The human smile is a complex expression driven by the contraction of specific muscles, primarily the zygomaticus major, which pulls the corners of the mouth upward. When coupled with the muscles around the eyes, it produces a genuine smile, universally associated with pleasure. Projecting this human emotional framework onto other species is called anthropomorphism. While many animals appear to grin, few possess the emotional and neurological wiring that connects this facial movement to the feeling of joy.
Anatomy: Why Some Faces Always Look Happy
The illusion of a perpetual smile in the animal kingdom is often a simple consequence of comparative anatomy. Many animals lack the intricate network of facial muscles that allow for a wide range of temporary expressions. A fixed, upturned mouth line is frequently the result of a species’ specific skull and jaw structure.
The shape of the bone, the placement of fat pads, and the absence of certain depressor muscles contribute to an upward curve at the mouth’s corners. This resting facial posture is incidental to the animal’s mood, serving a functional rather than an emotional purpose.
Animals With Permanent Resting “Smiles”
The Quokka (Setonix brachyurus) is perhaps the most famous example due to its perpetually pleasant expression. This small marsupial’s friendly look is a direct result of its unique jawline and muscle structure around its mouth. The upturned corners, combined with its habit of approaching humans, create popular photo opportunities. The open-mouthed look may also serve the physiological function of making panting more efficient for cooling in hot weather.
Dolphins also appear to wear a permanent grin, but this is a fixed anatomical design, not an emotional display, as they lack the necessary facial musculature to manipulate their mouth into an expression of mood. The rigid, upturned curve of their jaw is instead an adaptation related to hydrodynamics, helping water flow smoothly over their bodies as they swim. While some research suggests bottlenose dolphins may use an open-mouth expression during play to communicate, their iconic resting “smile” remains a purely structural feature.
Behavioral Expressions Mistaken for Happiness
Certain temporary expressions in animals can look strikingly similar to a human smile but serve an entirely different communicative purpose. The classic example is the primate “fear grimace” or “bared-teeth display” seen in chimpanzees and other apes. This expression involves the retraction of the lips to show the teeth, signaling fear, submission, or appeasement toward a dominant individual. It is a nervous gesture meant to de-escalate tension, which is the opposite of human joy.
The “submissive grin” in dogs is a common appeasement behavior, where the dog pulls back its lips to show its front teeth. This action is a conflict-avoidance strategy, communicating to humans or other dogs that the animal is not a threat. When accompanied by a loose body posture, a low tail wag, and soft eyes, the grin is a non-aggressive signal, but it is not the equivalent of happiness.
Another expression that mimics a smile or sneer is the Flehmen response, seen in ungulates like horses and felids like big cats. This behavior involves the animal curling back its upper lip, often holding the posture for a few seconds. This action draws air into the vomeronasal organ, a specialized sensory organ, to detect non-volatile chemical signals like pheromones. The Flehmen response is purely a sensory intake mechanism, allowing the animal to analyze chemical cues in its environment.