Facial expressions turn out to be one of dogs' main indicators of human fear, but popular belief of them "smelling" our fears can still be possible. Humans always thought that dogs are good in "smelling" fear, but several studies reveal that there's even more to that.
In an article posted in Animal Planet, to start off, dogs were thought to have "humanlike minds and moral sensibilities" and Greek philosophers Plato and Diogenes, who believed in this notion. According to a book called "How Dogs Think" by Stanley Coren, dogs learn to adapt to its human owners because they can easily interact with us.
In the same book, it is also mentioned that dogs' abilities include understanding human speech, having more than 150 words in their vocabulary, ability to solve complicated problems, and intentionally trick other dogs. There are also evidence that show that dogs actually study signals from human and interpret facial expressions. Even researchers from Azabu University in Japan observe that dogs can differentiate a smile from a blank look when shown in a series of photographs. If that's the case, it is highly possible that dogs can also understand changes in facial expressions such "clenched teeth and wide eyes" of a frightened person.
Quora user Jim Johnston has a lot of years in laboratory and field research of canine olfaction.
"Dogs are very sensitive to subtle aspects of human behavior and are likely to respond to such changes, especially because such circumstances have in the past had a significant impact on the dog's behavior," he said, according to Mirror.
Indeed, dogs can pick up from our actions more than how we smell for them to detect our fears. Johnston also suspected that there are some dogs who respond to our fear reactions, and visual cues that are related to human behavior is what they use as basis. Our behavior changes whenever we are afraid of something, and most of the time, we tend to be more tensed, our breathing changes and our movements quicker.
Then why is there a saying that dogs can "smell" our fear? When we are actually frightened about something, we tend to sweat more. And whenever we are alarmed, we have these chemicals called pheromones that we unconsciously give off, according to Alexandra Horowitz, author of "Inside of a Dog: What Dogs See, Smell and Know."
And with dogs' notoriously sensitive smelling abilities, it can be possible for them to detect these chemicals.