A history of thought on brain injury in head-hitting animals

Throughout time, humans have observed animals performing head impact behaviors in nature and have wondered whether they sustain brain injury. The resulting literature spans centuries and provides both valuable insight and misguided theories. Bighorn sheep and woodpeckers are the two main species studied in this regard and, simultaneously, in current popular culture they are often represented as being immune to brain injury. How did this narrative arise, and is it accurate? This historical review explores these questions by tracing the development of human thought on animal head impacts from prehistoric rock art, through medieval illumination, to the advent of natural history, and finally to modern-day research. As the study of these animals increased, contrasting hypotheses arose regarding both bovids and woodpeckers. While research from engineering and biomedical fields hypothesizes that specialized anatomical features prevent these animals from sustaining brain injury, research from the evolutionary biology field refutes the idea of any such shock-absorption mechanisms. Modern histochemistry techniques have provided cellular evidence of brain injury in both bovids and woodpeckers, and yet biomimicry research continues to seek inspiration for brain protection from these animals. The race for solutions to a growing neurodegeneration epidemic has led to a legacy of unsupported claims amongst the research taking inspiration from head-hitting animals. This review traces the development of these ideas, with a focus on persistent misinformation. By re-examining the literature, it calls for a shift towards evidence-based approaches to more effectively advance our understanding of animal brain injury, and ultimately human health.