Animal Emotions: Does Your Dog Know When Your Cat Is Angry?
In his seminal work About the origin of species, Charles Darwin describes natural selection, a theory that has become the foundation of modern evolutionary biology. But fewer people know about his third major work on the theory of evolution, The expression of emotions in humans and animals. In it, Darwin explains why emotions, like other traits, adapt and change over time.
Although unpopular at first, most scientists now agree that emotional expression has been conserved across species because emotional intelligence plays such a critical role for animals. Animals use emotions to react to events they find meaningful. When we express emotions, others pay attention. Sometimes our emotions trigger an automatic, subconscious emotional response in the receiver. Researchers describe this effect as “emotional contagion.” In other words, emotional contagion is the beginning of basic empathy, the ability to be affected by the emotional state of others and to share it. Empathy can amplify emotions within a group, strengthening social bonds.
We know that emotional contagion is a powerful force in humans. Research has also shown that it occurs in the social lives of dogs, bonobos, mice, and pigs. But whether emotional contagion could occur between species is another question.
Can animals understand other people’s emotions?
Because different species can be so familiar with each other – think of a dog and its owner – it makes sense that we have learned to perceive and discriminate between emotions to facilitate cross-species interactions. Several studies have sought to determine whether nonhuman animals can interpret vocal or facial emotional cues from humans. This work has provided us with empirical evidence to back up what all pet owners know – dogs, cats, horses, and even mice can understand and respond to our emotions.
But no studies have investigated whether nonhuman animals can distinguish between emotions in the vocalizations of other nonhuman species. So we know your dog can tell when you’re angry. But can he understand your cat’s annoyed meowing? This information is important: understanding how animals interpret the vocalizations of closely related species is essential to understanding the interspecific perception of emotions. It also gives us important clues about the evolution of emotional perception. To further this understanding, researchers from the University of Copenhagen and ETH Zurich observed a number of animals: domesticated horses, wild horses (in particular, a breed called Przewalski’s horse), domesticated pigs and boars. They tested whether these animals could distinguish between positive and negative emotions in members of their own species, related species and humans.
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Their results showed that all species except wild boars could distinguish between positive and negative emotional displays of members of their own species, members of another closely related species, and humans.
Research provides ample evidence in support of cross-species perception of emotions. It also shows that different animals probably learn emotional intelligence in different ways.
If you’re happy and you know it, wag your tail
First, the researchers recorded the sounds of different individuals of each species. Half of the animals were female and the other half male, and the sounds were recorded when they were in negative or positive emotional states.
Researchers placed animals in contexts believed to induce positive and negative emotions. For example, sometimes the animals were reunited or separated with other members of the group. In other cases, researchers provided or withdrew food, water, and toys. The researchers used accepted indicators such as body position, as well as physiological indicators – heart rate, respiratory rate, etc. – to confirm whether the animal felt pleasant or unpleasant emotions. When the researchers analyzed the recordings, they found that the acoustic structure of the vocalizations (neighs for horses and grunts for pigs) differed depending on the situation.
For the humans, the researchers used voice actors from a validated database as they expressed joy, amusement, anger and fear. In these recordings, the actors did not use meaningful phrases or words. This controlled for the possibility of pets responding to the word, not the emotion.
The researchers exposed the animals to all positive and negative recordings from members of their own species, closely related species, and humans. Thus, a domestic pig has heard noises from other domestic pigs, as well as wild pigs and humans. The researchers reproduced the noises on loudspeakers, ensuring that latency and time between recordings were equivalent across species. The only characteristics that changed were the speaker and the type of emotion expressed.
Measuring emotional contagion in animals
The researchers measured a series of physical cues to determine how the animals responded to sounds. They recorded the animals’ responses and then used a blinded study design – the researchers were unaware of the treatment being used when they recorded the animals’ responses. Observers looked for a suite of responses, including reactions to the speaker (approaching, looking at, or avoiding the speaker); movements (standing, walking, running or trotting); head movements (particularly ear movements, such as the proportion of time spent with the ears perpendicular, rear-facing or forward-facing); tail movements; and vocalizations.
Both domestic and wild horses responded more strongly to readings when the vocalizations were negative than when they were positive. They spent more time walking and paid more attention to the speaker. This happened regardless of whether the speaker played the sounds of conspecifics, related species, or humans. Domestic pigs also reacted more strongly to negative emotions of all species.
Interestingly, the boars did not react to the noises of other boars, nor to those of humans. However, when the boars heard the vocalizations of the negative or positive cries of the domestic pigs, they moved their heads more often, produced more cries and remained with their tails high and erect for a long time.
All species reacted most markedly in all cases to closely related species or conspecifics, and less markedly to a human voice.
Overall, domestic horses, Przewalski’s horses, and pigs distinguished vocal indicators of positive and negative emotions in all species, but boars only responded to calls from domestic pigs. These results suggest that the emotional responses of horses and pigs may arise from different mechanisms, such as relatedness and domestication.
The foundation of empathy?
In some situations, animals reflected the emotions they were exposed to, especially negative emotions. The researchers did not directly test this type of emotional contagion, which has long been considered the first step towards empathy. Yet their research will likely inspire other behavioral biologists to assess the emotional intelligence and capacity for empathy in these and other animals.