Can dogs really smell fear? A vet weighs in.

You’re watching a scary movie. Or dreading that dentist appointment you have in a few hours. Your heart’s racing—and you swear your dog seems to notice. Are you imagining things, or can your four-legged friend really sense your fear?

Science says your hunch is right. Dogs can sense human emotions—fear included—using their extraordinary sense of smell.

When researchers at the University of Naples Federico II collected sweat samples from volunteers who had just watched videos designed to trigger fear, happiness, or a neutral state and presented them to domestic dogs, the dogs exposed to fear-related odors showed more signs of stress, had higher heart rates, sought more reassurance from their owners, and were less friendly with strangers, compared to dogs exposed to happy or neutral smells.

In a later study at the University of Veterinary Medicine in Vienna, dogs exposed to a fear scent showed signs of hesitation or discomfort: They spent more time near the experimenter, held their tails lower, and took longer to approach new objects presented to them.

The scent of fear 

Fear starts in the brain but leaves chemical traces elsewhere in the body. When your amygdala—the brain’s fear center—senses a threat, it sends a signal to another part of your brain called the hypothalamus, which in turn prompts your adrenal glands to release the stress hormones adrenaline and cortisol. These changes in the human body alter the mix of chemicals in our breath and sweat, and dogs are able to pick up on this, says Dr. Zoe Parr-Cortes, a veterinarian and PhD graduate from the University of Bristol.

Compared to the 5 million scent receptors in human noses, dogs have about 220 million. They also have a special organ called the ‘Jacobson’s organ’ (or ‘vomeronasal organ’), which humans lack, that detects pheromones—chemical signals used for communication between members of the same species.

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“For example, dogs trained on scent by Medical Detection Dogs in the UK can alert people with Addison’s disease when their cortisol levels drop too low,” says Parr-Cortes.

In research led by Parr-Cortes with Dr. Nicola Rooney and Professor Mike Mendl at Bristol Veterinary School, the team found that dogs reacted differently to stress odors than to relaxed ones, even when dogs didn’t know the people who provided the samples. The researchers took odor samples from people after measuring their cortisol, heart rate, heart-rate variability, and self-reported anxiety. They then selected odor samples from participants with the strongest cortisol responses and then presented them to untrained dogs. Ultimately, the dogs exposed to a stressed person’s scent were more cautious than the dogs exposed to a relaxed person’s scent.  

This suggests that dogs may detect differences in cortisol, or other chemical changes associated with stress and fear, via scent—even without training, says Parr-Cortes. 

An evolutionary explanation

Do dogs benefit at all from being so emotionally in-sync with humans? 

In Parr-Cortes’s above experiment, all the dogs were initially given an empty bowl. Then, the dogs were exposed to the smell of a stressed or relaxed person before being given a new bowl. The dogs exposed to the stressed person’s smell were slower to approach the new bowl, indicating that the dog was less optimistic about finding a reward there. This effect was not seen when dogs were exposed to the smell of a relaxed person. “These findings suggest that the smell of stress may have reduced how willing the dogs were to take a risk in a situation where a positive outcome seemed unlikely,” says Parr-Cortes. “This could be a way for the dog to avoid disappointment and conserve energy.”

These reactions likely have deep evolutionary roots. “As one of our closest companions, dogs have co-evolved alongside humans for thousands of years,” says Parr-Cortes. “Detecting stress or fear response in others within a social group, sometimes called emotional contagion, is thought to be beneficial, especially if it signals a possible threat in the environment.”

The bottom line

Even though a trip to the dentist can’t compare to facing down a saber-toothed cat, it triggers the same ancient fear response—and your dog can smell it. Their sensitive noses don’t just read the world around them; they read us, too.

This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.

 

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