“Recognition of one’s own reflection,” he wrote, “would seem to require a rather advanced form of intellect… Moreover, insofar as self-recognition of one’s mirror image implies a concept of self, these data would seem to qualify as the first experimental demonstration of a self-concept in a subhuman form.”

Gallup’s mirror test is one of the most famous and controversial techniques in the study of animal intelligence. It has been administered to dozens of species, in much the same way. If an animal inspects the mark on its body (and not on the reflection), and if it interacts with that area more often than usual, it passes the test. At the very least, that’s a sign of self-recognition. It suggests that the animal has a sense of self—a mental representation of its own body or appearance. Perhaps, as Gallup has repeatedly argued, it indicates self-awareness.

Some animal species perform inconsistently on the test. Chimps and orangutans have unambiguously passed, but they don’t always. An Asian elephant named Happy passed in 2006 after staring into a really big mirror, and examining a mark on her head with her trunk—but two other elephants failed.

Killer whales and bottlenose dolphins have allegedly passed; they’re obviously less dextrous than apes or elephants, but they seemed to go to special lengths to inspect marked body parts by twisting, turning, and stretching. Two captive manta rays reportedly did something similar, but as with many of these studies, it’s hard to interpret the creatures’ behavior.

That’s especially true for the many species that have failed the mirror test—which is to say, most of them. Pandas failed. Dogs failed. Sea-lions failed. They’ll ignore, fight, or play with their reflections, but won’t act as if they recognize themselves. Gorillas have been inconsistent. For a few decades, they were thought to be the only great apes that failed the mirror test; a few individuals have since passed, but all had lived in enriched environments with extensive human contact.

Monkeys also had a terrible track record, beginning with Gallup’s seminal study and continuing for several decades after (including a prominent case of fraudulent monkey business). But in 2010, Abigail Rajala from the University of Wisconsin-Madison noticed that some of her lab monkeys, which had been fitted with head implants as part of an unrelated experiment, were checking themselves out in a mirror. Those same monkeys had previously failed a mark test, but now, they were investigating their weird skull adornments. Later, they even started examining unseen parts of their bodies, like their genitals (as in the video below). “We cannot objectively claim that these animals are self-aware, all the pieces are there to suggest that, in some form, they are,” Rajala and her colleagues wrote.

Gallup wasn’t convinced. He argued that even when monkeys fail the mark tests, they can use mirrors to locate hidden objects. They understand reflections, even if they can’t recognize themselves. So given that they had an implant stuck to their heads, they may just have been trying to examine the strange heavy thing that they could feel but not see. As ever, interpreting the mirror test isn’t easy.