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March 4, 2008

The art and science of smells

deodorant.jpgThe creation and use of fragrances and cosmetics is one of most ancient arts known to humans. Just think of the diverse tribal cultures that use facial and body painting, or recall the stories of the glamor of Cleopatra and the (in)famously luxurious Roman parties drenched in rose perfumes. In the natural world as well, colorful (peacocks) or highly odorous (musk deer) males do everything in their power to attract females.

Fast forwarding from the time of Cleopatra to the twenty-first century, we see that fragrance and cosmetics products have become a billion-dollar industry, promoted by stars of fashion and eagerly purchased by consumers all around the world. The key part that changed is the transition from naturally occurring ingredients in cosmetics to synthetic ingredients that, as readers of the Enviroblog well know, are not currently regulated for safety to human health.

These synthetic fragrance chemicals bear unpronounceable names, such as galaxolide, tonalide, musk xylene, musk ketone, linalol, lilial, or phenylethyl alcohol. In the Wild West of the cosmetics industry, consumers are left to fend for themselves when choosing amongst the potential dangers these ingredients may pose.

Many scientists and public health advocates express grave concern about this lack of government or public oversight for the cosmetics industry. Yet, whatever scientists say, people will continue using fragrances and cosmetics – to feel better about themselves, to conform with the social standards, and, of course, to attract potential mates. So, for this blog post, instead of sympathizing with the plight of fragrance-sensitive individuals (of whom I am one) or worrying about cosmetics ingredients going down into the wastewater stream, I will focus on the “individual olfactory signature” that is unique to each human being and every forest mouse.

Chemosensory recognition (or, in plain language, how the other individual smells) is important not only for identifying plants, animals, objects, or people. Body odors regulate the social, sexual, and endocrine responses of many species. Partner choice in mammals is influenced by the smell of air-born chemicals that convey a wealth of information, such as the sex, age, health and reproductive state, all of which contribute to the odor profile of the animal.

As an immunologist, I have been long fascinated with one particular research finding on smell signaling. It turns out that the immune make up of a person plays a role in mate selection. What immune genes a person has influences how they smell –- and which other individuals may find them attractive. On average, human subjects rate the odors of other individuals as more pleasant if they have less immune gene similarity rather than a high degree of similarity. Even though human mate choice is rather difficult to study, this research does support the long-standing notion of “chemistry” in a relationship.

So how do synthetic or plant-derived fragrances affect this natural process of mate selection based on immune fitness? At the moment, we don’t know, but it sure would be interesting to find out!

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