A study at the University of Kansas Medical Center "discovered that cats use nearly 300 distinct facial expressions to communicate with one another":Cats' Facial Expressions
In contrast, humans have 44 different facial expressions, and I was surprised to read that dogs have only 27. Feline expressions of emotion often involve ear movements and whiskers, however, so it's not so strange that they have more "distinct" expressions than we do. I was also surprised that cats' "facial signals" play such a large part in their communications with each other. As this article points out, cats are more social than people usually assume.
Chimpanzees convey a lot of information to each other by subtle facial movements:How Chimps Communicate with a Look
Lisa Parr, director of the Yerkes National Primate Research Center, discusses how small changes in expression can communicate different emotions. Chimps were tested on how well they could distinguish and identify the significance of other chimps' facial expressions. Studying these behaviors in chimpanzees may contribute to better understanding of human nonverbal communication.
Dogs may have developed some types of facial expressions specifically to communicate with us:How Dog Expressions Evolved
Of course, as this article mentions, a lot of canine communication occurs through body language. Maybe that's why they haven't evolved as many variations on facial expressions as we have. Also, scent plays a vital role in dogs' experiences of the world, a sensory dimension we almost entirely lack compared to canines.
Quora features questions about why animals of the same species tend to look so much "alike," while human beings have distinct individual appearances. Some answers explain, in addition to the human-centered bias that causes us to make finer distinctions among members of our own species, that many animals have less variation in facial appearance than we do because they rely on other senses such as smell to recognize each other.
If intelligent Martians existed, we might think they all look alike, as the narrator of Heinlein's DOUBLE STAR does at the beginning of the novel. On the other hand, the Martians would probably have trouble telling Earth people apart.
Margaret L. Carter
Please explore love among the monsters at Carter's Crypt.