Woodpeckers spend all day hammering their head on tree trunks, using their beak to make holes and digging insects out of those holes for a meal. The birds’ distinctive drumming and drilling had led researchers to hypothesize that the bone between woodpeckers’ beak and braincase must absorb shocks to protect their brain from concussions. But a new study suggests that their head and beak act like a stiff hammer for optimal pecking performance rather a shock-absorbing system to cushion the brain.
“What this bird has to do during the entire day is dig holes into the wood. It’s very important that this business be very efficient,” explains Sam Van Wassenbergh, an evolutionary biomechanicist at the University of Antwerp in Belgium, who led the new study. If the woodpecker absorbs some of the energy it directs at the tree, then less energy is imparted to the tree trunk, and it has to peck even harder to make holes. “So the more you think about it, the less it made sense that there was any shock absorption going on,” Van Wassenbergh says. “But it had to be tested.”
To investigate this question, European and Canadian researchers used high-speed video of three species of woodpeckers in action. They tracked the motion of different parts of the woodpeckers’ head as the birds hammered the trees and hypothesized that if there was any cushioning going on, it could be confirmed by detection of slower deceleration of the braincase in comparison with the beak upon impact with the wood. But that’s not what the team found. Instead the head functioned like a stiff hammer, with little to no dampening of vibrations. The results were reported on July 14 in Current Biology.
Previous work in the 1970s had examined this question from a theoretical perspective, but this study is the first to capture high-speed video to determine how much force is being loaded onto the woodpeckers’ bill and separately onto their brain, says neurobiologist Dan Tobiansky, who studies woodpeckers at St. Mary’s College of Maryland and was not involved in the study.
So how do woodpeckers avoid concussions? The researchers used simulations to calculate the impact on the brains of the birds and compared it with thresholds for concussion-causing forces in humans. For people, an impact of about 135 g’s produces a concussion. But woodpeckers are much smaller. The length of their brain is about one seventh that of a human, which means that they can withstand forces that are seven times higher, Van Wassenbergh explains. Based on the models, the forces woodpeckers’ brain sustains are below the danger threshold by a factor of two. So “they could hit the tree at higher speeds and still not suffer a concussion,” he says. “The key is that the head of the woodpecker is just much smaller than that of a human.”
But Tobiansky notes that among football players, the brain injury chronic traumatic encephalopathy (CTE) is most commonly found in offensive linemen who receive repeated, subconcussive shocks. So even subconcussive events can have a detrimental effect on the brain, and woodpeckers may have some yet-to-be-determined physiological mechanisms that protect their brain from these repeated subconcussive insults. The work of Tobiansky and his colleagues suggests that steroid hormones such as androgens and estrogens may have protective effects on the birds’ brain.
The physiology of brain protection is not yet well understood. It seems clear, however, that, given these unknowns, the woodpecker braincase is not a good model to inspire the design of helmets for humans. Surprisingly, perhaps, a crustacean, a member of a class of animal that specializes in a hardened exterior, might provide better insight on how to proceed.
The most distinctive feature of the bigclaw snapping shrimp (Alpheus heterochaelis), a small coastal marine crustacean that lives in tropical and subtropical waters, is its snapping claw—a weapon the territorial animal frequently uses to battle invaders and defend its home. The claw, made of a plunger and a hole, operates with a latch mechanism that shoots the plunger through the hole at blistering speed. That causes a jet of water to shoot out, creating an area of low pressure where the jet is boiled into a bubble of air. That so-called cavitation bubble collapses, creating a snapping sound and a brief burst of light. But the primary product of the snapping claw is a high-amplitude pressure wave that can cause damage to soft tissues such as those of the brain.
When two of the shrimp engage in combat, each snaps within a centimeter of its opponent, trying to intimidate it in a hydraulic game of chicken. The pressure waves from the snaps can inflict brain damage.
A team of researchers at the University of South Carolina and the University of Tulsa have now revealed these snapping shrimps’ secret to coping with the constant onslaught. They are equipped with a transparent, gogglelike structure, known as an “orbital hood,” which covers their eyes and protects their brain from the pressure waves. The researchers reported their findings on July 5 in Current Biology.
“There’s been tons of work on the evolution of weapons and relatively little work on ‘How do you defend against them?’” says Melissa Hughes, who studies snapping shrimp at the College of Charleston in South Carolina and was not involved in the study. In this case, the question is particularly interesting because the weapon makes a snap, which can be harmful to the shrimp itself, as well as to its opponent. “And so you need double protection—protection from others coming at you but also from your own use of the weapon,” she explains.
To understand how the snapping shrimp cope with frequent exposures to shock waves generated by rivals (and their own claws), researchers surgically removed the orbital hoods from some of the animals. The team exposed the hoodless shrimp to shock waves from other shrimp and found that they became disoriented and lost motor control of their limbs, sometimes permanently, an indicator of brain damage. In contrast, intact animals exposed to shock waves did not have these problems and showed normal behaviors.
Next, the researchers used tiny sensors to measure the pressure inside and out of the orbital hoods as the shrimp were exposed to a snap. They found that, on average, the orbital hoods cut the magnitude of the shock waves in half. “When we have an animal that has their helmet on, it’s pretty effective at dampening those shock waves, so we get less energy reaching the brain underneath the hood,” explains Alexandra Kingston, a biologist at the University of Tulsa and the study’s lead author. But when the hood is removed, the shock waves reach the brain at full strength, which translates into “no hood means no protection.”
How, exactly, are the goggles so effective at protecting the animals’ brain? Kingston and her colleagues hypothesized that a shock wave forces water out of the orbital hood, thus transferring its energy to water expelled down and away from the animal instead of traveling through its tissues.
To test this idea, they glued the goggles shut on some snapping shrimp, preventing water from exiting the orbital hood. The team found that when these shrimp were exposed to a shock wave, it propagated through the sealed goggles and hit the brain as if the orbital hood wasn’t really there. This showed that the ability to expel water out of the bottom of the orbital hood is critical to protecting the brain.
Understanding how these tiny helmets work could inspire the design of equipment that will protect humans from traumatic brain injuries, the researchers say. Shock waves from an explosion can cause devastating brain damage to a bystander. Mild traumatic brain injuries are some of the most common types to occur in military personnel. Even armored vehicles do not protect against shock waves. “They go right through them,” explains Dan Speiser, a visual ecologist at the University of South Carolina and senior author of the study.
Woodpeckers and snapping shrimp are “the two animals that seem to risk brain damage all day, every day,” Speiser says. But luckily, their physiological and biomechanical tricks to protect their brains may inspire new medical or engineering solutions to prevent brain injuries in soldiers and athletes, too.