SEEING a man with a scowl on his face has been known to make many a human pulse race, but it has now been discovered that it has the same effect on horses.

New research by the University of Sussex has revealed for the first time that horses can read human facial expressions.

When shown pictures of angry human male faces, horses studied by psychologists tended to focus with their left eye - behaviour associated with perceiving negative stimuli - and recorded an increased heart rate.

They also showed more stress-related behaviours, whereas happy or positive expressions did not generate such a pronounced reaction.

The study into emotional awareness in horses, published today (Wednesday February 10) in Biology Letters, concluded that "this response indicates that the horses had a functionally relevant understanding of the angry faces they were seeing."

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

Amy Smith, a doctoral student in the Mammal Vocal Communication and Cognition Research Group at the University of Sussex who co-led the research, said: “What’s really interesting about this research is that it shows that horses have the ability to read emotions across the species barrier.

"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.”

Research has shown that many species, including horses and dogs, view negative events with their left eye because it is linked to the right half of the brain, which specialises in processing threatening stimuli.

Amy continued: “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 recognise threats in their environment.

"In this context, recognising angry faces may act as a warning system, allowing horses to anticipate negative human behaviour such as rough handling.”

Professor Karen McComb, a co-lead author of the research, said: “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.

The horses were recruited from five riding or livery stables in Sussex and Surrey, between April 2014 and February 2015.

They were shown happy and angry photographs of two unfamiliar male faces.

The experimental tests examined the horses’ spontaneous reactions to the photos, with no prior training, and the researchers were not able to see which photographs they were displaying so they could not inadvertently influence the horses.