The most common distinction between hair and fur that people tend to make is usually between you and your pet. You have hair on your head, and your dog Fido has fur all over its body. Easy enough, right?
Well, actually, it's kind of complicated. "The way I think a lot of people understand fur versus hair is in the density of follicles," says Ross McPhee, Curator of the Department of Mammalogy at the American Museum of Natural History. "So for fur-bearing animals—which were converted into coats and so-forth—people always wanted it to be something that was very dense, so there’s the idea therefore that fur is dense, and it certainly is in [the] kinds of fur-bearers that we use for this purpose. Whereas hair is not so dense. There's less of it." The reason this distinction doesn’t work, however, is that humans have a similar follicle density to any other ape, and an adult scalp has the same follicle density as a mouse.
Chemically, it's all keratin—the same protein substance that also makes up skin, feathers, nails, hoofs, claws, and horns—that grows on mammals (either all over or just in certain places). So, MacPhee says, "fundamentally, they are all the same thing. Naturally, there’s specialization, just as there are in tissues all over the body."
There are three main types of hair: ground hair, guard hair, and whiskers. Two of them, ground and guard, are classified as fur. Ground hair is used primarily as insulation and is soft, while guard hair is for protection from the elements and tends to be coarse. Because it's in-between ground and guard, human hair can be reasonably called fur.
No matter how human hair is classified, however, the distinction between hair and fur shows up in the third category: whiskers.
Whiskers, although hair, are categorically not fur. Whiskers have a few differences, including that they tend to be longer and stiffer (but this is not always the case), and they're important sensory organs. "Every follicle has a certain amount of innervation," MacPhee explains. The way it works for whiskers is that they have mechanoreceptors, which means that when the whisker is disturbed by hitting an object, a signal is immediately sent back to the brain and analyzed there. Which is why whiskers are utilized by all kinds of mammals as a sensory apparatus in their environment. A manatee's face [PDF], for instance, is exclusively whiskers because every follicle has these key (and distinct) features. These types of whiskers are known as vibrissae—and humans don't have them. "The whiskers that you find on a male are just hair," MacPhee says. "They’re not richly endowed with these sensory nerves. Whether they’re on the muzzle, or the eyebrows, or on other places on the body. Very frequently, mammals have vibrissae on their wrists and ankles. Although cats and dogs have good vision, it’s clear they’ve retained vibrissae and some degree of information to fine tune their body."
Ultimately, though, it might just be a word thing. "I’m not sure that the hair versus fur distinction is always made in other languages," MacPhee says. "Humans universally only have hair, right? Not fur, even though there is no important way to distinguish the two. The differences are arbitrary."
THE GROWTH MYTH
“Wait a second!” you scream before you admit that you have fur on you. “I thought the common difference is that hair keeps growing while fur has a set length at which it stops!” That's not true: Human hair doesn’t keep growing and growing. All hair—whether it’s on your human head and arms or on the body of a chimpanzee or on beloved Fido—has a growth limit based on genetics, so your shorthaired cat will stay shorthaired even if you don’t get its fur trimmed.
The myth of the difference between hair and fur is perpetuated by a misunderstanding of the hair growth cycle in the body that regulates hair length. The “anagen” period of the cycle is the phase of constant hair growth; the “catagen” phase is a transitional time when the body tells the hair to begin to stop growing, shrinking the strands themselves and cutting the roots of the hairs so new strands won’t be produced; and the “telogen” phase is when the hair follicle rests and no new growth occurs. This leads to the “exogen” phase, in which the hair falls out to start the cycle over again.
So in humans, hair will stop growing after the cycle runs through each phase. It’s just that the cycle in you as a human is longer than, say, Fido’s hair growth cycle. The anagen period of active growth on a human scalp could run anywhere from 2 to 7 years (taking into consideration other factors such as baldness), while the telogen period in which the hair on your scalp is dormant could go up to 100 days.
The cycle itself is relative not only to different species, but also to different areas of the body, which explains why the hair on your arms and legs isn’t the same length as the hair on your head. For instance, the same 2- to 7-year anagen phase on your head lasts 30 to 45 days on your arms and legs.
On animals, the hair all seems to be a fixed length to us because their cycle is relatively shorter when compared to ours, and so the mistake of differentiating fur from hair is made. It becomes clearer when you consider that longer haired dogs just have a longer period of hair growth than something like a Lab, which has a shorter cycle.