Primates managed to keep most of their neurons the same size

Friday, August 10th, 2018

Eugène Dubois gathered the brain and body weights of several dozen animal species and calculated the mathematical rate at which brain size expands relative to body size:

Dubois reasoned that as body size increases, the brain must expand for reasons of neural housekeeping: Bigger animals should require more neurons just to keep up with the mounting chores of running a larger body. This increase in brain size would add nothing to intelligence, he believed. After all, a cow has a brain at least 200 times larger than a rat, but it doesn’t seem any smarter. But deviations from that mathematical line, Dubois thought, would reflect an animal’s intelligence. Species with bigger-than-predicted brains would be smarter than average, while those with smaller-than-predicted brains would be dumber. Dubois’s calculations suggested that his Java Man was indeed a smart cookie, with a relative brain size — and intelligence — that fell somewhere between modern humans and chimpanzees.

Dubois’s formula was later revised by other scientists, but his general approach, which came to be known as “allometric scaling,” persisted. More modern estimates have suggested that the mammalian brain mass increases by an exponent of two-thirds compared to body mass. So a dachshund, weighing roughly 27 times more than a squirrel, should have a brain about 9 times bigger — and in fact, it does. This concept of allometric scaling came to permeate the discussion of how brains relate to intelligence for the next hundred years.

Seeing this uniform relationship between body and brain mass, scientists developed a new measure called encephalization quotient (EQ). EQ is the ratio of a species’s actual brain mass to its predicted brain mass. It became a widely used shorthand for intelligence. As expected, humans led the pack with an EQ of 7.4 to 7.8, followed by other high achievers such as dolphins (about 5), chimpanzees (2.2 to 2.5), and squirrel monkeys (roughly 2.3). Dogs and cats fell in the middle of the pack, with EQs of around 1.0 to 1.2, while rats, rabbits, and oxen brought up the rear, with values of 0.4 to 0.5. This way of thinking about brains and intelligence has been “very, very dominant” for decades, says Evan MacLean, an evolutionary anthropologist at the University of Arizona in Tucson. “It’s sort of a fundamental insight.”

Comparative EQ

A century later, Suzana Herculano-Houzel found a (gruesome) way to count neurons efficiently:

An entire rat brain contains about 200 million nerve cells.

She looked at brains from five other rodents, from the 40-gram mouse to the 48-kilogram capybara (the largest rodent in the world, native to Herculano-Houzel’s home country of Brazil). Her results revealed that as brains get larger and heavier from one species of rodent to another, the number of neurons grows more slowly than the mass of the brain itself: A capybara’s brain is 190 times larger than a mouse’s, but it has only 22 times as many neurons.

Then in 2006, Herculano-Houzel got her hands on the brains of six primate species during a visit with Jon Kaas, a brain scientist at Vanderbilt University in Nashville, Tennessee. And this is where things got even more interesting.


As the primate brain expands from one species to another, the number of neurons rises quickly enough to keep pace with the growing brain size. This means that the neurons aren’t ballooning in size and taking up more space, as they do in rodents. Instead, they stay compact. An owl monkey, with a brain twice as large as a marmoset, actually has twice as many neurons — whereas doubling the size of a rodent brain often yields only 20 to 30 percent more neurons. And a macaque monkey, with a brain 11 times larger than a marmoset, has 10 times as many nerve cells.


The usual curse of an ever-expanding neuron size may stem from the basic fact that brains function as networks in which individual neurons send signals to one another. As brains get bigger, each nerve cell must stay connected with more and more other neurons. And in bigger brains, those other neurons are located farther and farther away.


A large rodent called an agouti has eight times as many cortical nerve cells as a mouse, while its white matter takes up an astonishing 77 times as much space. But a capuchin monkey, with eight times as many cortical neurons as a small primate called a galago, has only 11 times as much white matter.


Kaas thinks that primates managed to keep most of their neurons the same size by shifting the burden of long-distance communication onto a small subset of nerve cells. He points to microscopic studies showing that perhaps 1 percent of neurons do expand in big-brained primates: These are the neurons that gather information from huge numbers of nearby cells and send it to other neurons that are far away. Some of the axons that make these long-distance connections also get thicker; this allows time-sensitive information, such as a visual image of a rapidly moving predator, or prey, to reach its destination without delay. But less-urgent information — that is, most of it — is sent through slower, skinnier axons. So in primates, the average thickness of axons doesn’t increase, and less white matter is needed.

This pattern of keeping most connections local, and having only a few cells transmit information long-distance, had huge consequences for primate evolution. It didn’t merely allow primate brains to squeeze in more neurons. Kaas thinks that it also had a more profound effect: It actually changed how the brain does its work. Since most cells communicated only with nearby partners, these groups of neurons became cloistered into local neighborhoods. Neurons in each neighborhood worked on a specific task — and only the end result of that work was transmitted to other areas far away. In other words, the primate brain became more compartmentalized. And as these local areas increased in number, this organizational change allowed primates to evolve more and more cognitive abilities.

All mammal brains are divided into compartments, called “cortical areas,” that each contain a few million neurons. And each cortical area handles a specialized task: The visual system, for example, includes different areas for spotting the simple edges of shapes and for recognizing objects. Rodent brains don’t seem to become more compartmentalized as they get larger, says Kaas. Every rodent from the bite-sized mouse to the Doberman-sized capybara has about the same number of cortical areas — roughly 40. But primate brains are different. Small primates, such as galagos, have around 100 areas; marmosets have about 170, macaques about 270 — and humans around 360.

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