Researchers have for the first time confirmed horses are able to distinguish between angry and happy facial expressions of their human handlers. 

In the latest study, psychologists from the University of Sussex examined how 28 horses reacted to seeing photographs of two unfamiliar males with either positive or negative facial expressions, according to a news release. 

When shown an angry face, researchers found the horses exhibited a series of stress-related behaviors. For instance, the horses moved their heads to look at the images with their left eye -- a behavior often associated with perceiving negative stimuli. Their heart rate also increased.

"What's really interesting about this research is that it shows that horses have the ability to read emotions across the species barrier," co-leader of the study Amy Smith, a doctoral student in the Mammal Vocal Communication and Cognition Research Group at the University of Sussex, said in the university's release. "We have known for a long time that horses are a socially sophisticated species but this is the first time we have seen that they can distinguish between positive and negative human facial expressions."

One reason animals view negative events with their left eye is because visual information from the left eye is processed in the right hemisphere, which specialized in the processing of threatening stimuli, researchers explained. Such behaviors have also been seen in dogs. 

"It's interesting to note that the horses had a strong reaction to the negative expressions but less so to the positive. This may be because it is particularly important for animals to recognize threats in their environment," Smith continued. "In this context, recognizing angry faces may act as a warning system, allowing horses to anticipate negative human behavior such as rough handling."

Their study was recently published in the journal Biology Letters. 

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