Full Article:Animals remember previous facial expressions that specific humans have exhibited

Summary
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. While 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 may 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.
Shortterm 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 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.