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

We aim to test three questions regarding human eccrine sweat gland density, which is highly derived yet poorly understood. First, is variation in functional eccrine gland density ("FED") explained by childhood climate, suggesting phenotypic plasticity? Second, is variation in FED explained by genetic similarity (a proxy for "geographic ancestry"), implying divergent evolutionary pathways in this trait of ancestral populations? Third, what is the relationship between FED and sweat production?

To test questions one and two, we measured FED in 68 volunteers aged 18-39 with varied childhood climate regimes and geographic ancestries. To test question three, we compared sweat production to FED in our n = 68 sample. In addition, we examined the relationship between FED and whole-body sweat loss during cycling in warm conditions using a sample of eight heat-acclimated endurance athletes.

Interindividual variation in six-site FED was more than twofold, ranging from 60.9 to 132.7 glands/cm² . Variation in FED was best explained by body surface area and limb circumferences (negative associations) and poorly explained by childhood climatic conditions and genetic similarity. Pilocarpine-induced sweat production was unrelated to FED while whole-body sweat loss during cycling was significantly, though modestly, associated with FED.

We hypothesize that gland-level phenotypic plasticity, rather than changes in eccrine gland density, was sufficient to permit thermal adaptation to novel environments as humans colonized the globe. Future research should measure effects of FED in dehydrated states and the relationship between FED and salt loss, and control for effects of microclimate to rule out phenotypic plasticity effects.