Whether it's a tricky maths problem or an unexpected bill, life is full of stressful experiences. Now researchers have found that humans produce a different odour （气气味） when under pressure and dogs can sniff it out.
While previous studies have suggested dogs might pick up on human emotions, possibly through smell, questions remained over whether they could detect （察觉）stress and if this could be done through scent.
Writing in the journal Plos One, Clara Wilson, a PhD student at Queens University Belfast and the fiurst author of the research, and her colleagues report how they first constructed a stand bearing three containers, each topped by a lid with holes. The researchers were able to train four dogs to point out the container that was holding a particular breath and sweat sample taken at a different time of day, even when the lineup included unused gauze（纱布）.
With the team confident that the dogs understood the approach, they turned to breath and sweat samples from 36 people who had been asked to count backwards from 9, 000 in units of 17. The participants reported feeling stressed by the task and for the 27 who carried it out in the lab, their blood pressure and heart rate rose.
The dogs were taught to pick out samples taken just after the task from a lineup that included two containers holding unused gauze. The researchers then tested whether the dogs could do the same when the lineup included not only unused gauze but samples taken from the same participant just before the task, when they were more relaxed. Each set of samples was shown to a single dog in 20 trials. The results show that the dogs chose the "stressed" sample in 675 out of the 720 trials.
"It was pretty amazing to see them be so confident in telling me 'nope, these two things definitely smell different', " said Wilson. While it was unclear what chemicals the dogs were picking up on, the study shows humans produce a different odour when stressed. Wilson said it was possible that even untrained pet dogs might detect changes in odour when a human became stressed.
