A new study suggests horses may be able to read human facial expressions — especially anger.

University of Sussex researchers examined the reactions of 28 horses to photographs of people making positive and negative expressions. 

When the animals saw images of angry faces, they looked at the pictures more with their left eyes — a behaviour previous studies have shown is associated with perceiving negative stimuli, according to the study published in Biology Letters

That's likely because the left eye processes information for the right hemisphere of the brain, which in turn processes threats. 

Horsies

Doctoral student Amy Smith poses with Red the horse. (University of Sussex)

What's more, the angry expressions caused the horses to show signs of stress, including an increased heart rate.

"The reaction to the angry facial expressions was particularly clear," Amy Smith, a doctoral student in the Mammal Vocal Communication and Cognition Research Group at Sussex who co-led the research, said in a press release.

The animals didn't have strong reactions to the happier faces, which suggests their facial expression recognition abilities may be defensive in nature, the researchers say. 

"This may be because it is particularly important for animals to recognize threats in their environment," Smith said. "In this context, recognizing angry faces may act as a warning system, allowing horses to anticipate negative human behaviour such as rough handling."

Co-author Karen McComb, a University of Sussex animal behaviour professor, has a couple of theories as to why horses have learned to read human emotions. 

"Horses may have adapted an ancestral ability for reading emotional cues in other horses to respond appropriately to human facial expressions during their co-evolution," she said. 

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During the study, experimenter 1 (E1) held the horse while facing away from the images, while experimenter 2 (E2) stood behind a board and held up images for the horse to see. The triangles represent cameras. (Biology Letters)

"Alternatively, individual horses may have learned to interpret human expressions during their own lifetime," she said.

"What's interesting is that accurate assessment of a negative emotion is possible across the species barrier despite the dramatic difference in facial morphology between horses and humans.

"Emotional awareness is likely to be very important in highly social species like horses — and our ongoing research is examining the relationship between a range of emotional skills and social behaviour."

A University of Helsinki study about dogs last month found similar results. Canines also react most strongly to angry or threatening expressions.