Yet a human child learns to use a mirror almost intuitively, about the same time as certain other key developmental milestones are reached. Our closest relatives, the common chimp and the bonobo, also pick up the idea of a mirror almost as soon as they are exposed to it, as do most other great apes. (The gorilla is unusual in this regard, but for cultural reasons rather than cognitive -- in gorilla society eye contact is always an aggressive act. Polite gorillas avert their eyes, and thus never have an opportunity to realize that the image in a mirror is themselves. However, captive gorillas who have been socialized to human customs regarding eye contact learn to use a mirror just as easily as any other great ape).
As a result, it is generally taken to mean that being able to recognize oneself in a mirror is an indicator of a sense of self as separate from the rest of the universe. Our closest evolutionary kin share this skill, but more distantly related primates such as Old World and New World monkeys, lack this ability. (Although it also appears to have evolved separately in several other clades, including dolphins, elephants and ravens).
Furthermore, it is generally assumed that mirror self-recognition is a binary thing. Either a species has it, or it doesn't.
But a recent experiment with rhesus macaques is upending those assumptions. A number of these monkeys were put through a series of training exercises involving a mirror on which the experimenters painted laser targeting dots. After learning several skills related to mirror use, they appear to be able to recognize themselves in a mirror -- which raises serious questions about just what is going on inside their brains, and how this training technique has changed it. Did it unlock a behavioral potential that was latent all along, but just never activated in the environment of normal rhesus macaque society? Or did the experimenters somehow cause something else to develop? And does either explanation indicate the presence or development of a sense of self comparable to a human's?