What can slightly-sweaty primates teach us about that uber-sweaty primate, Homo sapiens?
Above: Chimpanzee. Photo by Jill Preutz.
Right: Elite human marathoner. Photo by Scott Mason Photography.
When you think about what separates humans from other animals you’d probably answer with intelligence, big brains, or walking on two legs. You may even recognize our (almost) naked skin as conspicuous. You’d be right about all these things but you’re forgetting one: sweating. Our tremendous capacity to cool through sweating is unmatched in other mammal and primate species. An ongoing Kamilar Lab project aims to shed light on this hallmark of human evolution.
First let's distinguish between the two types of sweat glands. Eccrine sweat glands, when found on the hands and feet, produce sweat to aid in gripping; when found on the body surface, they produce sweat for the purposes of cooling off the body. Apocrine sweat glands, also found in mammals and primates, produce odors and other secretions. These glands are not used for cooling the body except in horses and a few other species.
Humans aren’t the only primates who use eccrine sweating to cool off. Some monkeys and apes - those most closely related to us, but also a few distant cousins - cool by sweating, though the extent to which they rely upon this ability is unclear. To begin asking questions about the evolution of human sweating, we looked at characteristics of eccrine sweat glands in primates published in a series of papers in the 1960’s and 1970’s. After scouring 40+ papers we had a data set fit for addressing some questions:
1) To what extent are sweat glands similar between species?
2) Do differences in sweat glands align with degree of species relatedness (a.k.a., “phylogeny”), or have they instead evolved independently?
3) Can climate variables explain any of this variation?
4) What can these results tell us about the evolution of human sweating?
Perhaps unsurprisingly, a few characteristics seem to have evolved early at the base of the major clades, and changed little thereafter. Profusion of eccrine sweat glands on the surface of the body is one such trait (see figure at right). Having eccrine sweat glands confined to the hands and feet is the default mammal condition. Only the catarrhines (monkeys and apes of Africa and Asia, including humans) have eccrine sweat glands all over the body, with a few interesting exceptions in South American monkeys (more on that later).
Other traits seem to have evolved differently in individual primate species, likely after these lineages branched off and diversified. Two of these traits correlated with climatic variation: glycogen content and number of capillaries surrounding the gland. (See figures below.) Glycogen is a fuel used to power sweat production and capillaries supply glands with water and electrolytes needed for making sweat. Only four species in our dataset showed “moderate” instead of “high” glycogen concentrations and these species live in cool and wet climates. Similarly, for the nine species in our data set with body surface eccrine glands, degree of capillarization was significantly correlated with temperature and precipitation, with species in hotter and drier climates having more capillaries. These species are known to cool via sweating, though again, we don’t know how effective it is in these primates. Two South American monkeys also have sweat glands all over the body which are laden with glycogen, an interesting example of convergent evolution. (Capillary measures were not available for these species.)
Our results make intuitive sense. Species living in hot climates would benefit from sweating and this would be more effective in dry climates where evaporation is quicker. And in species where increased gland numbers have not evolved due either to lack of necessity or any other number of reasons (e.g. developmental constraint, lack of genetic diversity), increases in sweat production capability at the level of individual glands would be a good prescription. It would be helpful to measure thermoregulatory sweating ability in all these species to see if such gland changes correlate with increased reliance on sweating.
Perhaps most intriguing to us are the inferences we can make about the evolution of sweating in extinct hominins who constituted early parts of the human family tree. The observation that some primates sweat despite being quite hairy, and our results indicating that sweating capacity increases at the gland level due to climate pressures, led us to suggest two hypotheses.
1. Increases in sweating capacity may have evolved before hominins exhibited reduced body hair. Traditional explanations connect the origins of modern human-like sweating to body hair reduction, and it does make sense that our enormous sweating capacity probably didn’t fully develop until this time. But, gland-level changes such as those observed in our primate sample could have occurred earlier, before our ancestors became nearly hairless.
2. Climate, and not walking/running longer distances, could have driven initial increases in hominin sweating. Human sweating capacity is often linked with walking and running long distances in hot temperatures, behaviors which are thought to be important to the evolution of our genus. However, gland-level increases in sweating capacity, like those seen in our primate sample and evolved in response to hot/dry climates, could have come first, preceding the bigger increases associated with loss of body hair and longer walking/running distances.
Put together, our two hypotheses argue that the story of human sweating started earlier than previously thought.
We are now focusing our attention on sweat gland diversity found in modern humans. I am developing methods for counting sweat glands in human skin and hope to validate these techniques this spring. Then, I will begin a pilot study of gland counts in human volunteers, setting the stage for my dissertation research which will address several key questions, including: What factors influence gland activation early in life? We know that some glands remain inactive. What effect does active/inactive gland ratio have on an individual’s ability to cool via sweating? What is the evolutionary context of human sweating? Does the plasticity in sweat gland development reflect a tradeoff between the need to cool and the need to conserve water?
You can find more details of our study in the April 2018 issue of the Journal of Human Evolution. We hope you follow along with us on our sweaty journey!