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Big Brains Are Rare: Here’s Why

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The evolutionary ties around large brains have been subjected to scientific debate. However, recent study suggests that there are some associated evolutionary flaws to the mechanisms of physiologically supporting large brains. The research underlines that the high levels of energy expenditure could be responsible for the rarity of big brained creatures.

Much of the research on the consequences of having a bigger brain revolves around the cognitive capacities of people, who are believed to be highly superior to any other living beings. However, research published, Dec. 21, in the journal Proceedings of the Royal Society B suggests this comes at a cost.

Large Brains, Bigger Costs

There are substantial differences among species when it comes to the size of their brains, and there have been various hypotheses trying to shed light on the reasons behind the brain evolution. There has been scientific consensus on the fact that the brain is one of the most energetically expensive organ among vertebrates. But from there on, hypotheses started to differ.

For instance, a theory called the expensive tissue hypothesis (ETH) suggested that reducing the size of another organ which is (almost) equally expensive in terms of the body energy it consumes should compensate for the energy the brains use in order to work properly.

"We can think of tons of benefits to a larger brain, but the other side of that is brain tissue is incredibly 'expensive' and increasing brain size comes at a heavy cost," noted Kimberley V. Sukhum, a graduate student in biology in Arts & Sciences at Washington University in St. Louis.

Prior to this research, two hypotheses concerning the dynamics of energy expenditure have been tested. The first suggests a decrease in other energy demands, and the second involves an increase in the overall energy consumption.

Potential Metabolic Implications

The current research studied mormyrids, which are very brainy fish, as their brains account for 3 percent of their total body size, which is comparable to humans. Our brains have approximately 2 to 2.5 percent of our body size. The study analyzed 30 out of more than 200 mormyrid species, and observed that their brains differ in size.

Further, the team used the consumption of oxygen paired to the capacity to tolerate hypoxia in order to examine the energy use and demand. As a result, they saw that larger brains require a higher quantity of energy, translated in oxygen intake, while fish with smaller brains required less energy.

The results of this research are partially complementary to another study, published in May 2016, in the journal Nature. That research suggested that large brains are the result of higher metabolism.

"This research demonstrates that, pound-for-pound, humans use substantially more energy than do other apes. That may seem like a fairly simple conclusion, but precise data on energy use in apes had not previously been available. It has been widely assumed that humans would have had to cut corners in order to afford large brains, but that does not seem to have been necessary," noted one of the study authors.

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