Wherever we tread,
an invisible trace is left behind.
Showers of skin cells and bacteria
create an odour trail as unique as a fingerprint.
With a nose a million times
more sensitive than our own,
a bloodhound can identify and follow this scent
even after several days have passed.
We have bred bloodhounds to enhance
their sensitivity to human odours.
The long ears and hanging lips
are also designed to help trap smell.
In all dogs and many other animals,
this olfactory sense is used above all others.
Even our own odours can have hidden influence.
The salmon´s sense of smell
rivals the bloodhound´s.
After several years in the ocean, it now follows
an odour trail with a unique significance.
As rivers flow into the sea, they create
a characteristic plume of fresh water
that drifts for many kilometres.
The salmon can remember the chemical identity
of the river it left as a young smolt.
By swimming up this current of scent,
it can find the way back to its birthplace.
0nce in the river, the fish wait for
the optimum conditions of flow and temperature
before the smell leads them onwards.
The trail will take them to the spawning grounds
where they were born.
Each genetic race of salmon
returns to its own stream.
Driven by instinct
and locked onto this chemical signal,
the fish will overcome the torrential obstacles.
In air, odours carry
even more readily than in water.
Some birds that ride these air currents
have recently been found
to have a surprisingly acute sense of smell.
The fulmar covers great distances
gliding effortlessly on uplifts of air.
The same breezes carry with them
the smell of food.
The fulmar´s large tube nose
is packed with sensory cells.
It uses them to detect slicks of fish oil
from many kilometres away.
The same sense of smell will eventually guide
the fulmar back to its nest
but it now takes it far out to sea.
It joins other scavengers attracted by
the odour of the trawler´s fishy waste.
Like fulmars, storm petrels also appear
apparently miraculously wherever there is food.
They spend most of their lives
roaming the featureless ocean
and rely on a tube nose to guide them
to these unpredictable hand-outs.
Beneath the surface are other surprises.
The starfish is a hunter.
But as it moves, it cannot avoid releasing
an invisible signal into the water.
Like all animals,
it has its own characteristic smell.
This reaches the breathing siphon of its prey.
A spiny cockle reacts to the early warning.
Triggered by the same scent, a scallop competes
with the cockle´s fancy footwork.
An African impala reacts similarly
to the smell of danger.
The scent of a cheetah carries far on the breeze.
To the topi and other antelopes,
it signals danger.
Surprisingly, hunting cats do not take account
of wind direction when stalking prey
and are often smelt before they´re seen.
The cheetah relies on speed to compensate.
As the impala leap, glands on the ankles
produce a smell of alarm.
This communicates danger to the herd.
Many antelopes and deer produce
similar alarm scents when frightened.
These chemical warnings are also common
amongst insects.
A wasps´ nest is governed by many smells.
0ne of these signals danger.
The colony relies on workers
to find a source of food.
As the sting pumps venom,
it releases the alarm signal.
If the scent reaches the nest, it causes
the wasps to react aggressively to the threat.
(Man ) 0h, God!
(Siren )
Alarm scents of a different nature
are found in North America.
This striking black-and-white coat
is in reality a warning signal,
for the skunk possesses
the ultimate chemical deterrent.
Special anal glands create
a cocktail of pungent odours.
The effect is truly repellent.
But smells can also be used to attract.
The female Indian moon moth calls for a mate
by emitting a scent.
Her sexual perfume is released
from the tip of her abdomen
and may reach males
up to five kilometres away.
His feathery antennae comb the air
for her chemical traces.
A single molecule is enough.
He keeps on course by flying upwind,
only crisscrossing the air current
if he loses the trail.
His antennae guide him to the hidden female.
The mountain dusky salamander
also tracks his mate by scent.
But his courtship is far more bizarre.
He uses Dracula-like teeth
to lacerate the neck of the female.
There is a subtler side to this violent courtship.
The lacerations allow an aphrodisiac
from a gland on his chin
to pass into her bloodstream.
The female now becomes a willing partner.
As the male moves, she follows him,
straddling his tail.
The male then deposits
a package containing sperm.
As the male walks forward, the female follows
until she´s positioned over the package.
Fertilisation completes the courtship.
Canadian red-sided garter snakes
have equally strange courtship rituals.
The females are courted
by a whole entourage of smaller males.
To stimulate her,
they stroke their chins along her body.
She attracts them with a scent
exuded from between her scales.
Males have a different smell to the females
but there are cunning exceptions.
This male mimics the smell of the female
and lures other males towards him
with his deceptive odour.
As they near a genuine courtship gathering,
the fake female breaks away
from his confused suitors.
He joins the courtship frenzy and his smell
causes confusion to the males here, too.
This trick, which distracts rivals
away from the real female,
increases the impostor´s chances
of mating successfully with her.
This bizarre story of chemical subterfuge
evolved in the snake pits of Manitoba.
In this glacial sinkhole,
ten thousand snakes have been drawn out
by the spring sunshine.
They have wintered together
in hibernation dens below the ground.
The males are first to emerge,
so when a female appears,
she may be outnumbered 3,000 to one.
With this intense competition,
a male that smells like a female gains
a mating advantage over the other males.
In the wide expanse of the African savanna,
smells of identity have a special significance.
A lion is aware of many smells, but none are
more important than those of other lions.
Each individual has a unique odour.
As the lioness greets by head-rubbing,
she exchanges her scent with others
and creates the odour of the pride.
She recognises her cubs
by their individual odours.
During the first few weeks,
the cubs are kept separate from the males
but through contact with the females
they begin to acquire the pride odour.
(Squeals)
The smells are passed on
by licking and head-rubbing.
The smell from the adult males is the most
important ingredient of the pride odour.
The cubs need to acquire this smell
from the females
before they can safely approach the males.
A lion also uses smell
to mark the pride´s territory.
He transfers his odour to vegetation
by rubbing against it.
The smell is then reinforced,
leaving a musky reminder to other lions
on how often this area of the territory is used.
The Thomson´s gazelle uses an eye gland
to deposit his scent on grass.
0ver many days, thick deposits build up, leaving
a pungent territorial signal to other gazelles.
The topi has a similar gland,
and also uses it to anoint the vegetation.
He then manipulates the grass stem as a
paintbrush to daub the secretion onto his horns.
When thoroughly covered by the secretion,
he then transfers it to the ground,
at the same time picking up
the characteristic smells of the earth itself.
In this area, dotted with scent marks,
the male topi herds together
a large number of females.
These he has to protect
from the attention of other males.
A rival attempts to challenge.
This intruder can recognise the defender
by matching his smell
to the scent marks he left on the ground.
Even during a fight,
the challenger will sniff the ground
as if to check the identity of his opponent.
Scent marks don´t keep intruders
out of the territory,
but allow a challenger
to assess the status of the owner.
Using smell, the intruder can decide
whether to challenge or simply pass by.
0ther smells of a social nature guided
the construction of this natural chimney.
It is the air conditioning system
of the most complex of all insect societies.
A continual updraught of air
not only cools the interior
but also carries chemical messages
to its occupants.
These termite soldiers,
both major and minor castes,
are the defenders of the colony.
Worker termites tend the nursery.
The young can develop
either into workers or soldiers.
If many soldiers are killed, the decrease
in the concentration of their smell
causes more of the young
to develop into this warrior caste.
At the centre of the community lies the queen.
Without the influence of her odours,
the society would disintegrate.
Her bloated body
is continually tended by workers.
Totally immobile, this
pulsating sac is an egg-producing machine.
Every ten seconds of her 30-year life,
a new egg appears
and is carried to the nursery by a worker.
The destruction is caused by a mammal
with an unparalleled sense of smell.
The aardvark uses this heightened sense
to sniff out hidden termites.
As the workers repair the damage,
they are guided by scent.
Saliva chemicals in the globules of mud
attract others to the same spot.
Within hours, the nest is rebuilt
using smell as a guide.
This miniature eruption is created
by another social animal.
(Squeaking)
Like termites, the naked mole-rat´s
subterranean life is governed by smell.
There are even different castes,
including workers, soldiers and nurses.
The smaller workers do most of the digging.
Their spade-like incisors are used
to slice through the rock-hard earth
as they search for nourishing roots.
The colony also has larger soldiers
as well as nurses that tend the young.
Every mole-rat visits the same latrine.
Here, among the wet and pungent earth, the
odour messages of the colony are passed on.
(Squeaking)
Like termites, the mole-rats
are controlled by a single queen.
Her odour suppresses breeding in other females.
The mole-rats´ life is dominated by smell.
But do such chemical messages
have a significance in our own world?
A dog can detect odours
too subtle for us to notice.
Even in our obsessively clean modern lives,
some of these smells still emanate from us.
Glands on our body produce
a cocktail of seven chemicals,
resulting in a unique personal signature
that a dog can use to identify its owner.
House mice, too, have individual odours.
When one mouse greets another,
it sniffs along the whole body,
assessing not only personal scent,
but also status in the community.
Whatever the size of the colony,
only a small proportion of the females breed.
The smell of these dominant females
suppresses the breeding of others.
These and other chemical messages are left on
signalling posts created from dust and urine.
0dours left by the dominant male
have a profound influence on the colony,
reminding young males
of his priority over females and food.
Intruders to the colony
also use these scent posts
to discover the identity of the dominant male.
In an encounter, both the dominant male and
the intruder use smell to identify each other.
(Squealing)
The intruder loses the battle and will know
to avoid the victor´s scent in future.
(Yowling)
A cat is sensitive to many odours
of which we are not consciously aware.
(Purring)
As it rubs against our legs, it deposits
a personal odour from cheek glands
in the same way that lions
mark other members of the pride.
Although we are unaware of them,
our own scents have hidden influence.
The menstrual cycles of women who live
communally eventually become synchronised.
This is a result of smell messages
working on the hormone system.
Ironically, today we try
to disguise our odour identities,
replacing them with simulations
of other animal scents.
It seems that, in our sterile and crowded lives,
these odour messages from our biological past
have become unacceptable to us.
There are still other mysteries to discover
in this intangible world of smell.
Until recently, the olfactory sense of birds
was thought to be poorly developed.
It is now known
that some populations of pigeons
use smell as one of their navigational aids.
The sea has a unique odour,
detectable even by us.
(Birds calling)
To a pigeon, the smell of land
is equally distinct.
As the bird flies, odours from many different
sources are wafted to it on the breeze.
It´s thought that a pigeon locates these sources
of smell not only by flying through them,
but by associating their position
with winds from a particular direction.
(Cow mooing)
Using this knowledge, it can build up
an odour map of the local area
and use this in conjunction with information
from other senses to find its way home.
The most important smell of all
is that emanating from the loft.
0n our next Journey,
Super Sense explores different time worlds.
