Could Saddam Hussein's Smell Lead U.S. to His Lair?

Bijal P. Trivedi
National Geographic On Assignment
December 5, 2003
A posse of body doubles has long blurred the whereabouts of Saddam Hussein. But while it is easy to mimic the dictator by donning a uniform and a mustache, finding a "smell double" is a much tougher act.

With this in mind, the research arm of the U.S. Defense Department wants to develop technology that can identify individuals on the basis of a so-called smell signature.

Every individual has a chemical fingerprint or "odortype," said Gary Beauchamp, a biopsychologist and director of the Monell Chemical Senses Center, a primarily government-funded research institute in Philadelphia, Pennsylvania.

Beauchamp and his colleagues at Monell coined the word to reflect what seems to be a genetically programmed body odor.

"A dog can sniff clothes worn weeks ago and track an individual, which suggests that each has a unique odor," Beauchamp explained.

An odortype also has a distinct advantage over visual identification. Images and sounds are fleeting but odors—complex mixes of volatile chemicals—stick around for a long time.

Odortypes may also be useful for solving crimes and following terrorist activities where witnesses may be few but where chemical clues and odors abound.

"People are really good at identifying faces—they can recognize thousands of them," said Michael Leon, a neurobiologist at the University of California, Irvine, "but the way that people look is not a good indicator of their genetic make-up. Looks can be misleading."

By contrast, odortype reports directly on the genetic make-up of an individual, said Leon, who studies how odors are translated into chemical signals in the brain.

For scientists the challenge is how to convert a personal body odor into an ID tag. Odortypes are probably present in all parts of the body but are most concentrated in urine and sweat. The goal is to isolate a collection of chemicals in urine and perhaps sweat that reveal genetic information.

Experiments in mice have shown that the odortype is influenced by a set of about 50 genes called the Major Histocompatibility Complex. These genes are known for their essential role in the immune system, tagging every single cell in the human body with a "self" marker.

But these MHC genes also seem to affect how an individual smells—even a minute change in a single MHC gene can alter a person's odor. In animals, these odors influence recognition, mate choice and nesting behavior.

Mice are excellent at detecting differences between individuals. Beauchamp and his colleagues demonstrated that a mouse could tell the difference between two other mice genetically similar except for one MHC gene out of the 30,000 total genes in the mouse genome.

Though humans perceive far less sensory information through smell than other mammals—they have only 300 olfactory receptors compared to the mouse's 1,000—they still respond to odor signals even if subconsciously, says Diego Restrepo, a neuroscientist at the University of Colorado Health Sciences Center in Denver, Colorado.

Work done by scientists at the University of Chicago in Illinois and published last year in the journal Nature Genetics revealed that women can smell some genetic differences between men. Women preferred scents of men with similar—but not identical—MHC genes to their own. Scents from men with almost identical or starkly different MHC genes were not as desirable.

Other studies have revealed that women can also identify their infants by smell alone.

Beauchamp and his colleagues have devised an experiment that uses lab mice to detect the chemical markers in urine. The scientists are training mice to distinguish different MHC genes in human urine via scent.

A chemical analysis of the urine may indicate which volatile compounds—essentially a chemical profile—relays the genetic information, said George Preti, the organic chemist conducting the analysis. Once the scientists identify them, they'll synthesize a mixture and test it on the smart mice.

The goal is to build a sensor—an electronic "nose" that, coupled with a database of odortypes, can tell one individual from another.

The smell signature also applies to health. Beauchamp's team at Monell discovered that mice, for example, can distinguish older and younger genetically identical mice. They also use odor to identify animals infected with the Mammary Tumor Virus before any signs of disease are present. In Cambridge, England, dogs are being tested for their ability to sniff out traces of human prostate cancer in urine samples. Beauchamp anticipates that many diseases may have chemical signatures that may provide early diagnoses.

An understanding of chemical signatures may enable all kinds of sleuths to make heretofore impossible identifications.

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