Did primates opt for brains over brawn? Primate species with larger brains have reduced muscle mass, supporting a controversial theory that energy-hungry brains grew larger by stealing resources from other bodily tissues.
Brains are expensive to run. Some estimates suggest . As such, many researchers have wondered how primates – particularly humans – find the energy to keep their unusually large brains running.
An idea emerged in the mid-1990s: perhaps primates support their large brains by making energy savings elsewhere. In particular, the idea is that primate guts – another “expensive” tissue to run – evolved to be smaller as brains evolved to be larger.
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But it’s not yet clear that the is correct. For instance, in 2011 at the University of Zurich, Switzerland and her colleagues published questioning whether animals with big brains really have small guts.
Costly brains
Perhaps that’s because there is another piece to the puzzle, says at the University of North Texas in Fort Worth. She says muscle – particularly the slow twitch muscle used in sustained activities like long-distance running – is also an expensive tissue. She decided to explore whether primates with big brains have less muscle mass.
Muchlinski and her colleagues dissected the cadavers of ten primate species that had been held in captivity and died of natural causes. The species include the tiny , which weighs about 130 grams, and the , which weighs several kilograms. The researchers discovered that primates with larger brain volumes had less muscle mass, relative to their overall body mass.
Muchlinski “wouldn’t disagree” with the idea that primates have sacrificed strength for larger brains. “Primates, when compared to other animals, have very little muscle,” she says. “What I am seeing now is larger-brained primates have less muscle.”
That said, Muchlinski says these results are preliminary. A more complete dataset containing information from many more primate species should be published in the next year or so. “We have chimps and gorillas, capuchins and orangs – as well as human samples.”
“I think it may be possible that primates, as a group, are adapted to spend relatively little energy on locomotion,” says Isler. However, she is reserving judgement until Muchlinski gets more data.
Anatomical Record
