The smell of fear may make it hard for dogs to track some people

BALTIMORE, Md. — Some police dogs may smell fear. And that could be bad news for finding people whose genes make them more prone to stress, new data show.

Trained police dogs did not recognize stressed-out people who had inherited a form of a gene linked to managing stress poorly. The dogs had no trouble sniffing out these people when they weren’t under stress. Francesco Sessa reported their new findings here, February 22, at the annual meeting of the American Academy of Forensic Sciences. Their findings might help explain why dogs can perform flawlessly in training but have difficulty tracking people during real-world hunts.

Sessa studies genetics at the University of Foggia in Italy. He and his colleagues wondered whether fear might change someone’s normal scent. They focused on a gene called SLC6A4. It makes a protein that helps move signaling molecules in the brain and nerves. Studies had already linked different forms of this gene to how well someone manages stress. Those with a long version of SLC6A4 tended to handle stress better than did people with the short version, Sessa notes.

For its new study, his group recruited four volunteers. One man and woman each had the long version of the gene. Another man and woman had the short version. Each participant wore a scarf for a couple of hours a day. This left their scent on the garment.

Then the researchers brought the volunteers into their lab and gave them T-shirts. In the first session, the volunteers just wore one of the shirts. They weren’t subjected to any stress. The team then mixed the participants’ shirts with shirts worn by other people. They made two lineups of 10 T-shirts each. One set was from men and the other was from women. After sniffing the scarves, two trained police dogs had no trouble picking any of the volunteers’ shirts out of the lineups. One dog was a yellow lab. The other was a Belgian malinois. The canines identified each of the volunteers’ shirts in each of three attempts.

On their next visit, the volunteers wore new T-shirts. Then the researchers had them do public speaking to stress them out. The participants’ hearts raced and their breathing became shallow. Those are signs that these people were scared, Sessa explains.

That stress may have made their body odor change. Indeed, the animals had a harder time matching a volunteer to a stress-stained T-shirt. The dogs found the tees from the man and woman with the long version of the SLC6A4 gene in two out of three tries. But neither dog could identify the shirts from the stressed people with the short version of the gene. The result suggests that those people’s natural scent had changed more in response to stress.

The researchers need to confirm their findings in a larger study, Sessa says. The team has not yet studied how being scared or stressed changes body odor. In fact, more than one gene may be involved.

Still, the finding could help explain why dogs can find some people more easily than others, says Cliff Akiyama. He is a criminologist and forensic scientist. He also runs a forensic consulting company based in Philadelphia, Penn.

Fear can set off a flood of stress hormones in the body. Some people respond by freezing. Others fight. Still others may flee. Perhaps that same hormone flood could alter a person’s scent, Akiyama says.

Don’t give up on dogs yet, though. They may be useful for tracking people with the long version of SLC6A4. And they can help find people who are missing but not scared. For instance, Akiyama points out, some missing persons may be with relatives or others they know,. And if the people aren’t afraid, their scents may remain unchanged.