The Number of Neurons in the Cortex is Strongly Associated with Species Longevity

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.



Training a neural network takes time, and the neurons are very expensive gluttons. So a very short lived species will have much lower utility of a bigger brain, ceteris paribus. And since big brains are very expensive to maintain and develop, there is a huge selection pressure to shrink them. If they're is no marginal benefit, an individual with a smaller brain can have more resources allocated to muscle and mobility.

Another curious thing about the birds brains is that per size they are much more efficient then mammals, and us humans. If you could efficiently scale bird brain to human size and put it in a human body that brain might be much smarter then the two-three pounds we have in our skulls

Posted by: Cuberat at November 5th, 2018 8:02 AM

Hi Cuberat ! Just a 2 cents.

Excellent observation and explanation. I think there are shades of grey and outliers but overall this trend seems pretty solid over all animalia. If I were to compare a 211-year old bowhead whale vs a
111-year old blue ara amazon parrot, I would see 1 century difference between the two's MLSP, but the body is so different (size/mass can be uncoupled from metabolistic speed); but in a sense their developmental growth/speed of growth is about the same or the bowhead whale still developps two times slower than a parrot; and because of such, it reaches immense size. Or, it has better genetic detoxification or stem cell replacement. Humans, bowhead whales and parrots share this slow growth, which allows, as said in study, accumulation of cortical neuron/filling up the brain over long pubertal period. This fits perfectly with neoteny/neotenous features of all these animals. This would also tie in with cell dynamics/cell growth and stem cell replacement;

Neuregulin-1 (NRG-1) was found to correlate to MLSP in mammals, more neurons, would mean more brain BDNF and brain neutrophic factors that create neurogenesis, and upregulate NRG-1; NRG-1, under epigenetic control, would alter redox detoxification, metabolism and growth dynamic.

Yes, I believe that too, that birds and bats have this improved mitochondrial function and improved redox detoxification (one study foudn that birds had higher uric acid/bilirubin/biliverdine production, this is very powerful first line antioxidant that would curb oxidative stress during their energy requiring flights. Plus, they showed that most birds are just like humans and have 'torpor' during sleep, where the body induces bradycardia to slow down the heart and reduce the metabolism (controlled in part by pineal/thyroid genes whom control body temperature/there is a slight reduction in body temperature with sleep, especially at the terminaisons/hands or feets where there may be less warm blood flow reaching the member-ends creating 'cold feets' from sleep), in sleep; this reduces metabolistic burden during sleep and slows aging (for example, the hummingbird can in stationary flight alomst levitating, this extremely fast wings movement, is made possible by their heart beating at up to 1500 beats a minute in flight...yet they can live longer than a mouse (having 700 beats or so minute avg); that is because hummingbirds go into torpor during their sleep and heart rhythm goes all the way down to 100 or less beats; thus, they are not 1500 all the time, just a small time during quick flights from one point to another close point/they are not the type of bird to migrate the entire planet, they would die in flight from exhaustion).

In all these animals, I can bet my money's worth, that this neuronal increase is orchestrated by epigenetic means (brain IGF/NRG/BDNF/etc..all under epigenetic control) and that for example, a 500 year old quahog clam may not have as much neurons as a human but still lives 5-times longer, because of epigenetic signature preserving this animal into neoteny and ultra slow growth/long pubertal entry.
Likewise for Greenland sharks, I wonder about their brains, I have doubts that they are as brain cortical neuron rich as a Killer Orca Whale or a Dolphin, which are extremely intelligent marine creatures yet they never reach 500 years old like a Greenland Shark either. Again, for elephants, it seems this bigger brain/more cortical neurons associated with longer lifespan is a solid trend for mammals at least; and it makes sense, if you look at centenarians brains, they keep their brains healthy and that of a 'young elder adult' (like a centenarian brain equals a 60-year old brain or so), and the pruning of the cortex and everyother part is much reduced in their brains (so is obviously disease like Alzheimer's and parkinson's, they have less tau load and amyloid plaques, and just a bigger younger brain that preserved the total mass of cortical neurons since birth and have production of new ones as said via BDNF/NRG, BDNF was higher in sane non-senile centenarians, while demential ones showed more brain degeneration and neuron death). But then again, it's strange because there would be demential and non-demential Centenarians, meaning the brain aspect was important but it is certainly not all needed to reach 100 years, if there are demential ones with less neurons. In other words, you Can function with Less functions and still make it to a 100 despite so; just more damaged. This means one or two things;
epigenetic clock is very tricky and thus even supercedes these discrepancies and shows us that intrinsic aging by the methylation clock is whole other thing (and having more brain cortical neurons Will happen if you do live longer, the clock will take care of that maintenance, in concert with the brain's Circadian Clock; which is very much epigenetically controlled); secondly, it is highly likely that people whom all reach 100 years old all share a very important feature, which is that the maintained a rather brain-diseases free life for their entire lifespan or near-entire one Prior; and why there might be demented ones in the last 10 or so years of their life, while others might not become demented by simply having preserved their brains better (in the last 10 years or so; but for the Entire Life, but are about equal and both benefited; itS' just that one became 'sicker' quicker at the end). So, to resume, you need to be in stellar brain shape to reach 100 years old and not lose your cortical neurons.
They are not 100% the reason but definitely a large reason why we live this long as the study showed correlation with longevity. The brain is the center/Control Tower, as such it commands the entire body, if it shrivels to quick the rest of the organs suffer extremely and die; there is constant communication between all our organs; brain neurons direct this and operate so many mechanistic pathways in the rest of body (it'S why studies in flies that were specifically targeted to neurons saw lifespan extension - only, if targeted to brain neurons, not rest of organs....). Just a 2 cents.

Posted by: CANanonymity at November 5th, 2018 12:55 PM

A very interesting and highly intriguing observational study. To me, such a widely held correlation is consistent with, if not evidence of, the notion that aging is an adaptation.

Posted by: Anthonie Muller at November 8th, 2018 4:28 AM

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