Work at Cambridge trained sheep to select a familiar face from a pair of images by approaching the correct screen for a food reward. The animals learned to identify photographs of specific human faces at rates well above chance, and continued to perform when the familiar face was presented tilted rather than frontal, a test of genuine recognition rather than image matching.
Earlier work found that sheep can learn and remember around fifty individual sheep faces over periods of two years or more, with dedicated brain regions responding selectively to faces. Sheep also show measurable stress responses when isolated, which are reduced by the sight of a familiar flockmate's face.
The research was not undertaken to flatter sheep. Facial recognition circuits and their degradation are of interest for modelling Huntington's disease, for which sheep are used because their brains are large and long-lived enough to track change. The everyday consequence is that a flock is not an undifferentiated mass; individual animals track who is who.