The costs of a big brain outweigh the benefits

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Brains rather than braun may have guided our ancestors out of Africa, but new research suggests primates’ big brains are no longer the assets they once were.

A study published in the journal Evolution reports that larger brains are directly related to an increased risk of extinction in modern primates. Researchers led by Alejandro Gonzalez-Voyer at Doñana Biological Station in Spain, compared published data on 474 species of mammal, with their IUCN Redlist categorisations, to find out how different biological traits influence extinction risk. The team found that larger brains tend to be associated with a longer gestation period, longer weaning period and smaller litter sizes, all of which indirectly increase extinction risk.

Brain size isn’t everything

Big brains might offer intelligence, ingenuity, even language, but they come at a significant cost. Brains take a lot of time to grow and develop, and they use huge amounts of energy. The brain is the most energy-hungry organ in the body, consuming nearly 20% of an adult human’s energy intake but occupying just 2% of their body mass. So large-brained animals tend to live a long time (to reap the benefits of their smarts), have relative few offspring that develop slowly and have a long gestation period (to grow the big brain). All in all, this makes brainy animals more prone to extinction, particularly when times are tough.

But uniquely in primates, the authors found that brain size is directly correlated with extinction risk, even when these factors were taken into account. This direct correlation between brain size and extinction risk was not present when the researchers analysed all mammals together, but only emerged in analyses of primates alone. Although large brains may have previously been an advantage to primates, this study suggests that in present conditions, with pressure from human activities pushing their populations to the brink, big brains are a disadvantage.

Large-brained primates should be a particular focus for future conservation efforts, the authors say.

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