Horses and Pigs Hear Positive and Negative Emotions

You know your dog (and maybe your cat) can tell the difference when you speak to them in a nice or not-so-nice voice. But researchers have recently discovered that other animals can also differentiate between positive and negative sounds.

An international team of researchers studied whether animals could distinguish between positively or negatively charged sounds from humans and from members of their own species. They specifically concentrated on both wild and domesticated horses and pigs.

The study was part of a large project investigating the evolution of the vocal expression of emotions in hoofed animals, or ungulates.

“We were wondering if all the species we tested express emotions in the same way, and if as such, they can discriminate emotions in each other's calls (across species). We were also interested in whether domestication has had an effect on vocal expression of emotions,” study author and behavioral biologist Elodie Briefer of the University of Copenhagen’s Department of Biology, tells Treehugger.

They tested domesticated pigs and wild boars, as well as domesticated horses and wild Przewalski’s horses. Then they played human voices mimicking positive and negative emotions.

“A lot of research is done on dogs, and much less on farm animals. However, these animals have also been domesticated and are in daily contact with humans, so there are reasons to think that they can also share some abilities with dogs,” Briefer says. “This is already common knowledge for horses, but less for pigs. In addition, their emotions are a large part of their welfare, so knowing how we influence them while talking to them is crucial.”  

Listening to Positive and Negative Voices

For the study, researchers tested the calls of domesticated pigs with those of wild boars and vice versa. They also tested the calls of wild Przewalski’s horses with those of domesticated horses and vice versa. They studied the two wild species using animals that lived in parks or zoos. Those animals were familiar with people, but not with their closely related domesticated species.

Then they played recordings of human voices talking gibberish but said with positive or negative expressions.

They did this to see if the animals could distinguish between emotions in a distantly related species. That could be because animals express emotions with the same vocal cues as humans do, or the animals were familiar with hearing humans speaking around them. It could also be that through domestication, species that are aware of emotions in the human voice might have unconsciously been selected to continue breeding.

“Animals that live around humans learn to associate words with consequences or contexts,” Briefer says. “Our study was aimed at testing how voice intonation (‘emotional prosody’), in a similar way as changes occurring in animal calls, affect emotions.”

To rule out that animals may know the meaning of certain words, they used actors reciting nonsense words.

They found that domesticated pigs and horses and wild horses can tell the difference between sounds coming from distantly related species, as well as from human voices. Wild boars, however, responded differently to domesticated pig calls, but did not react to the sounds of other wild boars or humans.

Horses responded by the way they pointed their ears, stopped eating or walking, and with various head movements. Pigs and boars were measured by whether they stopped walking or eating, and by the position of their heads.

The results were published in the journal BMC Biology.

Voices and Animal Welfare

Researchers were curious whether animals mirrored human emotions, which is known as emotional contagion. In the study, that would’ve been apparent if animals experienced negative emotions when they played negative vocalizations, or conversely with positive emotions.

“Unfortunately, indicators of emotional valence (whether the animals are experiencing negative or positive emotions), are very subtle and tricky to assess (e.g. ear and tail position), and in particular in the wild species in zoos and parks that were sometimes filmed from further away, we couldn’t score these subtle changes,” Briefer says.

“We can thus only see that in general, the two horse species and pigs reacted stronger (e.g. faster) when we played first a negative sound followed by a positive one than vice versa. However, stronger means a higher level of arousal (bodily activation), not necessarily a negative emotion. We were thus not clearly able to investigate if emotional contagion occurs or not.”

Researchers were most surprised that wild Przewalski’s horses can discriminate between positive and negative human voices. They also found it interesting that wild boars didn’t change their behaviors for humans, but reacted with more calls and “freezing” when they heard positive calls of pigs, and vice versa.

The fact that so many of the animals acted more strongly to a negative sound first suggests to Briefer that the way humans talk around animals might influence their well-being.

“The most important finding regarding how we treat animals would be that we now know that the two horse species and pigs discriminate between positive and negative speech and react stronger when they first hear negative speech,” Briefer says. “This means that the way we speak around or to animals influenced their emotions, and hence their welfare.”