Since the 1970s, scientists have put a range of species in front of mirrors and recorded their reactions. They still can’t agree upon what the results mean.

The standard test of self-recognition involves painting a mark on an animal somewhere visible only in a mirror, such as its forehead. If the animal uses the mirror to investigate the mark on its body, it suggests it knows the reflection is of itself and not some other animal.

The debate is over what mirror self-recognition might imply about an animal’s cognitive abilities. Some scientists have linked success on the mirror test to self-awareness, while others argue that it might not require a complex sense of self at all.

The list of animals that have passed the mirror test is small but confusing. Great apes tend to pass, and at least a few individual dolphins and elephants seem to recognize their reflections. But in addition to these big-brained species, cleaner fish, roosters, and manta rays show some evidence of mirror self-recognition.

One problem with the mirror test is that it is biased towards animals that are primarily visual. Dogs, for instance, fail the standard mirror test. But when presented with an olfactory self-recognition task, they pass (investigating their own urine odor more when it is chemically “marked” with another scent).

Scientists at Wilfrid Laurier University in Ontario, Canada, adapted this paradigm to test snakes, another animal that interacts with the........

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