Horses are able to discriminate between happy and angry human facial expressions, according to research.
In an experiment using photographs of male human faces, scientists from the University of Sussex showed that domestic horses "responded negatively" to angry expressions.
The scientists say domestication may have enabled horses to adapt to and interpret human behaviour.
The findings are published in the journal Biology Letters.
The team carried out their tests at riding stables - presenting large photographs to a total of 28 horses.
"One person presents the photo while another holds the horse," explained researcher Amy Smith.
"The main result," she explained, "was that they looked [at angry faces] with their left eye."
Mammal brains are wired such that input from the left eye is processed by the right side of the brain.
"The right hemisphere is specialised for processing negative stimuli," explained Miss Smith. "It's really about the partition of energy - not using the whole brain."
The researchers also fitted the horses with heart monitors, which revealed that angry faces caused their heart rates to rise significantly.
Similar results have been reported recently in domestic dogs, raising questions about how living with humans might have influenced animals' abilities.
While horses might have an innate ability to recognise emotions in each other, their domestication by humans could have caused them to adapt that ability to humans.
"It also shows this extra ability of horses," said Miss Smith. "[It shows that] our behaviour around them has an impact."
Prof Debra Archer, an equine veterinary surgeon at the University of Liverpool, said that anyone who was around regularly with horses would recognise that their behaviour had a "marked impact" on the animals.
"But it's fascinating to see this tested scientifically; it's an interesting insight into the relationship between horse and human."
Gillian Higgins, an equine anatomist who runs the organisation Horses Inside Out agreed.
"When I'm relaxed, the horses will be relaxed. And if someone comes in who is stressed and nervous, they will pick up on that," she told BBC News.