51����

In primates such as humans, living in cooperative societies usually means having bigger brains — with brainpower needed to navigate complex social situations.

But surprisingly, in birds the opposite may be true. Group-living woodpecker species have been found to have smaller brains than solitary ones.

Cooperative societies might in fact enable birds to jettison all that brainpower otherwise needed on their own to constantly out-think, outfox and outcompete wily rivals, say researchers. Socialism in birds may therefore mean the individuals can afford to get dumber.

The results are based on a comparison of brain sizes in 61 woodpecker species.

The eight group-living species identified typically had brains that were roughly 30 per cent smaller than solitary and pair-living ones. “It’s a pretty big effect,” says lead researcher at the University of St Andrews in the UK.

Byrne’s explanation is that a solitary life is more taxing on the woodpecker brain than for those in cooperative groups, in which a kind of group-wide “social brain” takes the strain off individuals when a challenge arises.

Group-living acorn woodpeckers in North America, for example, are well known for creating collective “granaries” of acorns by jamming them into crevices accessible to the whole group during hard times.

“Red-cockaded woodpeckers cooperate to defend a tree from snakes by getting sap to run down all over the trunk,” says Byrne. He adds that other group-dwelling woodpecker species breed cooperatively, nesting near each other and jointly raising the alarm to fend off predators by mobbing them, all of which reduces the burden on individual birds.

Armed with such collective problem-solving rather than constant rivalry within a group, these species could afford for their brains either to shrink or not to grow as large as in solitary birds.

“Our result emphasises that a large brain is a costly organ, both to develop and maintain, so evolution readily acts to reduce its size when it isn’t needed,” argues Byrne. “Sometimes, people think having a big brain is just a ‘good thing’, but it is a major metabolic cost.”

We don’t yet know what it is about birds and mammals that makes group living affect brain size differently. “It must be that the benefits of cooperation [in birds] are so much greater than the costs of competition that they swamp any such effect, making Machiavellian manipulation a poor strategy,” says Byrne.

Brain drain

Byrne’s study is not the first to find brains shrinking in social animals. In 2015, a study led by at Drexel University in Philadelphia, Pennsylvania, found that among 29 species of paper wasps, .

“We found that social species invested less than solitary species in key, central-processing, higher-cognition brain regions called mushroom bodies, involved in learning, memory and sensory integration,” says O’Donnell.

“We suggested that extensive cooperation and overlap of interests in insect colonies may promote a form of distributed cognition, so it’s exciting to see the patterns we found replicated in a very different system, but potentially for similar reasons.”

And even humans might not be immune to this effect. “There is some evidence, albeit controversial, for a reduction in human brain volume in the last few tens of thousands of years,” says O’Donnell. “Skull measurements suggest about a tennis-ball’s-worth of brain volume has been lost.”

But he poses the question of whether aside from increased social collaboration, this is also down to cognitive offloading to developments and technology such as fire, oral traditions and writing.

And Byrne points out that Neanderthals seem to have had larger brains than us, even though they were not especially large-bodied. “If you really wanted to speculate, you might wonder if they were less genuinely cooperative than our ancestors, who then got the benefit of not needing such a large brain,” he says.

Normally in primates, though, the opposite is the rule, with brains expanding in tandem with the size of social groups.

at the University of Missouri in Columbia, who has studied trends in human brain size, says the new woodpecker finding is “surprising in light of the positive relation between group size and brain size in primates and many other mammals”.

The usual explanation is that as social groups expand, brains grow to enable individuals to snoop on their rivals, helping keep track of the additional social dynamics, stresses, strains and hierarchies within a larger group.

Journal reference: Biology Letters, DOI:

Read more: Did an ice age boost human brain size?;
Bigger brains help female fish outwit predators and live longer