Although only primates have the facial musculature to be truly expressive, many species use their appearance to convey information. For example, pufferfish expand their bodies to look more threatening, as do dogs and cats when they raise their hackles and puff up their fur. Cats, horses, and others also use their ears to relay information about their intentions. And certainly, many a pet owner has learned to read his dog’s “mood” by the movement of the animal’s tail.