Scientists have thought that the main determinant of maximal longevity in warm-blooded animals -- which varies from as little as 2 to as many as 211 years -- is a species' metabolic rate, which is inversely related to body size. It follows that at 2 years of life, small animals with high metabolic rates are already old, but large animals with low metabolic rates are still young.

New research published in The Journal of Comparative Neurology up-ends this theory and finds that differences in maximal longevity across bird and mammalian species are better explained by variations in the number of neurons in the brain's cortex than by body size and metabolic rate.

Specifically, it appears that the more cortical neurons a species has, the longer it takes to mature, and the longer it lives thereafter. This may relate to the importance of the cortex not only for cognitive capabilities, but also for adaptability of physiological functions related to heart rate, respiratory rate, and metabolism.

"These new findings imply that everybody's brains accumulate damages at a similar rate, and the longer the brain still has enough neurons that are sufficiently healthy to keep the body functioning as a well-integrated whole, the longer one lives," said author Dr. Suzana Herculano-Houzel, of Vanderbilt University. "This puts the brain squarely in the center of new initiatives to promote healthy aging and wellbeing throughout the lifespan."