Dogs can smell their humans' stress, and it makes them sad

Humans and dogs have been close companions for perhaps 30,000 years, according to anthropological and DNA evidence. So it would make sense that dogs would be uniquely qualified to interpret human emotion. They have evolved to read verbal and visual cues from their owners, and previous research has shown that with their acute sense of smell, they can even detect the odor of stress in human sweat. Now researchers have found that not only can dogs smell stress—in this case represented by higher levels of the hormone cortisol—they also react to it emotionally.