Lips form the muscular opening of the mouth and contain numerous tiny capillaries and nerve endings just beneath the thin membrane surface. The center of the lip is comprised of a muscle termed obicularis oris, which helps to hold food in the mouth and form facial expression such as smiling and frowning. Primates extensively use their lips to communicate a wide variety of emotional states and intentions. Expressions include drawing their lips upward in order to expose teeth, which may communicate a threat or warning. However, the same conduct may express submission in other situations. Monkeys and other members of the primate family are also known to engage in lip smacking behavior. Although they purse or pucker their thin lips, primates do not typically make direct contact with one another.