Among humans, laughter can signify a lot of different things, from intimacy to discomfort. Among animals, however, laughter usually communicates something along the lines of “this is playtime—I’m not actually going for your throat.”

According to new research from the University of California, Los Angeles, there are likely at least 65 different creatures, including humans, that make these vocalizations. They’re most commonly found in primates, but they have also been noted in distant relatives like birds. It’s not clear whether this is because laughter has arisen several times over the course of evolution or if it’s more widespread and we just haven’t noticed.

Laughter in the library stacks

To reach this number, Sasha Winkler, a PhD student in UCLA’s anthropology department, searched high and low for any mention of animals making noises during play sessions. Some of the articles she found were quite old—one paper on mink dates back to 1931—so she ended up dusting off some aged tomes in the university’s library.