Facial hair is one of the most sexually dimorphic features humans have, and it is widely read as a signal of masculinity and dominance. One proposal goes further: that a beard works like a , shielding the throat and jaw. The jaw is among the most commonly fractured facial bones in interpersonal violence. Beseris, Naleway and Carrier built a test rig.
It absorbs a blow better than bare skin. The team covered a with sheep skin in three states, , plucked and sheared, and dropped a weight on it. Peak force was sixteen percent greater and total energy absorbed thirty-seven percent greater in the furred samples than in the plucked ones, partly because the force arrived over a longer time.
The background is grim and quantitative. Human males perpetrate the vast majority of violence, most of it aimed at other males, and in hand-to-hand fighting the face is usually the primary target. Epidemiology of interpersonal violence puts facial injuries from fights far higher in males than in females.
That lines up with an argument from . The facial bones with the highest fracture rates from interpersonal violence are the parts of the skull with the greatest sexual dimorphism, in modern humans and in early hominins. That suggests those features emerged through male contest competition and act to protect the face. Some authors extend the argument to the beard, comparing it to a lion’s mane shielding vital areas like the throat and jaw.
The hypothesis tested here is that beards protect the skin and bones of the face by absorbing and dispersing the energy of a blunt impact. The rig was a fiber epoxy composite as a bone analog, covered with segments of skin dissected from domestic sheep. A drop weight impact tester fitted with a load cell struck it and collected force against time. Samples came in three conditions, furred, plucked and sheared, twenty of each.
Fully furred samples absorbed more energy than plucked or sheared ones. Peak force was sixteen percent greater and total energy absorbed thirty-seven percent greater in the furred samples than in the plucked. The authors attribute the difference partly to a longer time frame of force delivery, which is the principle a crumple zone works on. They read the data as support for the hypothesis that human beards protect vulnerable regions of the facial skeleton from damaging strikes. The work took an Ig Nobel Peace Prize.
wat. Hair spreads the blow over more time, so less of it arrives at once. The test was sheep wool over epoxy, not a jaw.
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The facial bones that break most often in fights are also the ones with the greatest sexual dimorphism, in modern humans and in early hominins alike.
How do we know?
The team laid segments of skin dissected from domestic sheep over a fiber epoxy composite standing in for bone, in furred, plucked and sheared conditions with twenty samples each. A drop weight impact tester fitted with a load cell delivered the blows.
What is still unknown
Sheep skin on epoxy, not a human face, and hair that is thicker than most beards. It shows what hair can do to an impact, not that this is why beards evolved.
Learn something?
Where to next
What are wrinkled fingertips for?evolution
Why does one animal produce droppings with flat faces and sharp corners?biomechanics
What is the appendix actually for?evolution