Men may have evolved to grow beards to protect their faces during fights, scientists believe.
Beards are often regarded as a symbol of masculinity, aggression and social dominance, wrote the authors of a paper published in the journal Integrative Organismal Biology. Past studies suggested that the beard may protect vital body parts like the jaw and throat from deadly attacks, similar to a lion's mane, the researchers at the University of Utah explained. This view matches up with the fact that the mandible is the part of the face most often fractured during fights.
Like other species of great ape, human males carry out most acts of violence, and mostly towards other males, according to the team. When human males fight, they usually target the face. This could explain why men and women's faces are structured differently.
The authors of the study hypothesized that thick facial hair could have evolved to absorb and disperse the force delivered by a sock to the face.
Read more
Millions of Human Cells Have Been Grown Inside Mice Embryos
Scientists Turned Parkinson's Disease Patient's Skin Cells Into Brain Cells
Man Discovers He Has Three Kidneys after Visiting Doctor for Back Pain
To test their hypothesis, the team created a model of human bone tissue, and cut it into 60 mm (2.3 inch) × 65 mm (2.5 inch) × 3 mm (0.1 inch) rectangles. They then covered the mocked-up bone with sheep hair comparable to three states of grooming in humans. A full beard was represented by so-called furred sheepskin, a trimmed beard by sheared skin, and hairless by plucked. The skin was soaked in liquid so it resembled living tissue, while the hair was kept dry.
The samples were taken from a local slaughterhouse. The team acknowledged that sheepskin isn't directly comparable to a human beard hair, but said it was "not practical to obtain fully bearded skin samples from human cadavers" and loose human hair wouldn't soak up the force of a punch in the same was as hair rooted in skin.
Using a machine called a drop-weight impact tester, the sheepskin was held in place with an anvil and a blunt 4.7 kg (10.36 lbs) weight dropped onto it from a height of 7.4 cm to mimic a blow to the face. The process was repeated for 20 samples of sheepskin.
On average, the furred samples absorbed almost 30 percent more energy than their sheared and plucked counterparts. All of the plucked samples broke, as did 95 percent of the sheared, and 45 percent of the furred.
The experiments revealed the biggest difference between the samples was the time it took to reach peak force and peak energy absorption. "This suggests that the greatest advantage offered by the hair is that it distributes the force of impact over a longer time frame," the authors wrote.
They went on: "The results of this study indicate that hair is indeed capable of significantly reducing the force of impact from a blunt strike and absorbing energy, thereby reducing the incidence of failure. If the same is true for human facial hair, then having a full beard may help protect vulnerable regions of the facial skeleton from damaging strikes, such as the jaw.
"Presumably, full beards also reduce injury, laceration, and contusion, to the skin and muscle of the face."
Men with beards may, therefore, have an advantage in fights, and this facial hair may be selectively favored in an evolutionary sense.
"This may also explain why facial hair is associated with high masculinity, social dominance, and behavioral aggressiveness, as it may function as a true indicator of level of invulnerability to facial injury," the team wrote. "The results of this study are consistent with the suggestion that the sexually dimorphic facial hair of humans may have evolved in response to selection on male-male fighting performance."
The study adds to evidence indicating fighting may have shaped how our skeletons evolved, they said.
However, the team acknowledged that while their data suggests hair can protect bone and skin front blunt strikes, "this may not be true in every case."
For instance, different populations have varying levels of hair thickness: with Middle Eastern and Northern European men more likely to have "thick, bushy beards," when compared with those of East Asian and American Indian descent. What's more, the thick sheep's hair used in the study is likely only comparable to a long human beard. Studies in the future should take these considerations into account, they said.
