sweat gland project

Metabolism-- the breakdown of molecules for energy-- is one of the most important processes for any organism.  A consequence of metabolism is heat production.  This can be very helpful:  by making their cellular metabolism less efficient ("uncoupled"), mammals exaggerate heat production to stay warm, and so can live in cold cliates.  But in hot climates, or during exercise, the need to dissipate heat becomes critical and limiting, primarily because an internal temperature increase of even a few degrees causes brain cell death.  As we evolved in a hot climate (Africa) and relied upon high physical activity for survival, Homo sapiens adapted to cope with both environmental heat and heat produced during walking, running, and other physical activity.

One way to avoid heat buildup is to avoid absorbing it from the environment.  This likely explains why we still have hair on our heads: it blocks solar radiation from heating up our brains.  Other adaptations are geared towards losing heat (which only works when the air is cooler than the skin temperature).  Peoples whose ancestors evolved in hotter climates tend to have a longer, leaner body shape.  However, most of our heat-dumping adaptations seem to be shared by all peoples, but are only fully activated when you grow up in or live in a hot environment, or produce lots of heat through exercise.  In this way, our heat-dumping biology is "plastic": we all have the potential to be good at getting rid of heat, but these features only fully develop when we expose ourselves to heat.  Reduced metabolic rate is an example; in the summer, many of us will burn fewer calories, which results in lower heat production. 

But sweating is the main way that we cool off.  There don't seem to be any definite differences between populations in terms of sweating.  Differences in sweating between ethnic groups are instead probably due to plasticity: in other words, long term acclimation.  It appears that most people are able to improve their heat shedding ability given the right acclimation, at first via increased and earlier-onset sweating and increases in sweat gland size, and later through changes in skin temperature and (somewhat oddly) a more conservative sweat response.  But an important element of sweating potential is determined early in life: eccrine gland number. 

The eccrine sweat gland (below) is very common on your skin-- you have probably 2 million of them.  When you need to cool, eccrine glands pump sweat (mostly water) onto the surface of the skin. Body heat delivered to the surface of the skin by blood vessels heats up the sweat until it evaporates.  This is very efficient and enables humans to produce and dump huge amounts of heat.  (We can run miles in 100 degree heat; only horses can do this too.)  It's also rare: even among primates, our profusion of these glands is unsurpassed.  Clearly we evolved to shed heat.
Sato et al., 1989.

Gland number, size, sensitivity (the threshold at which they begin secreting sweat), and sweat composition all vary between individuals.  Early in life, perhaps by age 2.5, some number of glands are activated; the rest will remain inactive probably forever.  Climate seems to play a role in determining how many glands are activated and how many remain inactive.  Because sweating was essential to early human survival, but also costly and dangerous because it bring about water loss, the plasticity in active/inactive gland number is surely a finely-tuned evolutionary feature, one that has not been fully elucidated. 

Project goals: 
1) Identify factors affecting eccrine sweat gland activation. 
2) Determine how active/inactive gland ratios affect heat shedding ability. 
3) Place human eccrine gland activation in an evolutionary context. 

In stages over several years, this project will include a phylogenetic analysis of primate eccrine characteristics, a pilot study to validate gland-counting methods, and finally a larger study on how active eccrine gland number in humans is mediated by environment and activity level and how this affects sweating capacity.  This research is overseen by Daniel Lieberman of Harvard and Brigitte Holt, Jason Kamilar, and Betsy Dumont of UMass. 

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