"Tell sheep they all look alike, and they may beg to differ. Like primates, sheep, in studies, recognize different faces (about 50 other sheep and 10 humans) and still know them two years later. They are calmed by familiar faces and can tell both happy and angry expressions (they prefer the former). These are sophisticated abilities in an animal not widely known for smarts, says Babraham Institute's Keith Kendrick. How the sheep brain encodes facial identity and emotional cues may be relevant in some human disorders, he says."
-- "Minds of their Own", National Geographic, March 2008