According to a group of researchers at the University of Sussex in Brighton, UK, domestic horses (Equus caballus) are able to distinguish between positive (happy) and negative (angry) human facial expressions.

The research group, led by Dr. Amy Smith and Prof. Karen McComb, studied how 28 horses reacted to seeing photographs of positive versus negative human facial expressions.

When viewing angry faces, the animals looked more with their left eye, a behavior associated with perceiving negative stimuli. Their heart rate also increased more quickly and they showed more stress-related behaviors.

“This response indicates that the horses had a functionally relevant understanding of the angry faces they were seeing,” the scientists said.

The effect of facial expressions on heart rate has not been seen before in interactions between animals and humans.

“What’s really interesting about this research is that it shows that horses have the ability to read emotions across the species barrier,” Dr. Smith said.

“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.”

“The reaction to the angry facial expressions was particularly clear – there was a quicker increase in their heart rate, and the horses moved their heads to look at the angry faces with their left eye.”

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,” Dr. Smith explained.

“In this context, recognizing angry faces may act as a warning system, allowing horses to anticipate negative human behavior such as rough handling.”

A tendency for viewing negative human facial expressions with the left eye specifically has also been documented in dogs.

There are several possible explanations for the findings, which were published in the journal Biology Letters.

“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,” Prof. McComb said.

“Alternatively, individual horses may have learned to interpret human expressions during their own lifetime.”

“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,” she said.

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Amy Victoria Smith et al. 2016. Functionally relevant responses to human facial expressions of emotion in the domestic horse (Equus caballus). Biology Letters, vol. 12, no. 2; doi: 10.1098/rsbl.2015.0907