Variation in human functional eccrine gland density and its implications for the evolution of human sweating

Area of Science:

• Human physiology
• Thermoregulation
• Evolutionary biology

Background:

• Human eccrine sweat gland density (FED) is crucial for thermoregulation but poorly understood.
• Investigating factors influencing FED variation is key to understanding human adaptation.

Purpose of the Study:

• To determine if childhood climate or genetic similarity explains variation in human FED.
• To explore the relationship between FED and sweat production.

Main Methods:

• Measured FED in 68 volunteers (aged 18-39) with diverse climate and ancestry backgrounds.
• Assessed pilocarpine-induced sweat production and whole-body sweat loss in athletes.

Main Results:

• FED varied more than twofold (60.9–132.7 glands/cm²), primarily influenced by body surface area and limb circumferences.
• Childhood climate and genetic similarity poorly explained FED variation.
• Pilocarpine-induced sweat production was unrelated to FED; whole-body sweat loss showed a modest association.

Conclusions:

• Phenotypic plasticity at the gland level, not changes in FED, likely facilitated human thermal adaptation.
• Future research should investigate FED in dehydrated states and its relation to salt loss.