12 October 2019 | New Scientist | 43drive to bond, like the “imprinting” we see in
birds and some other mammals.
The scent of newborns makes quite an
impression on other people too. The smell of
a new baby’s head activates reward circuitry
in adult brains, similar to when we receive a
treat or even take a drug. That is a potent way
for babies to remind us to look after them.
Newborn smell evaporates away by about
6 weeks of age, but as we get older, a vast range
of things influence our personal bouquets:
diet, age, fertility, illness, even state of mind.
So individual are our scents that some have
tried to use “odourprints” to identify criminals,
or “nose witnesses” to sniff out suspects in
a line-up. And it isn’t just how you smell to
others that is unique, but your sense of smell
too (see “Can you smell that?”, page 46).
But what exactly constitutes your particular
aroma, the scent of you? The realm of human
smells is vast, and most of our knowledge
about it is confined to the underarms. That is
what we generally negatively think of as body
odour (BO), although armpit odour is unlike
other noxious smells. “It is not a typical bad or
‘off ’ odour, even if we perceive it as such,” says
Andreas Natsch, a scientist at fragrance firm
Givaudan. In other words, it doesn’t contain
the small volatile chemicals associated with
things like sour milk, decay and faeces –
although our breath and foot smells may.
Instead, armpit odour has three key
components: thiols, steroids and a diverse set
of acids. All three are secreted as odourless
precursors and then converted into smellyI
AM standing in a bright and airy converted
barn in the English countryside sniffing
vials of pure armpit odour. The contents
of these five tiny bottles are so pungent they
actually knock me back. I’m getting top notes
of cheeses – stinky as they come – lots of
sulphurous onion and a hit of ammonia. The
least offensive has a citrusy undertone. The
bottles are provided by Camille Ferdenzi of the
French National Centre for Scientific Research
(CNRS) in Lyon, whose work includes recruiting
volunteers to sniff sweaty T-shirts. Clearly,
studying human smells isn’t for the squeamish.
Our bodily scents provide a channel of
communication that evolved to help us
survive and thrive, and in recent years Ferdenzi
and others have revealed this language to
be far richer than we realised. We have now
discovered that each person’s scent is unique –
not even identical twins smell exactly alike.
Each of us also has a one-of-a-kind nose for
smells. What’s more, we have learned that
scents wafting from our bodies and wisping
into our nostrils help us to forge family
bonds and draw us to partners, divert us
from danger, illness and aggression, and even
allow us to sniff other people’s happiness.
Yet throughout history and across cultures,
people have scrubbed, perfumed and
deodorised to disguise their natural smells –
perhaps never more than today. “Every
day, we control our olfactory image,” says
Ferdenzi. If these smells are such a powerful
form of communication, our aversion
to them is puzzling. And recent evidencesuggests we are getting less stinky and losing
the ability to detect certain scents. What the
smell is going on?
The role of smell in our lives begins before
we are born. Many odour chemicals from foods
can cross the placenta, says Benoist Schaal at
CNRS in Dijon, giving fetuses a taste of what
their mother is eating. That may help explain
why, after birth, babies find the smell of human
milk – also flavoured by their mother’s diet –
intrinsically attractive. Schaal’s team has
even found that a mother’s odour helps her
newborn’s brain to identify faces, suggesting
that from an early age, even mainly visual
processes incorporate smell. He speculates
that babies may have an early sensitive period
for their mother’s smell, which fortifies thePE >
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TR
AIN
“ There is no
doubt that
we can smell
fear, stress
and anxiety
in others”