How we speak matters to animals

To those with a unique talent for communicating with horses, such as Robert Redford’s character in the 1998 film The Horse Whisperer, it might come as no surprise to learn that they can recognise the difference between positive and negative-sounding human speech.

But according to a new study in BMC Biology, they aren’t alone, with pigs as well as wild and domestic horses being able to distinguish between positively and negatively charged sounds – and not just from humans.

“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,” explains Elodie Briefer, associate professor of ecology and evolution in the Department of Biology at the University of Copenhagen, Denmark.

The researchers studied domesticated pigs (Sus domesticus) and horses (Equus ferus caballus), as well as their closest relatives, wild boars (Sus scrofa) and wild Przewalski’s horses (E. f. przewalskii), while playing recordings of animal sounds and human voices from hidden speakers.

The animal sounds were recorded from animals of the same species as well as the closely related species. Human speech was performed in gibberish to avoid domesticated animals reacting to specific words with which they may have already been familiar.

The animals’ behavioural reactions were recorded on video, with scientists noting everything from ear position to movement or a lack thereof.

The researchers found that the domestic and wild horses, as well as pigs, but not wild boars, reacted more strongly when the first vocalisation played was negative compared to positive – regardless of which species was broadcast.

“Our results show that these animals are affected by the emotions we charge our voices with when we speak to or are around them,” says Briefer. “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.

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“In certain situations, they even seem to mirror the emotion to which they are exposed,” she adds.

These findings provide a concrete means for people to improve the daily lives of their working animals.

“When the animals reacted strongly to hearing negatively charged speech first, the same is also true in the reverse,” says Briefer. “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.”

Further research will look at how well humans are able to understand animal sounds of emotion.