Dogs are known for their powerful noses, sniffing out drugs and explosives as a safety measure.
But it's now been discovered they can actually smell when humans are stressed - even when it's someone they don't know, according to a new study carried out by Queen's University Belfast.
Researchers say this skill could be useful when training therapy and service dogs, as stress can be detected by the scent of a person's sweat and breath.
In the study, scientists collected samples of sweat and breath from 36 people before and after they undertook a hard maths problem.
They all reported their stress levels before and after the task, and researchers only used samples where the person’s blood pressure and heart rate had increased.
When presented with samples, all four pooches – Treo, Fingal, Soot and Winnie – were able to identify the stress samples.
Clara Wilson, a PhD student in the School of Psychology at Queen’s University Belfast, said: "The findings show that we, as humans, produce different smells through our sweat and breath when we are stressed and dogs can tell this apart from our smell when relaxed – even if it is someone they do not know.
"The research highlights that dogs do not need visual or audio cues to pick up on human stress.
"This is the first study of its kind and it provides evidence that dogs can smell stress from breath and sweat alone, which could be useful when training service dogs and therapy dogs.
"It also helps to shed more light on the human-dog relationship and adds to our understanding of how dogs may interpret and interact with human psychological states."
One of the pooches that took part in the study was Treo, a two-year-old cocker spaniel.
His owner Helen Parks said: "As the owner of a dog that thrives on sniffing, we were delighted and curious to see Treo take part in the study.
"We couldn’t wait to hear the results each week when we collected him.
"He was always so excited to see the researchers at Queen’s and could find his own way to the laboratory.
"The study made us more aware of a dog’s ability to use their nose to ‘see’ the world.
"We believe this study really developed Treo’s ability to sense a change in emotion at home.
"The study reinforced for us that dogs are highly sensitive and intuitive animals and there is immense value in using what they do best – sniffing."
In the study, each dog was taught how to search a scent line-up and alert researchers to the correct sample.
Stress and relaxed samples were then introduced, but at this stage the researchers didn't know if there was a difference in smell that the pooches could detect.
During each session of testing, the dogs were presented with one person's stressed and relaxed sample four minutes apart.
And each pooch was able to correctly identify the stress sample.
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