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.
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