by Apeksha Srivastava (ComSciCon-Virginia Tech 2023)
I remember seeing a woodpecker in my grandma’s home when I was around seven or eight. It happily pecked at a wooden pillar in the garden with its beak. “This is what they do all the time! It is their natural behavior,” my grandma said. After a long time, early this year, when we had to choose topics for an assignment as a part of our PhD coursework, I suddenly remembered woodpeckers again. I could immediately picture them pecking at trees with their beaks. Belonging to the bird family Picidae, woodpeckers are found worldwide. They are famous for tapping their beaks against hard surfaces many times a day for several reasons: finding and storing food, making nesting holes, for sexual display, and communication.
You might think, the forceful impact of the beaks on tough surfaces, followed by the abrupt slowing down of their heads, could hurt their brains! But surprisingly, there is no proof that woodpeckers experience brain injury from this tapping. Are these birds immune to brain damage? If yes, how?
Upon impact with a stationary object, the sudden braking of a moving head, can cause compressions at the impact site and expansions at the back side, potentially damaging the brain. The human brain gets injured at accelerations, one-fifth to half of those attained in repeated woodpecker drillings (Gibson, 2006). In the words of May et al. (1976), “… those who have seen the effects of even a limited degree of head-banging in patients with psychosis, epilepsy, or mental retardation, will wonder why the countryside is not littered with dazed and dying woodpeckers.” According to researchers, woodpeckers may have protective mechanisms contributing to their survival and fitness in such situations involving continuous forceful pecking.
Adaptations
A study in 1976 by May and colleagues compared the head and beak sections of two woodpeckers with that of a toucan, another related bird of the same order (Piciformes), which does not display pecking behavior. Their study highlighted four possible evolutionary adaptations aimed at protection from brain injury in our pecking-expert friends. Firstly, the woodpecker’s brain is tightly packed by a spongy and dense bone, which is light and “almost frothy” in a toucan. Secondly, woodpeckers possess robust muscles that serve as muscular shock absorbers and distributors to maintain the beaks’ rigidity. Thirdly, woodpeckers have very narrow spaces within their skulls, which may reduce the transmission of shock waves that arise due to the drilling behavior of these birds. Lastly, their skull is surrounded by a sling-like structure that might function as a shock absorber.
But What About the Pecking Efficiency?
Shock absorption is the major concept underpinning the points mentioned by May et al. (1976), and these mechanisms generally work by increasing the duration of impact and diminishing the force applied. Would it then logically follow that woodpeckers will suffer from decreased drilling efficiency? To counter this, Gibson (2006) suggested another set of three factors. Firstly, the small size of these birds lessens the stress exerted on their brain for any given acceleration. Secondly, the short duration of the impact of the beak on the surface augments the tolerable acceleration that avoids adverse effects on the brain. Thirdly, positioning of brain within the skull amplifies the contact area between the two, possibly contributing to protection from brain injury.
A Cooperative Phenomenon
In 2011, Wang and colleagues compared the Great Spotted woodpecker with the Eurasian hoopoe, a related bird of similar size that generally pecks at insects in the soil. They generated biomechanical models with the help of the 3D motion of these birds captured during pecking using high-speed cameras. Analyses revealed that a sling-like structure in woodpeckers (the long hyoid bone) extends to the top of the head and into the nasal cavity, acting as a “safety belt” after impact. Further, the unequal length of the upper and lower beaks may function as high-stress bearers or shock absorbers in these birds. The researchers emphasized that the protective system in woodpeckers is a cooperative phenomenon devoid of reliance on any single factor.
3D models by Jung and colleagues (2019) highlighted that a special part of the skull, the jugal bone, a “beam-like bar structure,” which is found in many reptiles, amphibians, and birds, plays a big role in protecting woodpeckers from getting hurt. The jugal bone is thought to reduce the skull weight and facilitate significant movement of skull bones relative to each other. Further, Jung et al. predicted that a large mismatch of the natural frequencies between the skull and brain reduces the load on these birds’ brains. These cool evolutionary protective adaptations that keep woodpecker brains safe can be utilized for several engineering applications. The concept of “woodpecker biomimicry” (e.g., Plessis and Broeckhoven, 2019), which uses certain features of the woodpecker’s skull structure, has been applied to produce safety equipment like helmets.
The Other Side of the Coin: Reconsidering Woodpeckers as Models for Safety Equipment
Head injuries rank among the most frequent causes of disability and death in human adults (Johns Hopkins, n.d.). With the woodpecker protective model gaining popularity, the designs of impact-related injury-resistant devices have been inspired by the skull morphology of these birds. However, the question remains: Are woodpeckers really immune to brain damage?
Not Shock Absorbers but Stiff Hammers!
According to Van Wassenbergh et al. (2022), many body parts of woodpeckers have evolved to maximize the kinetic energy required for their powerful strikes. If the skulls of woodpeckers evolved to absorb that energy, would it necessitate harder pecking? And if so, would it counteract the benefits of shock absorption? To investigate this, researchers filmed three woodpecker species using high-speed cameras and observed how parts of their heads moved relative to each other during pecking. Surprisingly, the results revealed that the entire head of woodpeckers, including their brain, comes to a halt at the same rate when they peck wood. This contradicts the proposed shock-absorbing mechanism in these birds, suggesting that their brains decelerate less than their beaks! Thus, based on numerical simulations, woodpecker heads can be considered “stiff hammers” and not shock absorbers as described by Gibson (2006). They appear to be rigid structures that evolved to preserve shocks, not absorb them.
The researchers explained that woodpeckers purposefully smash their beaks against trees, which is different from our needs — avoiding smashes and collisions. This means that these birds’ skulls should probably not be an inspiration for safety equipment. Woodpeckers have evolved over millions of years to minimize shock absorption to enhance their pecking performance, which is not something we would want in our protective helmets.
But then, another question arises: How do woodpeckers strike hard surfaces without sustaining injuries? The answer lies in their brains. Van Wassenbergh explained during an interview: “Think about a fly that hits a window and then just flies back again!” The tiny fly avoids brain injury in such instances. Similarly, woodpeckers have lighter and smaller brains than humans, significantly reducing the pressure exerted on each peck. Van Wassenbergh et al. (2022) calculated that to suffer from concussions, woodpeckers would need to strike wood at twice their usual speed or drill something around four times stiffer than average wood. Although woodpeckers might experience concussions from accidentally colliding with a metal surface, their smaller size prevents significant harm. This is unlikely to disrupt the natural behaviors of these birds, resulting in a relatively safe outcome.
Final Thoughts
Over the years, different aspects of the woodpecker’s skull and its associated areas have been assumed to be evolutionary adaptations protecting it against brain damage from its pecking behavior. However, research has given rise to even more critical questions: Do woodpeckers possess smaller brains than other birds of the same size, or do they possess differently shaped brains, which may be a better shock resistor? Are the associated tau accumulations responsible for woodpeckers’ behavioral changes and do they indicate potential brain damage? On the other hand, if we proceed along the “shock absorber” school of thought, the long hyoid bone, proposed as a “safety belt” after impact in woodpeckers, is essential for functions like carrying the tongue’s weight, swallowing, and even speech. Could it have undergone exaptation, a concept different from adaptation (Coolidge, 2019), to protect woodpeckers from pecking-induced brain injury? Adaptation pertains to behavioral or physical traits that, through natural selection, promote survival and reproduction. Conversely, exaptation involves repurposing a behavioral or physical trait from initial adaptive function and its subsequent enhanced fitness. For instance, feathers were selected (i) to regulate body temperature in birds through adaptation, and (ii) later found a new role in enabling greater mobility and flight through exaptation.
Our avian companions, woodpeckers, serve as intriguing subjects for studies in evolutionary biology and ornithology. With further investigations, we may gain a more profound comprehension of the mechanisms that safeguard against brain injuries in various woodpecker species, potentially leading to more effective strategies for preventing conditions such as traumatic brain injury and chronic traumatic encephalopathy in humans.
Glossary (in alphabetical order)
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Adaptation: A behavioral or physical feature that, through natural selection, aids survival and reproduction.
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Alzheimer’s disease: A progressive disease that starts with mild memory loss and eventually develops into the inability to carry out the simplest tasks.
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Chronic traumatic encephalopathy: A neurodegenerative condition associated with repeated trauma to the head
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Epilepsy: Neurological disorder in which nerve cells in the brain signal abnormally, leading to seizures
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Exaptation: A behavioral or physical feature coopted from its initial adaptive function and subsequently enhances fitness
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Jugal bone: A skull bone found in most amphibians, birds, and reptiles. It is thought to reduce the skull weight and facilitate significant movement of skull bones relative to each other.
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Kinetic energy: The energy possessed by an object because of its motion.
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Psychosis: Characterised by a disconnection from reality.
References
Coolidge, F. L. (2019). Evolutionary neuropsychology: An introduction to the structures and functions of the human brain. Oxford University Press.
Farah, G., Siwek, D., & Cummings, P. (2018). Tau accumulations in the brains of woodpeckers. PLoS One, 13(2), Article e0191526.
Gibson, L. J. (2006). Woodpecker pecking: How woodpeckers avoid brain injury. Journal of Zoology, 270(3), 462–465.
Jung, J. Y., Pissarenko, A., Trikanad, A. A., Restrepo, D., Su, F. Y., Marquez, A., … & McKittrick, J. (2019). A natural stress deflector on the head? Mechanical and functional evaluation of the woodpecker skull bones. Advanced Theory and Simulations, 2(4), Article 1800152.
May, P. A., Newman, P., Fuster, J., & Hirschman, A. (1976). Woodpeckers and head injury. The Lancet, 307(7957), 454–455.
Osborne, M. (2022). Woodpeckers don’t have shock-absorbing skulls. Smithsonian Magazine.
du Plessis, A., & Broeckhoven, C. (2019). Looking deep into nature: A review of micro-computed tomography in biomimicry. Acta biomaterialia, 85, 27–40.
Van Wassenbergh, S., Ortlieb, E. J., Mielke, M., Böhmer, C., Shadwick, R. E., & Abourachid, A. (2022). Woodpeckers minimize cranial absorption of shocks. Current Biology, 32(14), 3189–3194.
Wang, L., Cheung, J. T., Pu, F., Li, D., Zhang, M., & Fan, Y. (2011). Why do woodpeckers resist head impact injury: A biomechanical investigation. PloS One, 6(10), Article e26490.
What is a head injury? Johns Hopkins Medicine.
This article was initially submitted to the ComSciCon blog (ComSciConversation) and then further improved by the editorial process at Club SciWri. Club SciWri and ComSciConversation have a shared mission of making fascinating science accessible to everyone.
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