Subvert your cognition! Woodpeckers don't wear "safety helmets" when they hit trees

It's often thought that the spongy bones in woodpeckers' heads cushion their brains from impact, but in fact, their skulls are as hard as hammers.

A great spotted woodpecker returns to a tree hole after foraging to feed its chicks. Photo by Xinhua News Agency reporter Yang Qing

New research suggests that woodpeckers' skulls are not designed to cushion impacts, but rather to strike wood more forcefully and efficiently.

Woodpeckers bang their beaks against tree trunks to communicate, find food or build nests. The spongy bone between a bird's brain and beak was once thought to cushion the brain from repeated blows.

But this organization actually helps them strike quickly and deeply with minimal energy expenditure, like a well-designed hammer, says Sam Van Wassenbergh of the University of Antwerp in Belgium.

“We had a feeling that this shock-absorbing theory didn’t make any sense,” he said. “A hammer with shock-absorbing features is a bad hammer.”

Van Wassenbergh and colleagues analyzed 109 high-speed videos of six captive woodpecker species hammering on wood, including two black woodpeckers, two northern black woodpeckers, and two great spotted woodpeckers.

They found that in the milliseconds after the beak struck the wood, the bird's eyes and head moved at about the same speed as the beak, meaning that the spongy bone in front of the eye was neither compressed nor absorbing the impact.

The team then created digital models of woodpeckers to test what would happen if the spongy bone was impacted. While the cushioning would reduce the impact on the brain, it also means the bird's beak can't dig as deep into the tree, Van Wassenbergh said.

In fact, to strike deeper into the tree, the birds will have to hit the tree harder, which actually cancels out any benefit of shock absorption.

Despite the lack of shock absorption, the team found that the bird brains were not at risk of concussion because the impact was not strong enough.

The woodpecker's brain is located inside a fluid-filled skull, and given its size and weight, damage would only occur if they struck the wood at twice the normal speed, or if they struck a surface four times harder than natural wood.

“It’s normal for a smaller organism to be able to withstand these huge forces,” Van Wassenbergh said, comparing it to a fly hitting a window with much greater force, “they just take off and fly again.”

The term "spongy bone" doesn't mean the bone is soft or compressible, he said. Instead, it suggests the bone is porous and lightweight — crucial for a bird that can fly. "The bone is just strong enough to perform the functions it needs to."

The related paper was published in Current Biology on July 14.

Source: China Science Daily

Editor: Wu Tong

Review: Wang Fei

Final judge: Chen Lei