The scientific case for setting the right tone with horses and pigs

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"Our results show that these animals are affected by the emotions we charge our voices with when we speak to or are around them."
Image by Vanessa Väth

Fresh research has put forward a strong case for speaking kindly to horses and pigs. The way we speak matters to animals, the scientists concluded.

Researchers with the University of Copenhagen and ETH Zurich found that pigs, horses, and even wild horses, can distinguish between negative and positive sounds from their fellow species and near relatives, as well as from human speech.

Their recently published results provide insights into the history of emotional development and open up interesting perspectives with regard to animal welfare.

An international team, including researchers Anne-Laure Maigrot and Edna Hillmann, and behavioral biologist Elodie Briefer from the University of Copenhagen’s Department of Biology, investigated whether a range of animals can distinguish between positively and negatively charged sounds.

“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,” Briefer says.

The animals even showed the ability to distinguish between positively or negatively charged human voices.

While their reactions were more subdued, all but wild boars reacted differently when exposed to human speech that was either charged with positive or negative emotion.

Associate Professor Elodie Briefer. Photo: Kristian Bjørn-Hansen
Associate Professor Elodie Briefer. Photo: Kristian Bjørn-Hansen

In the study, reported in the journal BMC Biology, the researchers played recordings of animal sounds and human voices from hidden speakers.

To avoid having the domesticated animals react to specific words, positive and negative human speech was performed by a professional voice actor in a kind of gibberish without any meaningful phrases.

The animals’ behavioral reactions were recorded in a number of categories used in previous studies – everything from their ear position to their movement or lack thereof.

“Our results show that these animals are affected by the emotions we charge our voices with when we speak to or are around them,” Briefer says. “They react more strongly – generally faster – when they are met with a negatively charged voice, compared to having a positively charged voice played to them first. In certain situations, they even seem to mirror the emotion to which they are exposed.

Do animals have an emotional life?

Part of the aim of the study was to investigate the possibility of “emotional contagion” in animals – a kind of mirroring of emotion. Situations where one expressed emotion is assumed by another. In behavioral biology, this type of reaction is seen as the first step in the empathy category.

“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,” Briefer says.

The study was unable to detect clear observations of “emotional contagion”, but an interesting result was in the order by which the sounds were delivered. Sequences in which the negative sound was played first triggered stronger reactions in all but the wild boars. This included human speech.

This suggests that the way we talk around animals and the way we talk to animals may have an impact on their well-being, according to Briefer.

“It means that our voices have a direct impact on the emotional state of animals, which is very interesting from an animal welfare perspective.”

This knowledge doesn’t just raise ethical questions about how we perceive animals – and vice versa, it can also be used as a concrete means of improving animals’ daily lives, if those who work with them are familiar with it

“When the animals reacted strongly to hearing negatively charged speech first, the same is also true in the reverse. That is, if animals are initially spoken to in a more positive, friendly voice, when met by people, they should react less. They may become calmer and more relaxed,” she says.

The researchers had worked with three theories about what they expected to influence the animals’ reactions in the experiment:

  • Phylogeny: According to this theory, depending on the evolution of species, animals with a common ancestry may be able to perceive and interpret each other’s sounds by virtue of their common biology.
  • Domestication: Close contact with humans, over a long period of time, may have increased the ability to interpret human emotions. Animals that are good at picking up human emotions might have been preferred for breeding.
  • Familiarity: This is based on learning. The specific animals in the study may have learned a greater understanding of humans and fellow species, who they were in close contact with where they were housed.

Among the horse species, the phylogeny thesis best explains their behavior. In contrast, the behavior of the pig species best fits the domestication hypothesis.

The next step for Briefer is a switchover experiment. She and her colleagues are now looking into how well humans are able to understand animal sounds of emotion.

Maigrot, AL., Hillmann, E. & Briefer, E.F. Cross-species discrimination of vocal expression of emotional valence by Equidae and Suidae. BMC Biol 20, 106 (2022). https://doi.org/10.1186/s12915-022-01311-5

The study, published under a Creative Commons License, can be read here

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