Data for research article: Horses form lasting impressions of people based on facial expressions of emotion during a single encounter.

Abstract from article
For humans, facial expressions are important social
signals, and how we perceive specific individuals
may be influenced by subtle emotional cues that
they have given us in past encounters. A wide range
of animal species are also capable of discriminating
the emotions of others through facial expressions
[1–5], and it is clear that remembering emotional experiences with specific individuals could have clear
benefits for social bonding and aggression avoidance
when these individuals are encountered again.
Although there is evidence that non-human animals
are capable of remembering the identity of individuals
who have directly harmed them [6, 7], it is not
known whether animals can form lasting memories
of specific individuals simply by observing subtle
emotional expressions that they exhibit on their
faces. Here we conducted controlled experiments
in which domestic horses were presented with a
photograph of an angry or happy human face and
several hours later saw the person who had given
the expression in a neutral state. Short-term exposure
to the facial expression was enough to generate
clear differences in subsequent responses to that
individual (but not to a different mismatched person),
consistent with the past angry expression having
been perceived negatively and the happy expression
positively. Both humans were blind to the photograph
that the horses had seen. Our results provide
clear evidence that some non-human animals can
effectively eavesdrop on the emotional state cues
that humans reveal on a moment-to-moment basis,
using their memory of these to guide future interactions
with particular individuals.