Researchers at Vanderbilt University studied lifespan among different animals, and have found that an organism’s brain seems to be a greater influence on their age of sexual maturity and their lifespan than their body does. It appears that it was not animals with bigger bodies or slower rates of metabolism that were living longer lives. Animals that carry more neurons in their cerebral cortex live longer, regardless of body size.
"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," said study author associate professor of psychology and biological sciences Suzana Herculano-Houzel.
Body size or metabolic rate is often used as an easy way to compare animals. It only accounts for 20 to 30 percent of longevity, however, and is rife with inconsistencies. Birds, for example, live ten times longer than mammals of a similar size.
Humans are another anomaly, with lengthy childhoods and typically, many years in a woman's life after menopause. This report has placed humans neatly into this new framework; humans require as much time to mature as would be expected from the number of cortical neurons we carry, and we live for as long as would be assumed on the same basis.
Over 700 species of warm-blooded animals were assessed for this work, using the AnAge database and data on neuronal number in various species’ brains. Herculano-Houzel determined that parrots and songbirds had longer lives than primates with comparable body mass. The primates have longer lifespans than non-primate mammals with similar body mass.
"Likewise, for similar specific basal metabolic rates, parrots and songbirds live longer and take longer to reach sexual maturity than many mammalian species, especially non-primates," said Herculano-Houzel.
This pattern confirmed her previous work. Herculano-Houzel developed a way to accurately measure the number of neurons in a brain. She generates a "brain soup" by breaking down brain tissue, fluorescently tags the nuclei, and counts them.
She found that longevity gets longer across warm-blooded animals as the number of cerebral cortex neurons increases. "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,” explained Herculano-Houzel.
"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," says Herculano-Houzel. "The delay also gives those species with more cortical neurons more time to learn from experience, as they interact with the environment."
Species with more cortical neurons and longer lives also get a greater chance to pass information down to the next generation. "Which means that grandma is still fundamental in the lives of those with plenty of cortical neurons; she's just probably not the reason why our species is long-lived," added Herculano-Houzel.
It remains to be seen why having more cortical neurons is linked to a longer life. Herculano-Houzel suggested that researchers should now investigate the connection. Taking care of those neurons, she added, is a good way to help ensure we’ll live a longer life.
"The data suggest that warm-blooded species accumulate damages at the same rate as they age. But what curtails life are damages to the cerebral cortex, not the rest of the body; the more cortical neurons you have, the longer you will still have enough to keep your body functional,” said Herculano-Houzel.