[Can grief] extend to consciousness of one's own mortality? The Oxford mathematician Marcus du Sautoy cites the mirror-recognition research of Gordon Gallup, who first showed elegantly -- by surreptitiously applying a temporary red dot to the forehead and observing the reaction -- that only a few species realize they are seeing themselves in a mirror (even brainy parrots chatter with a perceived playmate) rather than another creature:
It is striking that chimpanzees start to fail the test once they reach 30 years old despite having some 10 or 15 years left to live. The reason is that self-awareness comes at a cost. Consciousness allows the brain to take part in mental time travel. You can think of yourself in the past and even project yourself into the future. And that is why Gallup believes that in later life chimpanzees prefer to lose their ability to conceive of themselves. "The price you pay for being aware of your own existence is having to confront the inevitability of your own individual demise.
"Death awareness is the price we pay for self awareness."