Anyone who has spent significant time around horses won't be surprised, but a study published in the journal Biology Letters has found that horses can easily distinguish between happy and angry human facial expressions.
The BBC recently published an article about the study, written by Science Writer Victoria Gill, on its website. Gill spoke with veterinarians and others who said that, while they've known horses have that ability, it's interesting to see that ability tested scientifically.
The study, which involved 28 horses, used large photographs of male human faces. The researchers, from the University of Sussex, noted that horses "responded negatively" to angry faces. The scientists also found that angry faces caused the heart rates of horses to rise significantly.
Here is the BBC story: http://www.bbc.com/news/science-environment-35522233