The smell of fear may make it hard for dogs to track some people | Science News for Students

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

Trained dogs had trouble sniffing out people with a short form of one gene that turns on in people who are stressed
Apr 16, 2019 — 6:30 am EST
a white labrador dog wearing a red harness is trotting on a green lawn, its head is lowered as though sniffing the ground

Being stressed or afraid may alter a person’s usual scent. That can throw dogs off the person’s track. People with a particular version of a gene may have a bigger change in odor when stressed, data show.


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.

Power Words

(more about Power Words)

annual     Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.

canine     Members of the biological family of canids. These are carnivores and omnivores. The family includes dogs, wolves, foxes, jackals and coyotes. 

colleague     Someone who works with another; a co-worker or team member.

criminology     A research field that focuses on understanding crime and criminals. Experts in this field are known as criminologists.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning.

forensic     And adjective referring to the use of science and technology to investigate and solve crimes.

gene     A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nerve     A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

signaling molecule     A substance created by an organism and released into the environment. In organisms that use quorum sensing, signaling molecules are used to broadcast an individual’s presence to similar organisms nearby.

stress     (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative.


Meeting: F. Sessa et al. The smell of DNA: How genetics and fear influence the human scent. American Academy of Forensic Sciences annual meeting, February 22, 2019. Baltimore, Md.