A snarl's a facial expression, where the upper lip's raised, and the nostrils widen, generally indicating hate, anger or pain. In addition to humans, other mammals including monkeys and dogs snarl, often to warn others of their potential bite. In humans, snarling uses the levator labii superioris alaeque nasi muscle. Snarling's often accompanied by or used synonymously with threatening vocalizations (it may also be used as an onomatopoeia for a threatening noise, as in the 'snarl' of a chainsaw… (More on Snarl)