If you ask someone what animal has the best sense of smell, chances are that most people will answer “Dogs”. Not a bad answer, though not quite right. At least two others are better.
Dogs have an amazing sense of smell. That's why we use
dogs to track and find people who are lost, buried in avalanches, trapped in
earthquake- or bomb-damaged buildings. We use dogs to sniff out illegal drugs, explosives,
buried landmines, soil contaminants, animals of specific endangered species, black
truffles, and even diseases. They couldn’t do it without their remarkable
ability to smell. But what’s also significant is that dogs are so willing to
be trained for these tasks. Some other candidates with great noses are less
convenient and obliging. More on that, later.
Why are dogs are better at smelling than human?
Humans have 5-6 million smell receptor cells in
our noses; dogs 100-300 million.
The area of the brain devoted to processing
smells is 40 times larger in dogs than in humans.
Dogs have a special organ — the “Jacobson’s
Organ” devoted to processing smells. We don’t.
As humans breathe in, we get new smells. As we
breathe out, nothing. As dogs breathe out, their breath causes some turbulence
which pulls new scents into the smelling area of their nostrils.
Dogs can distinguish whether smells are coming
from their left or right nostril. The difference of smell between the two nostrils
lets them know the direction a smell is coming from.
Professor Alexandra Horowitz, head of the Dog Cognition Lab
at Columbia University summarizes a dog’s ability to smell: “while
we might notice if our coffee has had a teaspoon of sugar added to it, a dog
could detect a teaspoon of sugar in a million gallons of water, or two
Olympic-sized pools worth.” Alexandra Horowitz has written a number of fascinating
books about dog cognition, most recently The Year of the Puppy, and Our Dogs, Ourselves.
Dogs can identify the intensity of a smell with incredible
sensitivity. By smelling 5 footprints they know which are less intense and
therefore older. From this they know which direction the person was walking.
believe that this is how dogs know when to expect owners back from work. If the
owner has a regular routine and returns, say, 9 hours after leaving, the dog
knows that when the owner’s smell has dissipated to a 9-hour-old level, it’s
time for their return. Ingeniously, this hypothesis was tested by quietly
reinforcing the owner’s scent during the day, by introducing clothing worn by
the owner. Dogs who habitually waited at the front door at the right time, were
For dog owners it’s important to understand that dogs’ sense
of the world is primarily through smell, not sight, as ours is. Your dog will
be happier and better adjusted if you allow it to spend a lot of walking time
following the interesting smells which are all around. And which you are
unaware of. When we meet people, we naturally look at them to get a sense of
who they are. Dogs smell each other to get that. Females tend to smell the front end of
other dogs; males the back ends. By smelling they can tell the age, gender and
health of the other dog. And more – like sexual attractiveness. But while we need the person present to see that
information, dogs can smell that information by sniffing where the dog has
been. When dogs sniff a tree, hydrant or telephone pole, they’re identifying
who has been there. And when they pee on it themselves, they’re leaving a
message to say “I was here”.
By the way, our vision is not just better than dogs’ — it’s better
than most of the animal world. Eagles can see more acutely than we can (“eagle
eyes” is an apt expression), but few others even come close.
So what animals out-smell dogs? In hindsight it’s not
surprising. Elephants’ noses are astonishing, not just for being long, but for
Elephants have been known to smell water from a distance of
20 km. They were tested for their ability to smell after it was noticed that
they avoided land mines in Angola. Elephants can smell the difference between
people from two Kenyan tribes: the Masai, who traditionally hunt elephants and
the Kamba, who don’t. Elephants have been tested on their ability to tell, by
smell, the quantity of sunflower seeds in a bucket with a perforated lid. The
elephants consistently choose the fuller container.
In a really interesting experiment, described by Ed Yonge in his fascinating book
An Immense World, scientists tested the ability of elephants to tell the
identity of other elephants by the smell of their urine. The scientists would follow
a herd of elephants and wait for one of them to pee. Then they would scoop up
the pee-soaked earth, guess where the herd was headed, and dump that
earth in the expected path. When the elephants reached that area, they would
smell the pee-soaked earth. Those elephants who were walking ahead of the donor of the specimen
were clearly confused by it. It was obvious that they knew whose pee they were
smelling, but that individual was behind them, so couldn’t have just peed in
front of them.
The other animal that can out-smell a dog is a bear. There are
lots of stories about bears being able to smell animal carcasses from huge distances
(some say up to 32 km), and male polar bears have been known to trek 100 miles
(160 km) following the scent of a sexually receptive female.
But for very practical reasons we do prefer using dogs to help us with smell-related
tasks, rather than elephants or bears.