The Man Bun: Energetic and Thermoregulatory Costs.

manbunhairstyle.net...seriously.

That's right-- it's time that everyone's burning question is finally explored: what are the costs associated with having a man bun?  Not the costs to his social standing, which are mixed, nor his manhood, also mixed.  But rather, energetic and thermoregulatory costs, particularly while locomoting.  I fully expect an NIH grant to explore this further.  You're welcome, science.

During particularly sweaty workouts I have questioned the wisdom of having extra hair attached to my dome.  Is it trapping heat?  Preventing sweating?  Adding excess weight? Let's try to get at some answers.

1. Energetic consequences of the man bun

All that extra hair only adds a trivial few ounces of weight.  It's not going to make much energetic difference to the whole-body cost of running.  It's like wearing slightly thicker shorts-- a few ounces there isn't going to add up to much.  However, the head moves around a bit when you run, so these extra few ounces on top of the head could count for more. Unlike extra weight at, say, the waist, which has to be carried forwards with each stride (plus a bit of up and down), weight at the end of a bouncing extremity incurs an extra cost through every cycle of its independent movement.  So a bouncing man bun is like wearing a heavier pair of shoes: there's a cost to carry the weight forwards, like any body weight, but also the cost of moving the part around during its extra movement cycle.


But it's not like heavier shoes.  Most of the work done to stabilize your head while running is performed by the nuchal ligament (right).  Unlike muscles, ligaments don't need energy to do their work, so the human head is restrained while running with little energy cost.  This structure is found in other animals that hold their head up all day, and in running animals; indeed, in humans, it may have developed largely because of running.  So, while a heavier shoe incurs a cost every time the foot is lifted, the man bun's weight is mostly controlled by the passive nuchal ligament and incurs little cost.  The extra weight of a sweaty man bun will add to this, but not much, and only if other muscles must be recruited.

2. Thermoregulatory consequences of the man bun

Except in very cool conditions, heat buildup is among the biggest factors limiting endurance performance (this is now the subject of my research).  Running generates massive amounts of heat; a trained athlete running a 1-hour race will elevate their metabolism ~19x over resting levels and heat is a byproduct. (In fact, you don't need to wait for summer to become heat acclimated: running even in moderate temperatures will get you partway there, as the endogenous (internally-produced) heat overwhelms your body even more efficiently than ambient heat.)  Because the body can only tolerate a small increase in internal temperature, much of our biology has been shaped in part by the need to dissipate heat: huge numbers of sweat glands (unusual and rare among animals), hairlessness (also weird), profusions of blood vessels supplying the skin, even our very body shape.  I'll note here that our unique heat dumping biology implies very strongly that our evolutionary past included strenuous heat producing activity.  Sustained running, or run-walking, could explain it, and few other things can.  Even carrying a heavy load in very hot weather, walking produces only half the sweat that running can; so arguing that our ancestors did not rely somehow upon running necessitates accepting that our cooling ability has huge safety margins, excess capacity.  And such excess capacities in complex and costly physiological systems are rare in the animal world.

Back to the head.  The brain lives here and is very temperature sensitive.  In fact, the body's chief heat-related concern during exercise is to cool the brain, as even a small temperature increase spells neural death.  In the cold, you lose roughly as much heat from the surface of the head as you do other parts of the body.  In hot conditions and during exercise the forehead sweats a lot, maybe even a bit more than other body regions, but overall the head isn't more of a radiator than you'd expect for a body part with its surface area to volume characteristics.  The brain is cooled primarily by taking hot blood elsewhere in the body for sweating, not simply by local sweating at the head, though other neat hypotheses abound: perhaps our larger sinuses or extra blood vessels permit brain cooling, or perhaps sweat on the scalp releases heat and then is re-absorbed into the skin instead of evaporating.

Machado-Moreira et al., 2008

So what about hair?  Does it prevent heat loss? Wearing a hat does, but hair?  You sweat everywhere on your head, though sweating under the hair isn't particularly effective because hair blunts evaporation and the sweat glands get clogged up (unless the weird hypothesis above holds true).   A 2008 experiment (left) confirmed earlier findings that the forehead sweats more than other head regions, with the top of the head having the lowest sweat rate.  Still, any sweat is helpful, the skin under head hair does do some sweating, and hair prevents sweat from doing its job (but again see the weird idea above).  However, this experiment involved shaved heads.  I suspect that the observed sweat rates would be lower with hairy subjects, maybe much lower, as sweat glands on a hairy scalp may clog and stop sweating very quickly.

The evidence is incomplete but it would seem that any hair on the head will blunt sweating to some degree.  But hair has another thermoregulatory purpose: it blocks solar radiation, shielding the brain from external heat.  In full sun, this heat-shielding mechanism might outweigh heat-increasing penalties incurred from reduced head sweating, having a net positive effect on body cooling.  In less sunny conditions I bet no hair is the best bet.  A man bun shouldn't be any worse than any length hair because it is up in a, well, bun, away from the forehead and neck and other sweat-producing skin regions.

What's the verdict?  A man bun will have a negligible effect on the energy cost of running and removes perhaps 10 square inches of skin real estate from the body's sweating arsenal.  This effect isn't any worse with any length hair, but in a competition where seconds count, this tiny added heat stress could matter.  For summer racing in the sun having some hair, even a man bun, is good, but a race entirely in the woods might call for a shaved head.

But none of this is as costly as the stigma that we man-bunners carry around.  Time to go to Whole Foods where I'll blend in.



References

Lieberman, D. E. (2015). Human locomotion and heat loss: an evolutionary perspective. Comprehensive Physiology.

Machado-Moreira, C. A., Wilmink, F., Meijer, A., Mekjavic, I. B., & Taylor, N. A. (2008). Local differences in sweat secretion from the head during rest and exercise in the heat. European journal of applied physiology, 104(2), 257-264.

Takeshita, K., Peterson, E. T., Bylski-Austrow, D., Crawford, A. H., & Nakamura, K. (2004). The nuchal ligament restrains cervical spine flexion. Spine, 29(18), E388-E393.

Wheeler, P. E. (1984). The evolution of bipedality and loss of functional body hair in hominids. Journal of Human Evolution, 13(1), 91-98.