The mirror test is held up as the gold-standard for testing self-recognition in both humans and nonhumans. Infants do not react to a mirror until they are around 18-months-old, and so it is theorized that infants do not possess self-recognition until that age. Being able to comprehend "self" means being able to recognize others as unique individuals with different thoughts and ideas from our own. It means being able to separate "me" from "you".
Self-awareness was once touted as a uniquely human trait. If other species possessed it, scientists argued, it would be primates, specifically apes like chimpanzees.
So primates have been studied extensively. Chimpanzees and orangutans recognize themselves in mirrors. Three elephants presented with full-size mirrors appear to recognize their reflections and one of the three recognized a novel mark on their bodies. Previous studies with elephants used smaller mirrors and did not show self-recognition. Monkeys have also been heavily studied. For the most part, they ignore mirrors or react as if the reflection is another monkey. But one study, a result of an accident during another study, indicates rhesus monkeys may pass the mirror test. Who to believe? Decades worth of studies indicating rhesus monkeys fail or one study that indicates they pass?
The mirror test has been studied in non-mammals too. Results with European magpies indicate they have self-awareness. At least one pigeon has shown self-awareness as well.
Pigs were studied with a modified mirror test. Pigs, when presented with a mirror, are fascinated by it. They will spend a long period of time staring at themselves, turning their head side to side. The research pigs did not act as if the pig they saw was another pig. But they failed the mark test in which a novel marking is placed on the animal. An animal that passes the mark test will focus heavily on the mark and/or try to remove it. The researchers modified the test by hiding a bowl of food behind a wall. A fan dispersed the scent of the food. The only way the pigs could figure out where the food was located was by using the reflection of the mirror. Seven of the eight pigs did so.
Mirror tests are inherently flawed, though. They are based upon human awareness and understanding of mirrors and our highly attuned and advanced visual system. For example, people who have been blind all their life and regain sight, may not recognize mirrors or themselves initially. This is not because they lack self-awareness but lack the cultural and social experience of the mirror itself.
Which is why it's imperative researchers focus more on the sensory abilities of the animals they are studying (of course, not studying them at all would be fine by me, but that's a different topic). For example, dogs fail mirror tests. But dogs are not truly visual creatures. They have finely tuned motion-sensing abilities, but their visual acuity pales in comparison to other species, like humans and birds. What they lack in visual ability, they more than make up for in olfactory ability. Testing self-awareness in dogs should involve the sense of smell. I would argue that for pigs too - they rely heavily on their sense of smell, not sight, to survive.
Failing a mirror test does not mean an animal or species fails at self-awareness. Certainly not all species are self-aware, but I would wager more mammalian and avian species possess some measure of theory of mind and self-awareness than not.
What do you think about mirror tests and nonhuman self-awareness?