Why Woodpeckers Don’t Mind Hitting Trees With Their Faces

Why Woodpeckers Don’t Mind Hitting Trees With Their Faces

Watching a woodpecker repeatedly smash its face into a tree, it’s hard not to wonder how its brain stays intact.

For years, the prevailing theory has been that structures in and around a woodpecker’s skull absorb the shocks created during pecking. “Blogs and information panels at zoos all present this as fact — that shock absorption is occurring in woodpeckers,” said Sam Van Wassenbergh, a biologist at the University of Antwerp. Woodpeckers have even inspired the engineering of shock-absorbing materials and gear, like football helmets.

But now, after analyzing high-speed footage of woodpeckers in action, Dr. Van Wassenbergh and colleagues are challenging this long-held belief. They discovered that woodpeckers are not absorbing shocks during pecking and they likely aren’t being concussed by using their heads like hammers. Their work was published in Current Biology on Thursday.

When a woodpecker slams its beak into a tree, it generates a shock. If something in a woodpecker’s skull were absorbing these shocks before they reached the brain — the way a car’s airbag absorbs shocks in an accident before they reach a passenger — then, on impact, a woodpecker’s head would decelerate more slowly compared with its beak.

Credit…Van Wassenbergh et al., Current Biology

With this in mind, the researchers analyzed high-speed videos of six woodpeckers (three species, two birds each) hammering away into a tree. They tracked two points on each bird’s beak and one point on its eye to mark its brain’s location. They found that the eye decelerated at the same rate as the beak and, in a couple of cases, even more quickly, which meant that — at the very least — the woodpecker was not absorbing any shock during pecking.

Dr. Van Wassenbergh said that if woodpeckers were absorbing some of the shock they were trying to deliver to the tree, “it would be a waste of precious energy for the birds. Woodpeckers have undergone millions of years of evolution to minimize shock absorption.” Maja Mielke, a biologist at the University of Antwerp and a co-author of the study, added that like a hammer, a woodpecker’s skull is “really optimized for pecking performance.”

But with one mystery solved emerged another: How do woodpecker brains withstand that repeated shock?

To calculate pressure in the birds’ skulls, the researchers created a computational model based on pecking movement and skull shape and size, and they found that the pressure created was far below what would cause a concussion in a primate. In fact, the birds would have to hit a tree at twice their current speed — or hit wood four times as stiff — to sustain a concussion. “We forget that woodpeckers are considerably smaller than humans,” Dr. Van Wassenbergh said. “Smaller animals can withstand higher decelerations. Think about a fly that hits a window and then just flies back again.”

“Traditionally, when people were coming up with hypotheses about how animals function, a lot of the time they never even looked at the living animal; they would just pull bones out of a drawer,” said Michael Granatosky who studies evolutionary biomechanics at the New York Institute of Technology and was not involved in the study.

Dr. Granatosky views this work as an example of how much is left to discover. “There are all of these things we think we know, and we just don’t,” he said.

But the findings don’t answer all questions about the birds — for instance, how a woodpecker maintains such stiffness between its skull and its beak during pecking and what other factors may be involved that could mitigate possible damage to the brain.

“You have to think about the complexity of these systems,” said Ryan Felice, an evolutionary biologist at University College London who was not involved in the study. “It’s not just bones and muscles, but maybe the amount of fluid in the brain and blood pressure, and even the ability to heal damaged neurons.”

Ms. Mielke sees this work as a call to action for scientists in any research field. “It’s always worth looking at phenomena that we believe we are already understanding, because sometimes, there can be surprises,” she said. “Intuition can fool us.”

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