The human smile is a complex facial expression, typically understood as a voluntary social signal tied to positive internal emotions like pleasure, joy, or amusement. This expression involves the contraction of the zygomaticus major muscle, which pulls the corners of the mouth upward. When combined with the orbicularis oculi, it creates the Duchenne smile, the perceived authentic marker of happiness. When people observe a similar upturned mouth or bared teeth in animals, they frequently engage in anthropomorphism, projecting this human emotional meaning onto the animal’s behavior. The central question is whether animal facial displays truly share this neurological link to subjective well-being or if they serve an entirely different, survival-based function.
The Human Smile vs. Animal Displays
The difference between human and animal facial expressions lies fundamentally in their purpose and control. The human smile can be an involuntary reflection of positive emotion or a deliberate signal used for social communication. Animal facial displays, by contrast, are generally more instinctual, involuntary, and less directly connected to a subjective sense of joy. While animals possess the necessary musculature for facial movement, the sophisticated neurological pathways linking these movements to complex positive emotions are largely absent.
Most animal expressions that appear “happy” are actually fixed action patterns tied to survival, social hierarchy, or a physiological need. These displays primarily signal intent, such as a warning, submission, or a desire to avoid conflict. Interpreting an animal’s bared teeth or retracted lips requires understanding the specific evolutionary context of the species, which frames the expression as a functional tool for social interaction, not a reflection of internal contentment.
Primate Expressions of Appeasement
In non-human primates, the display most often mistaken for a smile is the Silent Bared Teeth Display (SBDT), frequently referred to as the “fear grin.” This expression involves the lips retracting horizontally, exposing the teeth, and functions primarily as a signal of submission, fear, or anxiety. The SBDT is typically directed by a lower-ranking individual toward a dominant one to communicate a peaceful, non-threatening intention.
This appeasement gesture is a tool for de-escalating potential conflict and maintaining the established social hierarchy. The signal effectively communicates “I mean no harm” or “I submit,” allowing the subordinate primate to avoid aggression. While some research suggests the bared teeth display may occur in more relaxed, affiliative contexts, its most common function remains rooted in managing power asymmetry and stress. The primate SBDT is considered the evolutionary precursor to the human smile, but its original meaning was driven by tension and subordination, not happiness.
Domestic Animals and the Submissive Grin
The perception of smiling is particularly common in domestic dogs, where two distinct behaviors are frequently mislabeled. One is simple panting, which is a thermoregulatory mechanism where the dog opens its mouth and pulls back its lips to evaporate moisture and cool down. A relaxed, panting dog may appear to have a wide, happy grin, but the expression is a physiological necessity, not an emotional one.
The second common display is the “Submissive Grin,” also known as the canine grimace, which is a clear appeasement behavior. In this gesture, the dog wrinkles its nose and raises its upper lip to flash its front teeth, often accompanied by a lowered posture, soft eyes, and a sweeping tail wag. This action signals deference and a desire to avert conflict, especially toward dominant humans or other dogs. Some evidence suggests dogs have learned to use this expression because it often elicits a positive, rewarding reaction from the owner. Cats rely on far more subtle cues like slow blinking or ear positioning for communication, rarely displaying a facial expression that a human would interpret as a smile.
Anatomical Illusions in Other Species
Many animals appear to be perpetually smiling due to their fixed facial anatomy, which creates a permanent optical illusion. Dolphins are a prime example; their jaw structure and lack of facial muscles mean they are physically incapable of altering their expression. The natural upward curve of their mouthline, a hydrodynamic adaptation for moving efficiently through water, gives them a characteristic, unwavering “smile” regardless of their emotional state.
Another common example of misinterpretation is the flehmen response observed in horses, which looks like a dramatic grin. This behavior, where the horse curls its upper lip back and raises its head, is actually a physiological mechanism to draw pheromones and other scents into the vomeronasal organ (Jacobson’s organ). This specialized chemosensory organ allows the animal to analyze chemical signals, particularly those related to reproductive status, making the “horse laugh” a purely functional tool for chemical communication. The Australian quokka is another animal whose resting facial muscle structure gives it the appearance of a cheerful grin purely by anatomical chance.