Meet the creatures with the most powerful, sensitive, and sophisticated noses in the animal kingdom.
What animal has the best sense of smell? The question is not as easy as it might seem on first sniff. Different animals are most sensitive to different smells, so before we can answer questions about the best or strongest, we need to consider what we’re actually comparing and how we can do so.
Scientists who study smell and its variation across species have sometimes considered things like the number of olfactory receptors (ORs) an animal has in its nose, the number of functional OR genes in its DNA (we don’t want to consider nonfunctional smell genes which might be left behind when a species loses its sensitivity to various smells over its evolution), or the size of the olfactory bulb in an animal’s brain (on an absolute basis or as a percentage of brain size). The question then is whether these measures lead to valid predictions. Unfortunately, it turns out that cross-species comparisons of these things are not always good predictors of superior smell when put to the test by measuring different animals’ ability to discriminate smells. That said, there’s more to learn, and this study (for example) did find a positive correlation across species for the number of functional OR genes an animal has and its ability to discriminate subtly different odors.
Other factors we can look at include things like the presence of specialized smelling equipment (e.g., a Jacobson’s organ for detecting pheromones) and observed behavior such as how many miles an animal has been known to follow a scent. These are especially important given the shortage of cross-species olfactory discrimination studies.
The Human Baseline
Compared to many animals, humans have fewer OR genes, fewer scent receptors in our noses, less of our brains are dedicated to smell, our Jacobson’s organs are vestigial (preventing us from detecting pheromones), and smell just doesn’t play as central a role in our lives.
But while there’s no doubt that the sniffers on some animals outperform us, we might be underestimating ourselves. It turns out that humans can be trained to follow a scent trail, thanks in part to our stereo smell. There are also some scents we’re particularly sensitive to. For example, we’re better than dogs (who otherwise generally trounce us in the smell Olympics) at smelling some fruits and flowers, and we’re better than mice at detecting human blood.
The Animal Champions
You can find lots of articles online telling you how many thousands of times better dogs or bears are at smelling than humans, but it seems there hasn’t been a lot of science to back up those conjectures. However, we do know that many animals live in a different world of smell compared to us. A world where even when you can’t see or hear anyone around you, you’re aware of the presence of everyone nearby and others who were there before you.
But which animals do we think have the best noses in the world? Although this question hasn’t yet been definitively answered (for the reasons outlined earlier in this article), two candidates stand out: elephants and bears.
A team of Japanese researchers studying olfactory receptor (OR) genes found that elephants have the most documented in any animal so far. African elephant genomes contain nearly 2,000 functional OR genes—five times more than humans and more than twice as many as dogs. Although the study didn’t examine the function of each gene, the huge number of smell-related genes strongly suggests those long trunks contain highly sensitive smelling abilities.
Elephant behavior backs this up. Both African and Asian elephants are particularly good at smelling water, which they can detect up to 12 miles (nearly 20 km) away. A 2007 study even found that African elephants can distinguish between members of two tribes in Kenya (only one of which hunts elephants) partly based on smell.
Bears are well known for their uncanny ability to smell food over long distances, and silvertip grizzlies and polar bears are often described as having particularly strong senses of smell. There are lots of stories about bears being able to smell animal carcasses from miles away (some say up to 20 miles or 32 km), and male polar bears have been known to trek 100 miles (160 km) following the scent of a sexually receptive female.
These claims should probably be taken with a grain of salt since there’s been little research actually quantifying bears’ sense of smell, but what we know about their biology corroborates the idea that they’re in a league with the world’s best. For example, bears have remarkably developed snouts containing hundreds of tiny and highly dexterous muscles. Their snouts are also estimated to contain more scent receptors than bloodhounds (who are certainly no smell slouches—see below), and bears have massive olfactory bulbs (five times the size of ours, despite their brains being only one third as large).
Professional smellers: Dogs don’t quite make the list of champions, but they’re still ridiculously good sniffers and they’re a lot easier to train than bears. Dogs are employed to sniff out explosives, missing people, and even cancer. Among dog breeds, bloodhounds stand out as the best. They’re sometimes referred to as a nose attached to a dog, and for good reason, since they have more scent receptors in their noses (up to 300 million) than any other breed. That compares to 5–6 million for humans. Bloodhounds have been known to follow scent trails for more than 100 miles and detect smells over two weeks old. They’re so reliable that the nosewitness testimony of a trained bloodhound is admissible in most US courts.
Dogs aren’t the only ones with noses put to work by humans. African giant pouched rats are used in Cambodia and other countries to sniff out land mines.
An olfactory myth: It’s commonly thought that sharks can smell a drop of blood from far away (some say a mile), but is it true? Apparently not. Researchers from Florida Atlantic University put shark smell to the test and found that although shark smell is impressive and certainly better than your average human, it’s no better than a typical fish. A good article on the subject clarifies the myth: “sharks can smell a drop of blood in a volume of water about the size of a backyard swimming pool.”
Other Super Senses
What about animals with the best vision and hearing? Check out the next articles in this series to learn more: