A snarl is a facial expression

Facial expression

A facial expression one or more motions or positions of the muscles in the skin. These movements convey the emotional state of the individual to observers. Facial expressions are a form of nonverbal communication. They are a primary means of conveying social information among humans, but also occur...

, where the upper lip is raised, and the nostrils widen, generally indicating hate, anger

Anger

Anger is an automatic response to ill treatment. It is the way a person indicates he or she will not tolerate certain types of behaviour. It is a feedback mechanism in which an unpleasant stimulus is met with an unpleasant response....

 or pain

Pain

Pain is an unpleasant sensation often caused by intense or damaging stimuli such as stubbing a toe, burning a finger, putting iodine on a cut, and bumping the "funny bone."...

. In addition to human

Human

Humans are the only living species in the Homo genus...

s, other mammal

Mammal

Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s including monkey

Monkey

A monkey is a primate, either an Old World monkey or a New World monkey. There are about 260 known living species of monkey. Many are arboreal, although there are species that live primarily on the ground, such as baboons. Monkeys are generally considered to be intelligent. Unlike apes, monkeys...

s and dog

Dog

The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s snarl, often to warn others of their potential bite. In humans, snarling uses the levator labii superioris alaeque nasi muscle

Levator labii superioris alaeque nasi muscle

The levator labii superioris alaeque nasi muscle is, translated from Latin, the "lifter of the upper lip and of the wing of the nose". It has the longest name of any muscle in an animal...

. Snarling is often accompanied by or used synonymously with threatening vocalizations.

The word "snarl" may also be used as an onomatopoeia for a threatening noise, as in the 'snarl' of a chainsaw. This usage may derive from misunderstanding the common expression describing a dog as "growling and snarling". One undeniable literary use of "snarl" to mean a noise is in The Lord of the Rings

The Lord of the Rings

The Lord of the Rings is a high fantasy epic written by English philologist and University of Oxford professor J. R. R. Tolkien. The story began as a sequel to Tolkien's earlier, less complex children's fantasy novel The Hobbit , but eventually developed into a much larger work. It was written in...

in the encounter with the barrow-wight

Barrow-wight

Barrow-wights are wraith-like creatures in J. R. R. Tolkien's world of Middle-earth, based on the Old Norse Draugr. Barrow refers to the burial mounds they inhabited and wight is a Middle English word for "living being" or "creature", especially "human being"...

: "In the dark there was a snarling noise".