Humans are sweaty. With more than 2.5 million sweat glands distributed across the surface of our skin, we sweat more profusely than any other animal. Though the average person produces about 2 cups of sweat per day, this amount can increase to about 3 gallons with vigorous activity in high temperatures.
Sweat itself is 99% water, with traces of salts and metabolic wastes. When secreted onto the skin's surface, sweat evaporates, taking heat from our bodies as it vaporizes and cooling the blood that flows beneath our skin. This evaporative cooling system is likely the reason that human bodies are nearly hairless. And it turns out we can thank our efficient sweat glands for the trait that makes us uniquely human: our big brains.
Of all the organs in the body, the brain is especially susceptible to overheating. Evolutionary biologists have proposed that prolific sweating offered a competitive advantage to hominids foraging on the African savanna. With the ability to loose heat through sweating, early humans could travel far distances in high daytime temperatures and still keep their brains cool.
Sweat has another, more primitive, function. Our skin's watery secretion is the medium for an ancient language of chemical messages that affect our species' behavior and physiology.
Humans have two types of sweat glands: eccrine glands, which are distributed evenly throughout the body, and apocrine glands, which are densely packed in the underarm, genital and nipple regions. Apocrine secretions are milkier than eccrine secretions and are friendlier to bacterial growth.
"Sweat doesn't have much smell itself, but when apocrine secretions and microflora meet, it gives a unique smell," says Denise Chen, professor of neurology at Baylor College of Medicine in Houston who studies the nuances of our unique body odors.
Interested in how married people seem to understand each other without words, Chen and colleagues recruited couples, then used armpit pads to collect sweat from each individual in different emotional states: fearful, happy, sexual or neutral. Next, individuals blindly judged the sweat samples of their partner as well as the opposite-sex strangers in the group. The study found that individuals were significantly more accurate in distinguishing their partner's emotional sweat from his or her neutral sweat than they were in distinguishing the emotional sweat of a stranger. And their accuracy was directly related to how long they had been in the relationship.
"They do equally well with identifying a different scent, whether it's sex, happy or fear sweat," says Chen. "They can't name it, but they can determine that it's different."
Scent of attraction
A quick search on the Internet reveals a plethora of companies advertising human pheromones purported to help with attracting a mate. But does human sweat contain pheromones? Not that anyone has discovered yet, says Charles Wysocki, behavioral neuroscientist at the Monell Chemical Senses Center. He adds that it's a common misconception that pheromones are synonymous with sexual attractants, as "the original definition is a single compound or small set of compounds that elicit a behavioral or chemical response."
Though no single compounds as such have been identified in humans, human body odors do contain substances that affect hormone levels, physiology and behavior — including sexual attraction.
"The evidence really does suggest that there is some reproductive communication happening," says Martie Haselton, psychology professor at UCLA. In a study looking at how a woman's body odor changes throughout her fertility cycle, Haselton "found that men preferred samples of body odor collected from women at the high fertility phase of the cycle."
Some data have shown that when women who are ovulating are given samples soaked with men's body odor, they prefer those that are higher in testosterone. Other data show that women in this high-fertility phase prefer the odor of men whose bodies are more symmetrical, a trait that may be an indicator of higher reproductive fitness and that may also be linked to higher testosterone levels.
"Until recently, we've underestimated how important body odor is," says Haselton. "It might play a stronger role than we think in selecting a mate."