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 behavior previous studies have shown is associated with perceiving negative stimuli.

That's likely because the left eye processes information for the right hemisphere of the brain, which in turn processes threats. What's more, the angry expressions caused the horses to show signs of stress, including an increased heart rate.

The animals didn't have strong reactions to the happier faces, which suggests their facial expression recognition abilities may be defensive in nature.  In this context, recognizing angry faces may act as a warning system, allowing horses to anticipate negative human behavior such as rough handling.

Co-author Karen McComb, a University of Sussex animal behavior professor, 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 behavior."

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