125 million to 300 million or more, depending on the breed. This heightened sense of smell allows them to detect scents at incredibly low concentrations, allowing them to detect and differentiate a wide range of scents with great precision, even minuscule concentrations of odorous molecules. Their olfactory bulb, responsible for processing smells, is relatively large and well-developed compared to other mammals, comprising up to 5% of their brain's total mass, enabling them to process a wide array of scents with incredible precision and detail.

Olfactory bulb organ
Olfactory bulb organ

2. Jacobson's Organ (Vomeronasal Organ/System)

Dogs have a structure called Jacobson's Organ, or the vomeronasal organ, located on the roof of their mouths. Situated in the nasal cavity, this specialized structure allows dogs to detect pheromones, chemical signals used for communication between members of the same species. It plays a crucial role in their social interactions and reproductive behaviour and status. This additional sensory organ enhances a dog's ability to gather information about its environment too.

Vomeronasal System The system allowing the perception of pheromones in mammals is called the vomeronasal system or Jacobson-organ. The sensory neurons found in the vomeronasal organ (VNO) contain cell bodies that house receptors capable of detecting pheromones from the surrounding environment. Although very close and similar, this system is independent from the olfactory system. In fact, it projects to a separate bulb, called the accessory olfactory bulb, and from there to the hypothalamus via the vomeronasal amygdala. It also has a distinct class of genes, the vomeronasal receptors (V1R and V2R), along with Trp receptors. Those genes sustain the production and maintenance of the vomeronasal system receptors and proteins, allowing its functioning as a whole system. [8]   Schematic representation of a sagittal cut through the head of a mouse. Mice olfactory system (left). MOB: main olfactory bulb. MOE: main olfactory epithelium. AON: anterior olfactory nucleus. PC: pyriform cortex. OT: olfactory tract. LA: Lateral part of amygdala. EC: entorhinal cortex. Mice vomeronasal system (right). VNO: vomeronasal organ. AOB: accessory olfactory bulb. VA: vomeronasal amygdala. H: hypothalamus. Adapted from Dulac et al. 2003.
Schematic representation of a sagittal cut through the head of a mouse. Mice olfactory system (left). MOB: main olfactory bulb. MOE: main olfactory epithelium. AON: anterior olfactory nucleus. PC: pyriform cortex. OT: olfactory tract. LA: Lateral part of amygdala. EC: entorhinal cortex. Mice vomeronasal system (right). VNO: vomeronasal organ. AOB: accessory olfactory bulb. VA: vomeronasal amygdala. H: hypothalamus. Adapted from Dulac et al. 2003.

The Vomeronasal System or the Jacobson-organ allows the perception of pheromones even in other mammals. The sensory neurons found in the vomeronasal organ (VNO) contain cell bodies that house receptors capable of detecting pheromones from the surrounding environment. Although very close and similar, this system is independent from the olfactory system. In fact, it projects to a separate bulb, called the accessory olfactory bulb, and from there to the hypothalamus via the vomeronasal amygdala. It also has a distinct class of genes, the vomeronasal receptors (V1R and V2R), along with Trp receptors. Those genes sustain the production and maintenance of the vomeronasal system receptors and proteins, allowing its functioning as a whole system.


3. Long Nasal Passage (Turbinates and Nasal Conchae)

The canine nasal passage is relatively long and convoluted with complex bony structures within the nasal cavity that serve to increase the surface area inside the nose available for olfaction, allowing for better filtration, humidification, and warming of inhaled air. These bony structures are called nasal turbinates and are covered in a mucous membrane.

Turbinate bones in a dog
The Turbinate bones in a dog

The nasal turbinates protrude from the side walls of the nasal cavity and contain the venous network; thus 5–15% of air inhaled by dogs is redirected to them. The increased surface area allows for scent molecules to interact with the olfactory receptors, enhancing a dog's ability to detect and distinguish various odours. The long nasal passage and the bony structures, the turbinates, play a crucial role in filtering, warming, and humidifying the inhaled air, optimizing the detection of odours and preserving scent molecules. Nasal air flow turbulence, therefore, arises from anatomical and physiological factors. These factors influence humidification, warming up, and the flow direction of inspired air, guiding some of it in the direction of the olfactory epithelium.

Dog's respiratory structures - Animals (Basel). 2022 Feb; 12(4): 517.
Dog's respiratory structures - Animals (Basel). 2022 Feb; 12(4): 517.

4. Moist Nasal Surface

The inside of a dog's nose is moist due to the presence of a mucous membrane. This moisture helps capture and dissolve scent molecules, making it easier for the olfactory receptors to detect and differentiate between various odours.

3D illustration of the nasal mucosa lining the nasal cavity
3D illustration of the nasal mucosa lining the nasal cavity

5. Scent Discrimination and Memory

Dogs have an incredible ability to discriminate between different scents, even in complex environments with multiple odors. They can follow specific scent trails and identify individuals based on their unique scent profiles, excellent scent memory and scent signatures. This individual odour recognition ability is invaluable and enables dogs to identify and remember individual scents. This ability makes them valuable for tasks such as tracking, search and rescue, hunting, detection work including detecting diseases like certain cancers, COVID-19 or low blood sugar in diabetics.


6. Dual Airflow System

Dogs have a unique nasal airflow pattern that separates inhaled and exhaled air. During sniffing, the inhaled air in the dog’s nostrils separates into two distinct pathways. The upper flow path, approximately 12–13% of each breath, goes straight to the olfactory region responsible for scent detection, where odor molecules are deposited and accumulate, preventing them from being exhaled. The remainder of the air, in the lower pathway, flows down the pharynx into the lungs for respiration. This path is also used for exhalation, thus supporting prolonged exposure of inspired air to the chemoreceptor area of the olfactory epithelium as air flows through the olfactory area of the dog during expiration. This enables them to continuously sample the environment for scents, even while breathing. This capability is particularly useful in tracking and search-and-rescue operations. In dogs, this mechanism of nasal airflow patterns during inhalation allows the acquisition of separate odor samples in each nostril, allowing bilateral comparison of stimulus intensity and odor source localization.


7. Specialized Breathing Patterns

When dogs are engaged in olfactory tasks, they often exhibit a behavior known as "panting-sniffing." This involves rapid shallow breaths through the nose, which helps to maximize the flow of odorous molecules over the scent receptors.

A dog sniffing
A dog sniffing

7. Variability Among Breeds

The external anatomical structure of a dog's nose varies among breeds. While some have longer and more prominent noses (e.g., Greyhounds), others have shorter and flatter noses (e.g., Bulldogs). These variations can affect the dog's ability to smell, with longer noses and snouts often being associated with a heightened sense of smell because of larger nasal passages, potentially enhancing their ability to detect scents. Bloodhounds, for example, are renowned for their exceptional scent-tracking abilities due to their large, loose noses with many wrinkles, which trap scent molecules effectively.

8. Communication

For communication, dogs utilize their noses a lot. Scent marking and sniffing the scent trails left by other animals teaches them a lot about other dogs and their surroundings. Though present in most members of the animal kingdom, dogs' chemical sense of smell is one of the most crucial because it not only conveys information about the current state of the environment but also allows for the detection of signals from the past (such as the presence of prey, enemies, or new, undiscovered traces in the area).

Dogs sniffing fresh air
Dogs sniffing fresh air

Finding food, identifying hazards, and finding a partner for reproduction are all fundamental aspects of life that are maintained by this intricate network of smell combinations that forms a three-dimensional representation of the environment over time. One of the earliest senses to become active is smell, which even enables olfactory learning during pregnancy. Consequently, canine olfaction is used in dog intra- and inter-species communication, including with humans, and has been utilised by humans for a very long time.


How are Canine Olfactory Skills used?

 

The sensitivity of canine olfaction is closely connected with the structure of their olfactory system, which is both quantitatively and qualitatively very different to that of humans (for example, dogs have more olfactory receptor cells and a larger olfactory bulb, allowing for enhanced sensitivity of odor detection).


1. Dangerous and Illegal Substances Detection

An important subset of canines used as sniffer dogs around the world are those trained to find bombs and locate locations tainted with harmful chemicals like toluene. The fact that dogs are trained to find the lingering smell of combustible substances used to catalyze combustion and to ignore the scents of burned carpet, wood, or other pyrolysis products is crucial for the effectiveness of their work in identifying explosives.

Whereas traditional technology fails, they can detect these compounds in very small amounts (0.1 g) and over great distances. In order to detect illegal narcotics including cocaine, heroin, methamphetamine, and marijuana, trained service dogs are regularly used by customs officials and border controls to check millions of travelers and cargo at airports, seaports, and post offices.


2. Biological Scents Detection

A dog at a crime scene
A dog at a crime scene

Dogs are utilized more frequently in the detection of biological odors, such as human odor, because they can separate and identify a specific person's odor even when it is mixed with other, stronger scents or among several people. In certain nations, authorities employ the canine sense of smell to match odours at the crime scene with the accused's odor in order to identify offenders. Dogs can even locate a human odor trail in extremely crowded city centers up to 48 hours after it formed, with an average accuracy of 77.5%. Dogs are also used in search and rescue to locate victims of a variety of incidents, including avalanches, earthquakes, floods, plane crashes, etc. At accident scenes, a different breed of service dogs that have been trained to recognize dead bodies are frequently deployed.

These dogs can identify traces or remains of human bodies, such as bones, bodily fluids, and tissue, both above and below ground as well as in the water. Some subgroups of these canines are known as cadaver-detection dogs. These canines can cover a vast area fast and have a detection efficiency in the field that ranges from 30% to 81%, which helps law enforcement and emergency services save time and resources.


3. Detection of Other Living Organisms

  1. Preventing the accidental spread of the Brown tree snake (Boiga irregularis) is an example of dogs being used in biological safety and control, with an average detection effectiveness of 62%.

  2. Dogs’ detection abilities are also used in agriculture, such as for locating plant parasites such as the palm weevil, which massively affects the most important crop in the Middle East, Date palm (Phoenix dactylifera L.) plantations with a very high detection rate of 73%.

  3. Dogs can achieve 95% accuracy in detecting subterranean termites, which can cause close to two billion dollars in damage and pest control, annually. The dogs trained to find the termites at the early stages of development can discriminate between them and other burrowing insects (i.e., ants, cockroaches) as well as identify wood damaged by the termites achieving a 98% success rate.

  4. Specially trained dogs show a very high success rate (99.7%) in detecting screw-worm fly in both infected slugs and wounds on large animals infected via slugs. This fly can kill warm-blooded animals and cause serious economic losses.

  5. Interestingly, trained dogs can even detect microorganisms. Some cyanobacteria in commercial catfish farms produce compounds that accumulate in the fish flesh, causing an unpleasant smell reducing the cost of discarding affected fish stock that can run into millions of dollars.

  6. Dogs are effective in detecting the growth of mold and other fungi in buildings as they can have detrimental effects on peoples’ health and cause costly deterioration of the building. Dogs are effective if trained to detect mold and other fungi within construction materials, to identify 75% of all hidden samples containing microorganisms.

  7. Dogs are also helpful to conservation scientists when trained to find endangered wild animal species. Due to the low density and vast areas these species can potentially occupy, it is often very difficult to keep track of them. The so-called ‘Scat dogs’ are trained to find and follow the scats of a given endangered species. Molecular analysis of scats delivers data on the species, sex, diet, parasites, and the individual character of the animal. Breeding and stress hormones can indicate the fertility and reproductive rate as well as the influence of any disorders on the physiological state. Scats can also offer a better source of DNA samples than hair, skin, feathers, nails, bones, or saliva.

  8. Dogs are used in traditional bird hunting to locate and study those very endangered bird species. When compared to humans doing the same capture task, dogs are far less likely to injure themselves or the birds, as well as being more efficient. In New Zealand, dogs have been used for over 100 years to locate many endangered species such as Kiwi bird (Apteryzspp.), Kakapo (Strigop habroptilus), or Blue duck (Hymenolaimu malacorhynchos) The possibilities for using the canine sense of smell seem to be unlimited.


What Physiological States are detected by Olfaction?

 

The recognition of the features of the physiological state of other individuals is an important skill in the context of social interaction. These skills could be divided into unlearned, occurring spontaneously, and learned (often during specialized training). Olfaction can be used by dogs to recognize the following physiological states:-


1. Identification of the Reproductive Cycle Phase

Some stud dogs can distinguish between particular phases of heat in bitches, and will attempt mating only with females that are at the so-called optimal time for mating. Moreover, dogs are also able to detect changes in the odor of a female in estrus treated with antibiotics. Dogs can also identify dairy cows in heat by the scent of their vaginal fluid, urine, milk, and blood plasma at an accuracy of 78% to 99%. Dogs can also distinguish milk from a cow that is in proestrus, estrus, and diestrus phase.


2. Identification of Emotional State

As a synanthropic species, the dog is good at dealing not only with the visual expressions of emotion but also with interspecies chemo-signaling. Dogs can easily recognize human emotions, such as fear or happiness, by olfaction. Besides, human odorant stress signals provoke a longer reaction in dogs than happiness signals, and generally, this reaction is not gender-dependent, but a distinct odor is emitted with a distinctive behavioral syndrome. Dogs can also be used to predict aggressive outbursts in psychiatric patients using vision and olfaction a 100% accuracy.

3. Dogs can Detect Diseases in Humans and Animals The volatilome, understood as the composition of both the volatile organic compounds (VOCs) in an organism and the VOCs reflecting its unique current metabolic state (including the influence of infection), can be used to detect diseases and the presence of specific pathogens. Those substances released even in low concentrations in human blood and urine are certainly within a dog’s capabilities of detection, since dogs’ sense of smell has a lower limit of detection at concentrations of up to one part per trillion (ppt). When dogs recognize some disturbances in the physiological state of another individual (e.g., human) or another animal, they spontaneously behave differently. As such, dogs can detect cases of seizure (epilepsy), narcolepsy, hypoglycemic episodes in pet owners with type 1 diabetes, the presence of the malaria parasite, COVID-19 infection and different types of cancers (lung, breast, prostate, ovary, bladder, skin and large intestine). Dogs recognize specific changes in the odor of the person, and warn the owner based on subtle changes in behavior that are imperceptible to humans. Current studies have shown that most disease conditions are associated with olfactory-specific characteristics and can be easily detected with high accuracy by trained dogs. Trained dogs can detect patients’ distinct typical odor, e.g., mosquito parasites change the smell of an infected person, making him more attractive to mosquitoes, and trained dogs can detect the specific skin odor marker for this disease.

Disease-detecting dogs (DDD) are now trained to recognize diseases in exhaled air, urine, feces, and cancer tissue samples from patients affected with different types of tumors in both human and animal patients. In animals, dogs are trained to detect the scent of Sarcoptes-infected (sarcastic mange) animals and to differentiate between worm-infected and uninfected animal feces

What Factors Influence the smelling Skills of a dog?

 

The factors that influence the olfactory (smelling) skills of a dog can be either internal or external factors. These include the following:

  1. Genetic consequences One of the causes of breed-specific olfactory ability is the variation in the percentage of pseudogenes and the frequency of particular gene polymorphisms. Therefore, it appears that innate traits may play a role in the emergence of genetically defined dog lines that are more suited for olfactory tasks.

  2. Breed The vast variety of dog breeds and the fact that the olfactory receptor (OR) genes make up the largest gene families in animal genomes support the hypothesis that olfactory ability is correlated with breeds. Some gene alleles are breed-specific (or uncommon in the canine population), and some are the predominant allele in the breeds that make good sniffer dogs. However, several behavioral traits, such as innate drive, eagerness to learn, trainability, and ability to work with people, as well as hereditary predisposition, also greatly affect the general canine olfactory capability, for example, Sulimov dogs, a jackal-dog hybrid breed.

  3. Age and Sex Olfactory capacities decline with age as a result of atrophic alterations, with fewer olfactory cells and olfactory epithelium degradation shown in older dogs. Additionally, older dogs may have a far greater long-term recall of odours and be able to handle more complex olfactory information than young dogs. The cells in female dogs' olfactory bulbs are more active than those in male dogs, which is consistent with gender differences in olfactory ability.

  4. Environmental Conditions With regard to the effect of environmental conditions, the following can affect the tracking efficiency of a dog: -

    1. Humidity Though an important factor in improving olfactory skills in dogs, due to improved nasal humidity and odorant trapping, increased humidity is responsible for increased odour intensity, positively influencing the tracking efficiency of dogs. However, even though the increased humidity connected with light rain is usually perceived as a positive influence on odor detection, heavy rain is usually a negative factor, since it could force the scent down lower to the ground.

    2. Foggy weather Humidity associated with foggy weather is a negative factor, causing scents to hang in the air and forcing the dog to scan the entire area, slowing down the process of tracking.

    3. Higher temperatures also negatively influence canine olfaction due to a reduced ability of dogs to work, causing a decline in searching performance. Beyond the direct influence of temperature on working ability, there is also the risk of dehydration as a consequence of increased activity in hot conditions, which can reduce canine olfactory efficiency due to decreased enzyme activity and nasal mucosal fluidity.

    4. Changing environmental conditions throughout the day can also influence the efficiency of the sniffing dogs. Changes in the physical environment, such as sudden landform changes (e.g., where open ground changes into woods, or farmland where fertilizers or pesticides have been freshly applied), can also offer a further complication.

    5. Diet components can also influence negatively/decreasing (coconut oil) or positively/increasing olfactory acuity (corn oil, EPA, DHA, DPA, animal-based proteins). Generally, olfactory work, which is a combination of physical and mental work, requires large amounts of energy which must be provided in the diet.

  5. Diseases Improper air flow, affecting olfactory skills, could be caused by diseases such as nasal cavity tumors, local injuries, or specific infections such as canine distemper or parainfluenza. In the case of hyposmia in dogs caused by endocrinological disorders such as hyperadrenocorticism, hypothyroidism, or diabetes, a neural mechanism is probably involved, similar to the anosmia observed in some cases of COVID-19 infection in humans. "Hyposmia is a decreased sense of smell. It can be mild or severe and could point to health conditions ranging from allergies and colds to nasal polyps and hormonal imbalances. Hyposmia may develop gradually or suddenly. Treatment depends on the underlying cause." The term “anosmia” refers to the total loss of sense of smell. Anosmia may be caused by an infection, such as a cold or flu. It may also be caused by nasal polyps or other blockages.

  6. Substances and Drugs Influencing Olfactory Abilities The influence of drugs on the sense of smell could be connected with a loss of acuity and/or distortion of function (dysosmia). The mechanisms responsible for those events are thought to be connected with inhibition of odorant receptors, e.g., inducing abnormal persistence of receptor activity or by blocking receptor activation. Zinc sulfate has been used routinely for the destruction of the main olfactory epithelium in many species, however, zinc nanoparticles could potentially be used to increase canine detection capabilities in environments with very low concentrations of the odorants. Anesthetic and analgesic drugs used in dogs, isoflurane and propofol, as well as fentanyl followed by naloxone, have no negative effect on canine olfaction. However, some medications have been identified to have a negative effect on canine olfaction, e.g., oral administration of metronidazole (25 mg/kg every 12 h) degrades the ability of working dogs to detect the odors of explosives. Similarly, the use of steroids (dexamethasone or hydrocortisone) causes a significant elevation in the olfactory detection threshold of dogs, without any observable structural alteration of the olfactory tissue.

Factors affecting the efficiency of canine olfactory detection.
Factors affecting the efficiency of canine olfactory detection.

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