Scent of a Man added 3/17/01
New Scientist Feb 10, 2001
Does taking the Pill disrupt a woman's animal instincts when it
comes to choosing the perfect partner to father her babies? Alison
Motluk sniffs out a controversial theory
IT ALL started with a bit of harmless T-shirt sniffing. A group of
women were asked to sniff T-shirts that had been worn by a group of
unknown-and unwashed-men. All they had to do was say which shirts
smelled best. The experiment, run by Claus Wedekind, a young Swiss
scientist then at the University of Bern, was designed to find out
if humans, like mice, use body odour to identify genetically
appropriate mates. But it revealed something more worrying.
As Wedekind predicted, most female volunteers had equivalent
preferences to their rodent counterparts. Numerous studies in mice
have shown that, given the choice, they sniff out mates with genes
for immunity that differ from their own, and that this seems to
increase their chances of producing healthy offspring. But among
Wedekind's volunteers there was a startling exception to this
trend. Women on the contraceptive pill showed the reverse
preference. Instead of being attracted to the scent of dissimilar
men, they chose men whose genes for immunity were closest to their
own.
If humans are using smell to find a good partner for reproduction,
and the Pill is turning things upside down, then there could be
serious consequences. By tricking users into falling for the wrong
guy, the Pill could be giving women better protection than they
bargained for-making it harder for them to have kids long after
they stop taking it.
It's a contentious idea. For a start, not everyone agrees that the
human brain can register such smells, let alone that they might
influence our behaviour. But if they do, then does the Pill
interfere with this form of sexual chemistry? "It's a reasonable
question," says Wayne Potts, a biologist at the University of Utah
in Salt Lake City, who specialises in mate choice. His colleague,
Dustin Penn, goes further: "It wouldn't surprise me if sabotaging
our reproductive machinery would lead to faulty mate choice."
The genes at the centre of the debate are a large cluster called
the major histocompatibility complex, or MHC. Each person has a
unique combination of MHC genes that encode various components of
their immune system. These include genes for histocompatibility
antigens-which determine, for instance, whether your body will
reject a foreign tissue graft-and for proteins called complements,
which help fight off disease. The idea is that the more varied an
individual's MHC, the more robust their immunity. And if your
parents have widely varying MHC genes, your own MHC is more likely
to be diverse. Parental differences may even increase the chances
of your being born in the first place.
Kunio Yamazaki at the Monell Chemical Senses Center in Philadelphia
has found that mouse embryos conceived by parents with dissimilar
MHCs are more likely to make it to term. And at the Reproductive
Science Center of the Bay Area in San Ramon, California, Louis
Weckstein found that human couples who fail to conceive after two
or more attempts with IVF shared significantly more MHC genes than
successful IVF couples.
If olfactory cues about MHC genes are so important, why do women
taking the Pill seem to respond so differently from other women?
"My guess is that the Pill simulates pregnancy and that maybe
during pregnancy, odour preferences change," says Wedekind. This
appears to be what happens in mice, he points out. The theory is
that during pregnancy, a female is attracted to the smell of her
own relatives, who have similar MHCs to her own. After all, family
members have the greatest interest in seeing her offspring survive.
Pregnant women-and women whose bodies are tricked into thinking
they're pregnant-might have the same tendency, says Wedekind.
The implications are not lost on Marian Petrie from the University
of Newcastle upon Tyne. "Potentially, women might affect their
chances of becoming [pregnant], not by something that's an obvious
means," she says. "It might be important generally to infertility
problems." In the next few months, she and her colleague Craig
Roberts will run a follow-up to Wedekind's study to find out
whether the Pill really does skew women's smell preference. They
will ask 80 women who are planning to start taking oral
contraceptives to do the T-shirt test, both before and a few months
after they go on the Pill. The researchers will also run the same
test on 80 women trying to get pregnant, both before and after
conception.
To date, almost all the work on MHC differences, mate choice and
their effects on offspring viability has been done on rodents. But
geneticist Carole Ober from the University of Chicago has come up
with a cunning way to explore some of the same questions in humans.
For over a decade, she has been studying a North American religious
community known as the Hutterites. Following Wedekind's revelations
about the women's preference for certain odours in an artificial
laboratory setting, Ober aimed to get information about real
partnerships.
In many ways, Hutterites are the perfect study group for "natural"
mate choice among humans. They are a socially isolated group who
work on communal farms, marry within their own community, and can
trace their roots back to a handful of European ancestors in the
16th century. Single men and women visit neighbouring Hutterite
colonies, and often end up working or celebrating special occasions
alongside potential spouses. They don't use perfumes or deodorants.
"They are absolutely maximising the importance of MHC," says Ober.
They marry once-divorce is strictly prohibited-and, claims Ober,
they marry for love. Once paired off, they place a high value on
big families, and seldom use contraception.
