They can’t talk back and they probably can’t understand the words we’re saying, but that doesn’t mean that animals don’t feel what we’re saying to them. In a new study, researchers at the University of Copenhagen found that horses, pigs, and wild horses were able to distinguish between positively and negatively charged sounds, including those in human speech, as evidenced by their behavior.

“The results showed that domesticated pigs and horses, as well as Asian wild horses, can tell the difference, both when the sounds come from their own species and near relatives, as well as from human voices,” behavioral biologist Elodie Briefer of the University of Copenhagen’s Department of Biology said in a statement.

Humans are innately emotional and social creatures. In fact, we’re so social that during our interactions, our emotions can synchronize with the people we’re talking to. If I feel sad and we interact, you may feel sad too. If I’m smiling and being very positive while we have a conversation, chances are the emotion will rub off on you. This phenomenon is known as emotional contagion or mirroring and can be triggered by facial expressions, indirect human interactions, and by observing other people’s behavior in direct and indirect interactions.

Part of the reason why the researchers embarked on this study was to see whether animals could also be affected by emotional transfer. The animals from the experiment involved privately owned horses, pigs from a research station, as well as wild Przewalski’s horses and wild boars from zoos in Switzerland and France.

The animals were subjected to animal call sounds and human voices played from high-fidelity hidden speakers. The sounds were played in sequences with either a positively or negatively charged sound first, followed by a pause, and then sounds of opposite emotions to the initial record. In order to avoid any bias introduced by reactions to specific words that domestic animals might know, the human voices were recorded by a professional actor who uttered emotionally-charged gibberish, which conveyed joy and amusement (positive), as well as anger and fear (negative), but without a specific meaning attached to the phrases.

The animals showed signs that they could distinguish between the positively or negatively charged sounds, including the human voices. For instance, the animals react faster and more anxiously when they hear a negatively charged voice. In certain situations, they even seemed to mirror the emotion to which they were exposed. The exception was wild boards, which didn’t seem to react differently to the human voices. However, the boars did change their behavior accordingly when subjected to positively or negatively charged animal sounds.

“Should future research projects clearly demonstrate that these animals mirror emotions, as this study suggests, it will be very interesting in relation to the history of the development of emotions and the extent to which animals have an emotional life and level of consciousness,” says Briefer.

According to the researchers, their findings suggest that the way to speak and interact with animals may matter a lot, with important implications for their well-being. From an ethical perspective, this means that people working directly with animals, be it in the zoo, research lab, or farm, should try to be more mindful of their language. Conversely, if people want to actively improve animals’ daily lives, they can try to use calmer, more positive language and attitudes.

“It means that our voices have a direct impact on the emotional state of animals, which is very interesting from an animal welfare perspective,” “Should future research projects clearly demonstrate that these animals mirror emotions, as this study suggests, it will be very interesting in relation to the history of the development of emotions and the extent to which animals have an emotional life and level of consciousness,” said Briefer.

The findings appeared in the journal BMC Biology.