The image of a monkey appearing to grin is a common fixture in popular culture, yet this human interpretation is misleading. When a primate pulls back its lips to expose its teeth in what looks like a grin, it is actually communicating a state far removed from joy or amusement. This facial expression, which scientists call the Silent Bared-Teeth (SBT) display, is a complex social signal that humans consistently misinterpret through a lens of anthropomorphism. Understanding the true meaning of this display requires examining the underlying social and emotional context in which it occurs. This reveals a significant difference in the function of this expression across the primate family tree.
The Critical Difference Between the Human Smile and Primate Display
The human smile primarily functions as a welcoming, affiliative signal, communicating pleasure, delight, or non-aggression. This expression is anatomically defined by the contraction of the zygomatic major muscle, which pulls the corners of the mouth upward. A genuine, spontaneous human smile, known as the Duchenne smile, also involves the contraction of the orbicularis oculi muscles around the eyes, signaling authentic positive emotion. The smile is a signal of relaxation and comfort within a social setting.
In contrast, the primate bared-teeth display is a signal of tension, stress, or fear, functioning as a defensive or appeasement mechanism. While visually similar to a human grin, the underlying emotional state is typically the opposite of happiness. The visual resemblance is superficial, creating confusion for human observers who project their own emotional meaning onto the primate’s face. This display is homologous to the human smile, meaning they share an evolutionary ancestor, but their modern functions have diverged significantly.
Decoding the Bared-Teeth Grimace
The Silent Bared-Teeth (SBT) display is widely considered a signal of submission and appeasement across many non-human primate species. The expression is used by a subordinate individual toward a dominant one to signal non-hostile intent and defuse potential conflict. In species with strict dominance hierarchies, such as rhesus macaques, the display actively communicates that the lower-ranking animal recognizes the other’s superior status. This act serves to deflect potential aggression from the more powerful group member, allowing the subordinate to remain in the area without provoking a fight.
The grimace is often seen in contexts where the animal is cornered, anticipating harm, or bracing for a fight it cannot avoid. It is a defensive signal that communicates vulnerability and a lack of intent to challenge the other individual. The baring of the teeth in this context is not a threat, unlike other primate expressions such as the open-mouth stare or a deep yawn, which can signal impending aggression. The grimace is specifically a non-aggressive signal.
The exact function of the SBT display can be nuanced, varying based on the social structure of the species. In more tolerant species with relaxed dominance hierarchies, like certain groups of chimpanzees and crested macaques, the bared-teeth display can be used in a wider variety of social contexts. In these cases, the grimace may signal reassurance or be used during affiliative interactions, such as grooming or reconciliation. Even in these more tolerant species, the expression remains fundamentally rooted in communicating a lack of intent to harm.
The Evolutionary Divergence of Facial Expressions
The scientific hypothesis for the origin of the human smile suggests it evolved directly from the primal, defensive bared-teeth grimace of our ancient primate ancestors. This evolutionary path involved a gradual re-purposing of a signal that once communicated fear and submission. The original function was to communicate a defensive lack of threat, essentially saying, “I am harmless.”
Over time, social pressures within the evolving hominid lineage led to the process of “ritualization,” where the signal was adopted for new purposes. As social groups became more complex and cooperation became more important for survival, this defensive grimace was gradually transformed into an affiliative greeting. The signal shifted from communicating “I will not attack you” to a positive indicator of “I welcome interaction with you.”
In higher primates, like chimpanzees, the bared-teeth display is sometimes observed being flashed between equals in a way that suggests affection or a positive greeting after a separation. This transitional use in apes provides a behavioral “missing link,” showing how the grimace began to move beyond its role as a purely subordinate signal. This shift established the foundation for the human smile, which is now a primary tool for positive social bonding and affiliation.