SCIENCE27 - Sense of Smell in Animals

Pat and I were recently intrigued with headlines about certain animals being able to detect human diseases (like cancer) with their sense of smell.  We wanted to learn more about this, so I decided to research the subject and write this blog, starting with a better understanding of the overall sense of smell in animals.

 

After a short introduction, I will discuss how various animals smell, then identify the “best” smellers in the animal kingdom, before talking about how some animals can detect human diseases, and finally how artificial intelligence is helping make animal detection of human diseases even better.

As usual, I will list my principal sources at the end.

 

Introduction

Animals have evolved highly specialized senses of smell (olfactory systems) tailored to the specific demands of their environment and survival strategies. 

Animals use smells to "talk" to each other and to protect themselves in the world.  Some animals have noses for smelling, and others use an organ that works like a nose to sniff or sense information.  Animals can also release smells to send a message to other animals.  A scent can be used to help find food, communicate, mark an animal's territory, find a mate, and keep away another animal.

Each basic category of animal has its own unique way of smelling.  For instance, amphibians like frogs use nares, which are holes starting in the mouth and leading to the face, like human nostrils, able to smell both airborne and waterborne odors.  Fish also use nares, taking in water that moves over sensory pads in their heads that tell their brains what they are smelling.  Birds smell through nostrils found on their beaks for foraging and navigation.  Mammal nostrils (including humans), on the other hand, are located on the nose to find food and avoid predators.  Insects use receptors on their bodies that contain pores, or holes, that pick-up scents from the air which provide chemical cues when there is food, potential mates, or threats nearby.  Reptiles like lizards and snakes, are even more unusual because they use their tongue to smell.  They flick their tongue to pick up scent particles from the air, and when their tongue goes back into their mouth, it rests on the roof of the mouth, where special sensors interpret the scent.

Human sense of smell is generally considered weak compared to many animals.  However, recent research suggests the human sense of smell is more sophisticated than previously thought, with the brain's powerful interpretation of scents playing a crucial role, and humans are particularly adept at identifying certain smells that are not as important for other animals.  The human brain's complex processing power allows us to differentiate and interpret a huge variety of odors that are crucial for our own survival and behavior.  Humans are more sensitive to certain scents, such as components of fruit and flower odors, that were evolutionarily relevant to a foraging diet.  While most animals rely on smell for core survival tasks, the human sense of smell contributes significantly to flavor perception, emotional memory, and kin recognition. 

 

How Animals Smell

There are many factors that determine how strong and accurate an animal species’ sense of smell is.  Scientists look at the number of olfactory receptor cells in the animal’s “nose,” the number of olfactory genes, the size of the brain processing area for smell, and anatomical adaptations to determine how well an animal can smell the world around them.

Number of Olfactory Receptors:  Animals that rely heavily on smell have a much higher number of scent-detecting receptors.  These receptors detect airborne particles and send signals to the brain for interpretation.  A dog has 220 million to 300 million receptors, while humans have only about 5 million. 

Number of Olfactory Receptor Genes:  The number of genes that code for olfactory receptors also contributes to an animal's sense of smell.  Animals have a specific number of olfactory receptor genes in their genome, which varies greatly by species, reflecting their adaptation to different environments. African elephants have the highest number with 1,948 olfactory receptor genes, compared to 396 in humans, and 1,207 in rats.  See the chart below.

Number of olfactory receptor genes in the genomes of 20 mammals.   A pseudogene is a DNA sequence that resembles a functional gene but has lost its ability to code for receptors due to mutations.

 

A single gene can produce thousands of copies of its specific receptor, producing millions of olfactory receptors in a single animal.  For example, dogs have between 100 and 300 million olfactory receptors, while humans have approximately 5 to 6 million.

Size of Brain Processing Area:  The olfactory bulb, the part of the brain that processes smells, is proportionally much larger in animals with a strong sense of smell.  For instance, a dog's olfactory bulb is 40 times larger than a human's, and a mouse's is 200 times larger. 

Anatomical Adaptions:  Many animals have anatomical adaptations for smell, such as specialized structures within their noses that increase the surface area for scent detection, or special organs in the roof of their mouths to process scents.

 

Best Animal Smellers

While a definitive ranking of animal smell capability can vary by the specific odor being detected, here are 15 animals (In alphabetical order) consistently recognized for having the most exceptional senses of smell, based on factors discussed above: 

African Elephants:  An African elephant's trunk measures up to 7 feet long.  With such a massive nose, it’s no surprise that these pachyderms have more olfactory receptor genes than any known species and have the strongest general sense of smell.  They use the receptors to locate food, water, and sense danger.

African elephants can detect water sources from up to 12 miles away.  In one study, elephants were able to distinguish between different quantities of food using smell alone.  There have been studies where elephants could differentiate between two different types of sunflower seeds before eating them.  African elephants can even distinguish between different human tribes by scent, and detect the presence of human poachers in their area, allowing them to flee or prepare a defense.

Elephants also use their sense of smell to learn about one another. They can sniff out information about the health, social status, and location of another elephant, and find family members from afar.

African elephants have the strongest general sense of smell in the animal kingdom.

 

Bloodhounds:  Smell is a dog’s most well-developed sense, and the breed with the strongest sniffing ability is the bloodhound.  Dogs have an olfactory bulb 40 times larger than a human. Once they pick up a scent, they follow it relentlessly.  Their legendary tracking ability is so reliable that a bloodhound's "nose-witness" testimony is admissible in some U.S. courts.

While the nose is strong, another major reason why bloodhounds can smell so well is due to their ears. Their drooping ears are quite large, and they help funnel the scents to their nose.  The wrinkles in their ears are also help to trap the scent particles, which is part of the reason they can follow the smell for so long.  It stays with them.

Dogs in general are known for their strong sense of smell, and are familiar helpers to law enforcement, with their use for tracking criminals, locating missing persons, and detecting narcotics, explosives, and accelerants in arson cases.  There have been many stories of dogs finding their way home from miles away.   They can also detected certain human diseases like cancer and diabetes (see below).

Catfish: Compared to the mighty African elephant and the famous bloodhound, catfish might seem unassuming in its smelling powers.  But catfish are among the best underwater sniffers.  Fish detect scents in the water via folds of sensitive tissue in an organ called a rosette inside their nostrils.  Some catfish species have more than 140 folds in their olfactory tissues, whereas other fish species tend to have fewer than 100 folds, and often less than 10.

Cows: Cows possess an unexpectedly acute sense of smell, with more olfactory receptor genes than dogs.  They use this to locate the most nutritious grasses, avoid poisonous plants, and detect the presence of predators.

Cows can detect scents up to six miles away, they may be able to smell you before you drive past them on the highway.  This amazing ability is due to the over 1,000 olfactory receptor genes that allow them to sense smells better than dogs and humans.  To smell things, cows curl back their upper lips.  Doing so closes their nostrils, which brings the scent right to their noses.  This skill is essential for detecting nearby food and alerting them to predators. 

Cows possess an unexpectedly acute sense of smell.

 

Honey Bees: Many insects sense odors with their antennae, which have microscopic receptors called sensilla.  Sensilla can detect odor molecules in the air that pass over them. 

Insects rely strongly on their strong sense of smell for survival and communication, but honey bees are on another level.  They have more odor receptors than other insects. Honey bees have approximately 170 receptors, while mosquitoes have 79, and fruit flies have 62. 

While they can do incredible things, bees mostly use their senses to tell the difference between different types of flowers.  They also use it to locate their hives, which can be miles away, by following the pheromones the hives emit. Their scents also show each other who’s in charge. The queen bee emits a specific pheromone that the other bees can recognize.

Their senses are so strong that some researchers have been attempting to use bees to detect illnesses like like cancer and diabetes (see below).  They’ve also been known to smell bombs and drugs, among other impressive feats.

Kiwis: This flightless bird from New Zealand has an exceptional sense of smell.  Among birds, kiwis have one of the largest olfactory bulbs for their size.  The kiwi is the only bird that has nostrils at the end of its beak.  Such nostrils help find insects under the soil.  Kiwis are nocturnal ground hunters and they do not use their eyes as much as other birds do.  Instead, their nose guides them through the dark.

Opossums: Like many of the animals on this list, opossums have a long snout.  A long nose can fit more olfactory receptor cells than a short or small nose can. The opossum's lengthy snout bestows it with an exceptional sense of smell, which it uses to forage and hunt for food.  These marsupials are omnivores and eat a wide variety of foods, including insects, small rodents, eggs, and fruit, but there are some smells, like garlic and molasses, that their sensitive noses cannot stand.

Pigs: Pigs have some of the most powerful noses among domesticated animals.  In the wild, pigs spend much of their time rooting around in the dirt, sniffing out tasty treasures.  The unique shape of their snout allows them to sniff deeply into the ground.  With their well-developed olfactory organs, pigs can smell food that is 25 feet under the dirt.  That is why they are prized as truffle hunters. 

Polar Bears: Bears are known to have an incredible sense of smell, and the polar bear is the most exceptional among them.  The polar bear’s secret for smelling lies in the bear’s nose, which is equipped with scent receptors and nasal tissues that can detect scent particles.  Polar bears can reportedly smell a seal through three feet of ice or from up to 20 miles away.  This ability is essential for finding prey in the harsh Arctic tundra.

The polar bear’s acute sense of smell is essential for finding prey in the harsh Arctic tundra.

 

In conjunction with their strong noses, bears also have large olfactory bulbs in their brains that quickly process complex scent information.  Since a bear loads up on calories before going into hibernation, they must be able to hunt and find a lot of food quickly, and their strong nose is key.  Their sense of smell also helps them to find potential mates during mating season.

Rats: Rats also have an incredibly strong sense of smell.  With over 1,200 olfactory receptor genes, this is yet another animal that has many more receptors than humans.  Since rats are born blind, their keen sense of smell begins very early in life when they have to find their mother by smell in order to eat.  Rats can’t smell scents from as far away as some of the other animals on this list, but they can smell that you have food in your house while they’re still outside. 

Many scientists also believe that rats can combine their sense of smell with their other senses to sense upcoming temperature changes, which leads them to seek shelter before the adverse conditions arrive.  Rats also use their sense of smell to detect when predators are in the vicinity, so they can find shelter before they’re caught.  A female rat will release pheromones during mating season.  Male rats can smell it, and the result is reproduction. 

Rats are known for their precise sense of smell.  One study has shown that they can smell in stereo,, meaning that they can tell through smell alone where an object is.  Their olfactory abilities are so accurate that they are trained to sniff out landmines.  Rats have also been trained to smell such diseases as tuberculosis in medical samples (see below).

Salmon: The salmon famous homing migration capability would not be possible without its remarkable sense of smell.  When salmon are young, they learn specific scents from their birthing place that they follow to return home each year.  Salmon have such a fine-tuned sense of smell that they can detect an odor in one part per 80 billion in water.

Sharks: Sharks have such a great sense of smell that they are sometimes called “swimming noses.”  Sharks have the largest olfactory bulb of any fish.  A great white shark can smell certain substances in water about 1 part per 10 billion parts water.  This helps them track and hunt wounded or dying animals.

Sharks can detect a single drop of blood from a mile away.  Sharks have very highly developed olfactory senses that are found between the top and bottom nasal passages.

While swimming, the shark pulls water into its nasal sacs, where its receptors compute the source of the scent so they can quickly find it.  On their way to the source of the scent, sharks move their heads back and forth to further pinpoint the scent.  Sharks don’t always know what kind of food they’re smelling until they bite into it.  This is why many people survive shark attacks.  The shark realizes that it’s not eating food it likes and moves on.

Silk Moths: Like the great white shark, the small but amazing silk moth can detect odors in parts per billion.  Silk moths use their feather-like antennae lined with olfactory receptors to sense odors in the air.  If even just a few molecules of a mating chemical pass through the sensitive sensilla on their antennae, silk moths can pick up the scent from up to six miles away.

Snakes:  Snakes have an acute sense of smell thanks to a highly specialized olfactory system.  A snake flicks its forked tongue to collect scent molecules from the air, which are then delivered to a special organ called the Jacobson's organ on the roof of their mouth.  This organ sends signals to the snake's brain, allowing it to interpret the smells to find prey, a mate, or a home, and even to track a scent trail in "stereo.”  They also have nostrils for basic odor detection. 

Snakes use their forked tongue to collect scent molecules.

 

Turkey Vultures: Since this large bird specializes in eating carrion (dead animals), it is no surprise that the turkey vulture has an exceptional sense of smell to help it find its meals.  This due to their large olfactory bulbs, which can detect carcasses from up to a mile away or more while they’re flying high in the air.  It’s also not uncommon for the species to detect a carcass of an animal as small as a mouse in a densely forested area. 

Another reason for their strong sense is due to their nostrils.  The nostrils of turkey vultures are wide open, and they don’t have an inner wall that separates their nostrils, like humans do with their septum.  The lack of a septum allows more airflow, which brings the scent to their olfactory bulbs.  Once the turkey vulture detects potential food, it will approach the source and fly in small circles until it can pinpoint its meal.

 

Some Animals Can Detect Human Diseases

Incredibly, dogs, rats, and bees can smell human diseases by detecting changes in a person's scent profile, which are caused by volatile organic compounds (VOCs) released by illnesses.  

Human illnesses can cause the release VOCs that may act as diagnostic biomarkers. These compounds are produced by the body's metabolism or immune response to disease and can be detected in breath, urine, or blood. Changes in VOCs can indicate infectious, metabolic, or other diseases, such as cancer. 

Dogs, honey bees, and rats have an incredibly powerful sense of smell that can detect diseases like cancer, COVID-19, tuberculosis, malaria, and diabetes - often before they are detected by traditional medical tests.  

Dogs, honey bees, and rats have been trained to detect human diseases

 

Dogs, rats, and bees have been trained to recognize these disease-specific odors.  These animals are used as a fast, efficient, and low-cost screening tool in some countries.  Here are some examples of diseases detected by animals. 

Cancer:  Dogs have shown high accuracy in detecting various cancers, including breast, prostate, colorectal, and lung cancer, by sniffing samples of urine, breath, saliva, or blood.  Researchers have trained honeybees to detect lung cancer by recognizing VOCs in human breath. In one study, a bee brain-based sensor was 93% accurate in distinguishing between synthetic healthy breath and synthetic lung cancer breath.  The bees were also able to differentiate between different types of lung cancer cells.

Infectious diseases: Dogs can accurately distinguish the odor signature of COVID-19 from sweat samples, sometimes even in individuals who are not yet symptomatic.  During the pandemic, studies showed bees could be trained to identify the scent of SARS-CoV-2 in human samples.  In one trial, trained bees correctly identified positive samples with 92% sensitivity.

Tuberculosis: Rats have been trained to detect tuberculosis in sputum samples. The superior olfactory senses of bees and wasps have also been used in research to detect tuberculosis.

Malaria: Dogs can detect the sweet scent of the malaria parasite from socks worn by infected individuals. 

Diabetes: These animals can detect low blood sugar levels.  In 2014, researchers trained bees to detect acetone, a biomarker for diabetes, in breath samples.  

Seizures: Dogs can detect the imminence of an epileptic seizure based on changes in their owner's body odor. 

How Artificial Intelligence Helps Make Animal Detection of Human Diseases Better

Artificial Intelligence (AI) helps in animals' detection of diseases in humans by acting as a powerful analytical tool for vast amounts of data (genomic, physiological, imaging, and behavioral) that the animals or monitoring devices collect.  It enhances their natural abilities and the surveillance process in several ways: 

Standardizing and Scaling: While a trained dog can get tired or bored, an AI system can work continuously. AI models can be trained on vast datasets from these sensors to provide consistent, high-accuracy screening methods that can be integrated into clinical use, potentially via handheld or smartphone-integrated devices.

Identifying Specific Biomarkers: By analyzing the data that animals respond to, AI helps researchers pinpoint the specific chemical compounds (biomarkers) associated with a disease, which can then improve traditional diagnostic tests. 

AI also helps in a broader sense by integrating data from animal populations to predict and manage disease outbreaks that could affect humans: 

Predictive Modeling: AI algorithms analyze diverse data sources (animal health records, environmental sensors, weather patterns, genetic data, and even social media patterns) to predict disease outbreaks and transmission patterns, allowing for proactive public health responses.

Image Analysis: AI-powered image recognition systems help veterinarians and researchers quickly and accurately identify disease indicators in medical images (X-rays, MRIs, tissue samples), often with higher accuracy than human analysis.

Physiological Monitoring: Wearable sensors on livestock or pets continuously collect physiological data (heart rate, temperature, activity levels), which AI analyzes in real-time to detect subtle deviations indicative of early illness, often before visible symptoms appear. This allows for earlier intervention for both animal and human health.

AI is also accelerating the development of drugs and vaccines by identifying potential drug targets and optimizing candidates much faster than traditional methods, with some AI tools also serving as alternatives to animal testing in preclinical research. 

Scientists are working today to identify the specific chemical compounds that animals detect, which could lead to the development of new screening instruments that mimic the animal's olfactory abilities.  Researchers are developing "artificial noses" (e-noses) and other biosensors that can detect chemicals at similar low concentrations as animals.  AI algorithms then analyze the complex patterns in this chemical data to identify the unique "scent signature" of a disease, replicating the animal's brain function in a more consistent and scalable way than relying on a trained animal.

Artificial Intelligence systems can sniff out human diseases as well as dog can.

 

 Conclusions

Animals possess incredible smelling capability, evolved to perform in their specific environment.  It is almost unbelievable to me that diverse animals can be trained to detect human diseases.  Artificial Intelligence will certainly help to enhance, improve, and expand this capability for the great benefit to human health.

 

 

This subject gives new meaning to “Dogs are man’s best friend.”

 

 

Sources

My principal sources include: “Sense of Smell,” wikipedia.com; “How Do Animals Use Smell,” fragrance.com; “The 20 Animals with the Strongest and Best Sense of Smell,” animals.com; “Animals with the Strongest Sense of Smell in the World,” a-z-animals.com; plus, numerous other online sources, including answers to many queries using Google in AI Mode.