There is ample evidence that non-human animals can experience and understand complex emotions. Evolutionarily, this makes sense, since it would be easier for animals that understood the feelings of other members of their species to survive. To some extent, this even happens between certain species and humans, including cows, dogs, sheep, cats, goats, chimpanzees, and horses. However, there’s only evidence that a few of these species can understand emotions through human voices, namely dogs, cats, and horses. 

In general, researchers still don’t understand whether (and how) animals differentiate emotional cues in voices beyond their own species. As such, a recent study expanded this area of science in two ways. First, the researchers tested whether two wild and two domestic species of horses and pigs could understand the vocal expression of human emotions. Second, the team tested whether these species could understand the vocal expressions of one another’s emotions (e.g., whether domestic pigs can detect emotion in domestic horse, wild horse, and wild boar calls, and vice versa). Ultimately, the study found wild and domestic horses, and pigs but not wild boars, were able to distinguish positive versus negative emotional tone regardless of species.

The researchers tested three hypotheses of interspecies communication: the familiarity hypothesis, the phylogeny hypothesis, and the domestication hypothesis. The familiarity hypothesis argues that simply by spending time together (for example, two different animal species living in proximity in a zoo), some species might be able to understand others’ emotions. The phylogeny hypothesis contends that since communication and emotions could be “conserved” through evolution, species that are more closely related would have a greater ability to under interspecies expression of emotions. Finally, the domestication hypothesis says that species that were domesticated by humans may be more likely to understand human emotions.

The actual experiment went as follows. The participant animals were selected based on two “taxa” that have both wild and human-domesticated species — horses and pigs. Specifically, the researchers worked with wild Przewalski’s and domesticated horses, and wild boars and domesticated pigs. The researchers then gathered recordings of animal sounds with previously scientifically established emotional tones, coded as either “negative” or “positive.” For the human voices, to avoid the possibility of the animals recognizing and reacting to specific words or phrases, the team hired a professional voice actor to speak gibberish in an emotionally negative or positive tone. Using hidden speakers and high sound quality to match the natural frequencies at which the animals could hear, the researchers then played the recordings in a randomized order and videotaped the animals’ reactions. Later, the team observed the recordings to code the animals’ emotional states (defined by their behavior and body language), looking for specific cues like ear position, whole-body movement, and facial expression shifts.

In the end, the researchers found evidence that the wild Przewalski’s horses and domestic horses, as well as the domestic pigs (but not wild boars), were able to distinguish positive versus negative emotional tone through each others’ and humans’ voices. The team concluded that for the horse species, this was best explained by the phylogeny hypothesis, whereas for the pigs, the domestication hypothesis made the most sense. They came to the conclusion about pigs because wild boars were unable to differentiate between positive and negative emotions even among other wild boars. The findings here were significant for two reasons. First, to date, no study has explored whether non-human animals can distinguish vocal expressions of emotion across species (other than humans). Thus, in addition to having complex emotions themselves, at least some species can show signs of empathy toward individuals outside of their own species. Another significant conclusion is that the emotional valence of human voices can impact the emotions of both domestic and captive animals. 

This has important implications for how we, as humans, treat and interact with non-human animals. In other words, as humans, our voices, as well as our actions, can have a direct impact on the feelings of the other animals we live, work, and play with. To this point, animal advocates would do well to not only fight for the physical well-being of animals, but also for their emotional and mental health, especially those held in captivity. The conclusions of this research raise a concrete ethical issue that we can act upon without much effort in our regular interactions with such animals. Put simply, how we talk to animals matters.