The Sweet Smell of Species Success

Hold the deodorant: Funky body odors may have some value.

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Sometimes the smell of body odor means more than just “Wash me!” A person whose sweat starts to smell fruity may have developed diabetes, and an ammonia smell may indicate liver or kidney disease. Odor of rotting fish may signal trimethylaminuria — a rare syndrome caused by a defective gene that prevents people from metabolizing trimethylamine, a natural byproduct of digestion of certain foods like saltwater fish, eggs, and liver.

Body odors have a way of making a lasting impression, even when they don’t signal illness and even when we try ignore them. I’ll never forget the powerful scent emanating from Father Brady, the Irish priest at the church where I grew up. He never looked sweaty, but whenever he would lean over to shake my hand with his own squat, papery one, a smell that made me wrinkle my nose wafted through his robes. My family always joked that we should give him Old Spice for Christmas.

We all emit body odors of some sort, of course, but some of us just cover them up better than others. I must have gone through a particularly stinky stage when, during my early adolescence, my mother took me to the grocery store and showed me where to find Tussy. The spicy-smelling, aqua-colored stick deodorant was the forerunner to Teen Spirit. Apparently, I needed it.

No matter how much soap and water, perfume and lotion, antiperspirant and deodorant we use, some chemical signals hidden in body odors can’t ever be completely covered up or washed away. This is a good thing, because scientists are learning that olfactory stimuli communicate ecologically important information that can influence a range of emotions and behaviors, including kin recognition and mate selection, among others.

A few years back, for instance, research psychologist Jill M. Mateo revealed how chemosignals play a crucial role in relationships. She discovered that the reason a species of ground squirrels called Belding rub cheeks when they first meet is to sniff out the odors emitted from facial scent glands. These odors contain information which tell the squirrels precisely how related they are. In this species, the animals use this information to determine favoritism — they will risk their lives to help mothers, sisters, and daughters, but not nieces, nephews, or more distant kin.

The discrimination of genetic differences by odor recognition is believed to be due to the major histocompatibility complex (MHC), a group of genes involved in immune function. Related kin are known to have similar MHCs. And it’s not just squirrels: Matteo’s recently corroborated these findings in humans by showing how MHCs are more similar among related humans than among those who are not related. It’s not clear, however, if the squirrel’s cheek rubbing is related in any way to human cheek kissing. Just what MHC compounds contribute to a “family” odor and how animals and humans measure variations in the MHC and unfamiliar odors also remains largely mysterious.

It is fascinating to think that people detect complex, subtle chemical signals hidden in body odors without even noticing that they’re doing so. “Our brain is so sensitive to chemosignals from body odors that it can detect even
 minute genetically-based differences, which in turn guide our behavior,” says Johan N. Lundström, Ph.D., an olfactory scientist at the Monell Chemical Senses Center in Philadelphia. Lundström recently used PET scanning to show that body odors are processed by a neuronal network separate from that which processes common odors, and that people can accurately smell the difference between a stranger and a friend. The smell of a stranger’s body odor actually activated the area of the brain associated with fear and danger. This is perhaps a mechanism to ensure that it grabs our attention, Lundström said.

Other work from his lab has shown that women who are fertile are more sensitive to androstadienone, a social odor produced by men, than are nonfertile women. And research from other labs has shown that men preferred the underarm aroma of women who were ovulating (and fertile) compared to those who were menstruating (and less likely to be fertile). Furthermore, studies have revealed that people generally prefer the smells of those with immune genetic complexes different from their own, a useful mechanism for avoiding inbreeding.

These findings, along with others from the field of olfaction research, make it clear that we underestimate the sense of smell at our own risk. Olfaction is far more sophisticated than we give it credit. The course of our lives is probably far more swayed by body odors than we would ever imagine — or like to admit. We might even be masking body odor at our own risk. • 3 June 2008

REFERENCES: “Kin-recognition abilities and nepotism as a function of sociality.” Mateo JM. Proc Biol Sci. 2002;269:721-7. “Rats assess degree of relatedness from human odors.” Ables EM, Kay LM, & Mateo JM. Physiol Behav. 2007;90:726-32. “Functional neuronal processing of body odors differs from that of similar common odors.” Lundström JN, Boyle JA, Zatorre RJ, & Jones-Gotman M. Cereb Cortex. 2008;18:1466-74.

 

Jennifer Fisher Wilson is the science reporter for Annals of Internal Medicine. Her stories are available at www.annals.org.
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