Including eyes and barring cases of ambiguous genitalia, men have 8 and women have 9. Done and done, right? Not really. Arguably the most prominent holes in the human body — beyond the obvious ones — are those associated with hair follicles (these produce hair that extends out of the skin via a canal) and sweat glands (which are connected to the skin's surface via tubular sweat ducts). Yes, "sweat ducts." Biology is weird at the dermatological scale.
We humans are positively riddled with these holes, and how many we have is actually an incredibly interesting evolutionary question. How many do we have, exactly? Well…
Counting holes is challenging. The density of openings associated with hair follicles, for instance, varies enormously across each person's corporeal landscape. In an email to io9, biological anthropologist Nina Jablonski — an expert on the evolution of human skin — explains that follicle density is highest on the scalp and lowest on the back, chest and limbs. This includes otherwise hirsute men rocking shiny, hairless cueballs; some follicles — as in cases of male pattern baldness — don't produce hair at all. The holes in the skin through which they would otherwise sprout strands, however, do exist, though they are often barely perceptible.
The fact that some follicles produce thick, "terminal" hair, while others sprout finer, less conspicuous "vellus" hair can also make finding them easier or harder, respectively. General distribution patterns of each hair type exist, but vary considerably between genders and age group. According to Dr. George Cotsarelis, a professor and head of dermatology at UPenn's Perelman School of Medicine, Even hair color can come into play when it comes to putting a figure on follicle density. In an email to io9, Cotsarelis says that blondes tend to have a higher hair-follicle density, on average, than redheads.
All this is to say that narrowing in on an average number of hair-holes per human is more difficult than you might at first imagine; but people have definitely tried.
According to Jablonski, one of the first researchers to closely examine the nature of hair density and distribution was anthropologist Adolph Schultz. Schultz cast his investigation in an evolutionary light by tallying the number of hair follicles covering the bodies of humans and other primates.
On the scalp, Schultz found that humans harbor about the same number of hair follicles per square centimeter as great apes, averaging 312 and 307, respectively. Gibbons, however — more distantly related to humans, in an evolutionary context — manage to pack just over 2,000 hair follicles per cm2. On the chest, where hair is least dense across all primates, humans average around 1 hair per cm2, where great apes average 90. Gibbons cram in close to 500. If you take the average hair densities for all the human body's various parts and add them up, you wind up with a ballpark figure of five million holes from hair follicles alone.
But for sweat glands, the numbers are even less clear. Debate over the amount and distribution of human sweat glands has carried on at varying degrees of intensity since 1844, when German anatomist Karl Friedrich Theodor Krause first turned his microscope on the human body and asked how many sweat-factories were contained therein. For instance, Krause concluded that the sole of the foot harbored 300 sweat glands per cm2. In the 1960s, cadaver studies concluded that this number was closer to 600 per cm2. When we asked Cotsarelis for a figure, he told us the human body has around 3 million sweat glands. In this recently published article on the mechanisms of sweating during exercise, the authors reported that "the number of sweat glands in humans can [vary] greatly, ranging from 1.6 to 4.0 million." Jablonski told us that "humans have many more sweat gland openings than hair follicles." (Recall that the human body is thought to average 5 million hair follicles.)
To be sure: many of these contradictions likely stem from natural human variations. Others arise out of different counting methods, differences in sample size, or perhaps even variation between sample populations. If there's any consensus to be had when it comes to sweat glands, it's that humans have a lot of them. And in an evolutionary context, that's the most important and intriguing thing of all.
Studies on hair density across a variety of primates have revealed the more massive a species is, the fewer and fewer hairs per unit of body surface it tends to possess. "Considering the absence of effective sweating in monkeys and apes," write primatologists Gary Schwartz and Leonard Rosenblum, this pattern "may represent an architectural adaptation to thermal constraints imposed by the decreasing ratios of surface area to volume in progressively massive primates." But humans serve as a weird exception to this rule.
Much of our hair is considerably finer than that of our ape cousins (even if we often share a similar number of follicles) making us effectively hairless. We also sweat to cool ourselves down — and we do it a lot. According to Jablonski, humans average five times as many sweat gland openings as apes. The fact that hair follicles and sweat glands originate from the same epidermal stem cells, but undergo different processes of differentiation, suggests that their evolution and development are closely linked. Some of the most popular hypotheses maintain that hairlessness and perspiration evolved in tandem as a means of regulating body temperature while traversing the African Savanna on two feet, though Jablonski points out that the reasons for the physiological differences between humans and some of our closest evolutionary cousins are still unclear.
To that end, researchers continue to investigate various tiers of biology in search of answers about human and non-human primates and their assorted hair- and sweat-holes (or lack thereof) — from the genetic level all the way to awesome looking whole-body "sweat maps" like the one picture above. Published in 2011 by Caroline Smith and George Havenith, it's one in a series of some of the most comprehensive maps of "regional sweat rates" ever charted.