In my third year of reporting on the coronavirus pandemic, I find woodpeckers, which can ram their heads against hard surfaces about 20 times a second, to be incredibly relatable. But the birds’ extraordinary behavior raises an obvious question: Why, as one team of scientists wrote in 1976, is the countryside “not littered with dazed and dying woodpeckers”?
The just-as-obvious answer is that woodpecker skulls have adaptations, such as spongy bone in the front of their skulls, that absorb or dissipate the shocks from their pecks, protecting their squishy brains. This explanation features in books, news articles, zoo displays, and scientific papers. “You can’t avoid it,” Sam Van Wassenbergh, a biologist at the University of Antwerp, told me. It’s so accepted that some scientists have tried to work out exactly which parts of the skull absorb shocks, while others have designed helmets and other protective technology inspired by the birds. There’s just one problem: As Van Wassenbergh and his colleagues have now shown, woodpecker heads don’t absorb shocks at all.
Although the shock-absorption idea seems superficially sound, “the more you think about it, the less sense it makes,” Van Wassenbergh said. Woodpeckers peck trees to send messages, dig out hidden insects, and excavate nesting holes; many of their body parts—strong beaks, grasping feet, and stiff, strut-like tails—have evolved to maximize the kinetic energy they deliver with each blow. If their skulls absorbed that energy, they’d just need to pound harder, which would negate any benefits from the absorption. If what you need is a hammer, why strap a cushion onto its head?
To check his suspicions, Van Wassenbergh and his colleagues filmed three woodpecker species as they hammered into wood, using high-speed cameras that could capture 4,000 frames every second. The team then analyzed every frame to see how parts of the birds’ head move relative to one another. If the skull really was absorbing shocks, then upon each peck, the brain should decelerate far less than the beak—just as when a car hits a bump, its body jerks less than its wheels do. But the videos revealed that, in fact, when a woodpecker pecks wood, its entire head, including the brain, comes to a stop at the same rate. (The team used the position of the eye as a proxy for the front of the brain, because the two are jammed closely together in woodpeckers, with little room for movement.) “That really lays to rest the idea that some part of the head is acting as a shock absorber,” Margaret Rubega, an ornithologist at the University of Connecticut who wasn’t involved in the study, told me.
Even if woodpeckers did absorb shocks, it wouldn’t help them. Using simulations of a black woodpecker’s head, Van Wassenbergh showed that a shock-absorbing skull would force the bird to spend more energy on pecking for no benefit. As Rubega said, “You don’t use a spring to hammer a nail with.” Instead, you use … well … a hammer, which is what the woodpecker’s head essentially is—a rigid structure that has evolved not to absorb shocks but to preserve them. “This makes intuitive sense,” says Lorna Gibson, an engineer at MIT who has studied woodpeckers and was always skeptical of the shock-absorption idea. “I’m not sure why [it] was accepted.”
The zoological literature is full of similarly false ideas that persisted for years or decades before being corrected: that hummingbirds drink by using their tongues as straws; that cheetahs overheat when hunting; that mantis shrimps have kaleidoscopic rainbow vision; that honey badgers follow birds to honey; or that Komodo dragons kill with bacteria-laden bites. Some of these factoids began as assumptions that somehow calcified into received wisdom without anyone checking them. Others were outright fabrications, or arose from preliminary studies that were exaggerated or overgeneralized. Many are still repeated today.
In the case of woodpeckers, few previous studies ever filmed and analyzed live birds, relying solely on digital simulations or observations of their skulls. This is typical of humans’ knowledge of birds: We make a lot of assumptions but do little actual testing. “We have no idea how the stress of pecking, or biting, or anything really, is accommodated by the skull, nor how this varies between species with different skull and beak shapes, diets, and behaviors,” Jen Bright at the University of Hull, in the U.K., told me.
But Van Wassenbergh also suspects that many researchers have been misled by a simple form of anthropomorphism. “It’s logical to think that, if I was this bird, I’d like to have a helmet or an airbag,” he said. But although we use such tools to protect us from unwanted impacts, a woodpecker is trying to smash its face against a tree. Its needs are completely different from ours, which means that features in its skull are probably not analogous to safety equipment. The spongy mass of bone at the front of the skull looks like it could be an airbag—but clearly doesn’t act like one. The long tongue, which, when retracted, wraps around the back of the skull and into the bird’s forehead, looks like a possible seat belt—but, again, clearly isn’t one. Engineers who are looking to woodpecker skulls for inspiration might think twice, Van Wassenbergh told me: “This bird has gone through millions of years of trying to minimize shock absorption … which isn’t what you want in a helmet.”
But if woodpeckers lack some built-in helmet, then how do they peck wood without sustaining traumatic brain injuries? A human who headbutted a tree at woodpecker speed would absolutely be concussed. But we have extremely large brains—a fact that, ironically, we seem to forget. Woodpeckers have smaller and lighter brains than ours, which greatly reduces the pressure that they experience upon each peck. According to Van Wassenbergh’s calculations, a woodpecker would have to hit a tree at twice its normal speed, or peck something four times stiffer than the average tree, to get a concussion. “If by accident they hit a piece of metal, I can still imagine that they’d suffer a concussion, but for their natural behavior, what they do is relatively safe,” he said.
Van Wassenbergh hasn’t yet checked if woodpeckers have evolved especially small brains for birds of their size, or differently shaped ones: A more spherical brain, he noted, would be better at resisting shocks than an elongated one. The birds may also have adaptations that help them cope with the damage that even subconcussive impacts can create; perhaps their brains have little fluid so that they can’t slosh around too much. Whatever the case, the secret to the woodpecker’s percussive powers appears to be deceptively simple: They just have small brains. Maybe I should try that the next time I’m tempted to smash my face into the nearest solid object.