Researchers recently reported a most interesting finding: there is a good correlation between the number of neurons in the cortex and life span when comparing species. This holds up between classes of species, as well for a number of well known exceptions to other associations between physical characteristics and life span. For example, you might compare these results with the relationship between metabolic rate, mitochondrial composition, and life span that largely holds in mammals, save for bats, which are distinguished by their ability to fly. Flight imposes enormous demands on metabolism, and flying species are as a result biochemically quite different from even near cousin flightless species. Further afield in the taxonomic tree of life, birds tend to have far greater life spans than similarly sized mammals, and once again this is probably because of the demands of flight. Nonetheless, this association with cortical neuron count holds up well for birds and mammals alike. Why does this relationship exist? At this point researchers have nothing but educated guesses. I would imagine that we will hear more on this topic in the years ahead, however.
Whether you're looking at birds or primates or humans, the number of neurons that you find in the cortex of a species predicts around 75 percent of all of the variation in longevity across species. Body size and metabolism, in comparison, to usual standards for comparing animals, only predicted between 20-30 percent of longevity depending on species, and left many inconsistencies, like birds that live ten times longer than mammals of same size. Most importantly, humans were considered to be a "special" evolutionary oddity, with long childhood and postmenopausal periods. But this research finds that is not accurate. Humans take just as long to mature as expected of their number of cortical neurons - and live just as long as expected thereafter.
Researchers examined more than 700 warm-blooded animal species from the AnAge database which collects comprehensive longevity records. They then compared these records with data on the number of neurons in the brains of different species of animals. The researchers found that parrots and songbirds, including corvids, live systematically longer than primates of similar body mass, which in turn live longer than non-primate mammals of similar body mass. Previous studies determining what brains are made of showed that parrots and songbirds have more cortical neurons than similar-sized primates, which have more cortical neurons than any other mammal of comparable body size.
"The more cortical neurons a species has, the longer it lives - doesn't matter if it is a bird, a primate, or some other mammal, how large it is, and how fast it burns energy. It makes sense that the more neurons you have in the cortex, the longer it should take a species to reach that point where it's not only physiologically mature, but also mentally capable of being independent. The delay also gives those species with more cortical neurons more time to learn from experience, as they interact with the environment." What is the link between having more neurons in the cortex and living longer lives? That's the new big question researchers need to tackle.