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Animals may make a funny-looking face while playing or after investigating a scent. An open mouth during play can act as a social signal; a curled upper lip after sniffing may be the Flehmen response, which helps with scent investigation. Neither expression is a reliable translation of a human emotion. What a face means depends on the animal’s species and what is happening around it.
Why do animals look like they’re smiling when they play?
Some animals hold their mouths open during play. Researchers describe these “play faces” as part of social behavior: in the right setting, the expression may signal that an interaction is play. It can resemble a human smile, but that resemblance does not show that the animal is feeling or expressing happiness in the human sense.
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A 2022 review discusses playful expressions in primates and other animals, including similarities between primate open-mouth play faces and expressions described in several carnivores. Similar movements across species do not, by themselves, establish that every animal uses the expression in precisely the same way. Read the 2022 review in Philosophical Transactions of the Royal Society B.
What a gorilla study found
A 2012 study of 10 captive western lowland gorillas found that the full play face occurred more often during intense play than during gentle play. Intense bouts featuring the full play face also lasted longer than bouts featuring only a relaxed open-mouth play face. These findings describe the gorillas and play bouts studied; they are not a universal rule about animal expressions. See the study in the American Journal of Primatology.
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Why does my cat make a funny face after smelling something?
If a cat or dog curls its upper lip after sniffing, it may be showing the Flehmen response. The expression can look like a grimace, but it is associated with investigating scent—not proof that the animal dislikes what it smells. Zoetis Petcare explains the Flehmen response in dogs and cats.
This scent-related lip curl is different from an open-mouth play face. Look at what happened immediately beforehand: a sniff followed by an upper-lip curl points toward scent investigation, while an open mouth in an ongoing playful interaction may be part of play behavior.
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How to interpret an animal’s expression
A single mouth shape is not a dependable emotional label. Consider the whole interaction rather than reading a face as if it had a direct human translation.
- Facial movement: Is the mouth open, or is the upper lip curled? Describe the movement before assigning it meaning.
- What happened around it: Did the animal just sniff something, or is it interacting with a playmate?
- Other signals: Consider posture, vocalizations, and how the animal and its interaction partner respond.
- Species and evidence: Ask whether the interpretation comes from a direct study of that species or a broader account of animal behavior.
A 2025 review describes AnimalFACS, an adaptation of facial action coding applied to eight animal species, including domestic animals and nonhuman primates. Such coding helps describe facial movements; it does not provide a simple, definitive human-language translation for every expression. Read the 2025 review in Frontiers in Veterinary Science.
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Why cat play faces need careful wording
Do not assume that a cat showing an open mouth during play is making a scientifically established “play face.” A 2021 review of social play in cats noted that the proposed domestic-cat play-face description had not been established through scientific observation in the literature it reviewed. That is a limit on what the review could confirm, not proof that cats never make such a face. See the 2021 review of social play in cats.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When a funny-looking face may need attention
A photo or brief expression cannot establish whether a movement is ordinary behavior or a health concern. The behavior studies discussed here do not provide a cross-species guide to medical causes of unusual facial movements. If a facial change is new or persistent, or appears alongside other concerning changes, consult a veterinarian rather than assuming it is harmless or trying to diagnose it from appearance alone.
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