While it may seem like pets can understand our emotions at times, scientists have found that your pet dog gets stressed out when you cry more than other animals.
Dogs—and pet pigs—were discovered to be able to tell the difference between humans vocally crying and humming, but the dogs were seen to get more stressed out by the crying than the pigs, according to a new paper in the journal Animal Behavior.
This discovery may suggest that dogs evolved to have a greater understanding of human emotions during their domestication.
Previous work has also found that dogs are capable of something called "emotional contagion," aligning their emotional states with that of their owners.
"We were curious whether the emotional contagion observed between dogs and humans was related to the well-conserved acoustic structure of some emotional vocalizations across species, or if it was promoted by the dogs' specific selection for cooperation and dependency on humans," study author Fanni Lehoczki, an animal behavior researcher at Eötvös Loránd University in Hungary, said in a statement.
To investigate if these stress responses were universal across intelligent pet animals or specific to domesticated dogs, the researchers compared the reactions of pet dogs and companion pigs—which don't have a selection for cooperation in their domestication history like dogs do—to the sounds of humans crying and humming. Humming is a non-negative, but strange sound, while crying is emotionally negative, but normal sound for the animals to hear.
"For collecting this data we used a so-called 'citizen science approach'," co-author Paula Pérez Fraga, also a researcher at Eötvös Loránd University, said in the statement. "The animals were tested remotely at their homes by their owners, facilitating the inclusion of more subjects in the study from various locations worldwide."
They discovered that the pigs were more stressed when they heard the humming, while dogs were stressed by hearing the human crying.
"Our study revealed a remarkable difference between dogs' and pigs' reactions to human sounds: dogs exhibited more stress responses and vocalizations to the crying, whereas pigs exhibited fewer stress responses and vocalizations to the crying," the researchers wrote in the paper. "To our knowledge, this is the first study comparing the behavioural reactions to human emotional vocalizations of two social domestic species kept as companion animals."
This could be because as dogs were domesticated over thousands of years, there was a strong selection pressure for them to cooperate with humans and understand their emotions.
The fact that the dogs were stressed by the sounds of crying supports them having emotional contagion from humans. The pigs becoming stressed by the humming may be an example of neophobia, with them becoming anxious due to hearing an unfamiliar sound.
"Our findings suggest that decoding the emotional content of certain human vocalizations, particularly those lacking a conservative acoustic structure and possibly perceived as less relevant for animals, such as humming, might be challenging for some domestic species," said Pérez Fraga.
"While we cannot exclude that pigs also experienced emotional contagion when listening to human crying, one explanation for their reaction might be that humming is highly unusual, and surprising sounds stress out pigs more than human sadness. On the other hand, one thing is clear, dogs seem to be specially keen on catching the emotional content of differently valenced human vocalizations, even of the more unusual ones," she said. "Therefore, selection for cooperation with humans might be key in promoting human-sound induced emotional contagion in domestic animals. However, to confirm this suggestion further research is needed involving physiological measurements."
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