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It's the Pits!
POSTED 9 APRIL 2009

In the pits, women win by a nose!

What good is body odor? We spend billions on perfumes, anti-perspirants and body washes to defeat it, but for about 10 years, scientists have suspected that BO plays a subterranean role in mating: Women tend to prefer the scent of men with genetic differences in their immune systems, suggesting that subliminal odor preference revs up their children’s immune system by giving it more genetic variation.

The computer illustrations look like hundreds of gobs of pink and peach chewing gum stuck together
Like a diverse stock portfolio, a diverse immune system should protect against all sorts of nasties. Evolutionarily, it’s a mistake to over-invest in a few immunity genes when selecting a mate.  Your kids will benefit if you sniff out a mate with different genes for an immune structure called the major histocompatibility complex (MHC). Sweat-sniffing research suggests that women prefer the scent of men with dissimilar MHC genes.

Now we hear that women are less easily confused in their ability to detect body odor. In research at Monell Chemical Sense Center, Charles Wysocki, a behavioral neuroscientist, and George Preti, an organic chemist, exposed subjects to a potent elixir of armpit. Then they tried to mask those odors with a series of pleasant fragrances, supplied by a firm that supplies manufacturers of products that control or mask underarm odor.

In the competition to be more sensitive to BO, women won big:

Among all male and female smellers, only nine of the fragrances masked male-derived odors, compared to 25 that masked the female odors.

When the results were broken down by the smeller’s sex, 19 of the 32 fragrances prevented male noses from detecting the stink of either sex’s armpit, but only two fragrances did that to female noses.

None of the masking agents could prevent women from detecting the male-derived odors. These differences were not due to a different response to plain body odors, which both men and women rated equally for intensity and likeability. (Nor was there any indication that people preferred an odor from the opposite or the same sex.)

A black bottle of Axe body spray altered to read “Aaak!” body spray, with Why Files log

Getting adapted?

A man wafts in a scent released from a silver conical apparatus attached to tubes toward his nose
The science of smell is not just about armpits and perfume. This fellow is performing “olfactometry” – measuring smells.

The masking effect is a form of what scent-sleuths call “adaptation.” When we smell the same odor for a while, the nose’s olfactory apparatus adapts, and we stop noticing it. When we pump gasoline, we immediately smell it, but after a minute or two, we become “adapted,” and the odor seems to disappear.

The Monell study looked at a related phenomenon called “cross-adaptation,” which creates the same effect through exposure to two different chemicals. Cross-adaptation is used in household air fresheners and the products that shield us from funky odors of human waste or rotting scraps of bologna in bathrooms and buses.

Armpit odor normally forms after bacteria metabolize compounds in sweat to create a complex brew of stinky chemicals. The researchers standardized their odor samples by stimulating sweat glands to release a compound and then altering it into a brew with a Class-A body-odor.

A pool party?

To begin the research, the scientists pooled sweat from volunteers, then performed the chemical transformation to create separate male-derived and female-derived odors.

The cross section of a nose tip from above shows the organ is shaped like a two pronged boat anchor
Graphic: Public domain, from Gray’s Anatomy
Many mammals (and the human embryo shown here) have a vomeronasal organ. The VNO detects pheromones, chemicals that carry information, especially related to mating, among members of a species. Scientists argue whether the vomeronasal organ survives in humans after birth. If so, it could be a receptor for chemical communication, a true sixth sense.

The research supports our existing picture of differential smelling talents, Preti says. “Most studies suggest that women have a much better sense of smell. They pay more attention to odor and are more impacted by odorants than men. The more differences we’ve looked for, the more differences we see.” In one study, for example, women were better at smelling “fear” in armpit samples taken from people who had watched a scary movie.

A woman appears to bury her face in the armpit of a man who has his back to us and his right arm up
Photo: Jan Tik
We don’t exactly know what to write here. Does this woman sport a super-schnozz?

An evolutionary outlook may explain this instance of female superiority, says Preti. “In humans, there is lots of evidence that people use odor as part of mate selection. The female in particular uses it to help distinguish who might be the best mate, genetically speaking.”

Preti concedes that men also need to select mates, but notes that evolutionary biologists say that “women have a higher investment in reproductive success, because they have fewer reproductive events, and so they need to be more careful in who they select.”

Despite the existing evidence that women have a better smeller than men, the most surprising result “was this very marked gender difference,” Preti says. “Many more chemicals block odor perception among males, and not many do so among females. We’d never seen that before.”

One final note: If you want to sound scientific when talking about the armpit, call it the axilla.

- David J. Tenenbaum

Related Why Files

• Smelling trouble: losing olfaction could put you in traction!

• All you ever wanted to know about sniffing mouse pee but were afraid to ask.

• See a parasitic plant get seduced by the scent of a tomato.

Bibliography

• Cross-adaptation of a model human stress-related odour with fragrance chemicals and ethyl esters of axillary odorants: gender-specific effects, Charles J. Wysocki et al, Flavour and Fragrance Journal, 2009.

Monell Chemical Senses Center


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