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If your animals are anthropomorphized, you can come pretty close to describing human-level expressions, depending on the animal in question. If your animals are not sentient, you have to study the...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/25768 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If your animals are anthropomorphized, you can come pretty close to describing human-level expressions, depending on the animal in question. If your animals are not sentient, you have to study the animal in question to be able to describe its body language. I've had cats my entire life, so I can describe feline facial expressions (eye movements, pupil widening, sniffing, flehmening, ear movements), vocalizations (meows, chattering, purring, growling), and other body language (posture, stalking, stillness, creeping, butt-waggling, tail lashing, happy tail, kneading, wrestling, swatting, use of claws) in great detail to show what a particular cat is thinking or feeling.