Behind the Cover: January 2024

Each month, Physics Today editors explore the research and design choices that inspired the latest cover of the magazine.

In the cover photo of Physics Today’s January 2024 issue , wood chips fly as a red-bellied woodpecker prepares its nest in a palm tree. With each peck, a woodpecker’s head decelerates at up to 400 times the acceleration of gravity, far exceeding the roughly 135 g that would render a concussive blow to a human brain.

So how do woodpeckers avoid injuring their brains? Some biologists suspected that spongy bone serves as a shock absorber between the beak and cranium to cushion each blow. Yet, as Sam Van Wassenbergh and Maja Mielke explain in their January Quick Study , recent research reveals that no such shock absorption exists. They and their colleagues used high-speed video to track the movement of parts of the beaks and braincases of three different woodpecker species and calculated the peak decelerations. The researchers consistently found the braincases decelerated just as much as the beaks. “The woodpecker’s head functions as a stiff hammer—not as a shock absorber,” Van Wassenbergh and Mielke write.

With its much smaller size, a woodpecker brain can tolerate much stronger shocks. The pressure on the brain during each peck is proportional to the product of its deceleration, mass density, and length. A woodpecker’s brain is roughly one-seventh the length of a human brain, so a woodpecker’s deceleration threshold for a concussion is about seven times as great as a human’s.

Physics Today senior designer Jason Keisling chose the cover photo because it shows a woodpecker in action. The scene was captured at a shutter speed slow enough to make evident the dynamics of the wood shards, he says. Keisling also liked the multiple shades of green in the background. For the typography, “I sampled a color from the bird’s belly to make the cover look more cohesive,” he says. “I chose the bold typeface Neue Haas Grotesk and the more dynamic Alda italic typeface to display both sturdiness and movement.”