“Adam catched Eve by the fur below,” goes an old English song from the swampy fens of Norfolk, “and that’s the oldest catch I know.” The ditty may not be suitable for work or for younger audiences, but it points to a tricky matter of trichoid distinction: namely, the finer-than-frog’s-hair distinction between hair and fur.
Frogs have no hair, of course, not that it mattered to the coiner of that fine Southern phrase. Indeed, only mammals have hair, a structure that, like our fingernails and the horn of the rhinoceros alike, is made of the protein called keratin. Hair grows from follicles deep within the skin, in greater or lesser profusion depending on the individual mammal. Creepily, like nails, it grows for a time even after the mammal has died.
By linguistic convention, the Norfolk songwriter notwithstanding, humans grow hair and not fur. That convention has scientific backing, for in the strictest biotechnical terms, fur has two required components. If you can get close enough to a gorilla or a mountain lion to inspect it, you will see, first, a thick undercoat called ground hair, atop which lies a finer and sparser layer made up of what are called guard hairs. An animal that has these two layers of hair is said to possess fur, whereas an animal that does not has hair. A young goat has no ground hair, while some varieties of lamb have no guard hair, and for each quality the hair of kids and lambs is prized. Some dog breeds have guard hairs, some not. Some, such as the xoloitzcuintli, have scarcely any hair at all, the result of genetic mutation.
A polar bear, on the other hand, has plenty of both, which is an evolutionary necessity: Ground hair primarily serves to regulate body temperature, while guard hair serves as a kind of natural downspout to help the body shed water in the form of rain or snow. Poor humans, we stand shivering in the cold, for, lacking ground hair—and therefore lacking fur—we do not have that advantage of temperature regulation, leading us, among other things, to seek the fur of animals that possess it when making our homes in colder climates.
There are other differences between the two kinds of hair that make up fur, but these tend to be matters of degree rather than sharp divisions. For one thing, ground hair grows to a certain length and then stops, while guard hair keeps growing, for which reason dogs go to the groomer and humans to the salon or barbershop. Human guard hair is finer than animal guard hair—as fine, one might say, as a frog’s.
The distinction between hair and fur, then, involves a little nitpicking—something we humans have not had to do, strictly speaking, for a very long time. Mutations in the MC1R gene, which regulates skin color, resulted in human furlessness—ground hairlessness, that is—about 1.2 million years ago, giving the lice that plagued our forebears nowhere to nest. At that time, paleontologists calculate, there were already some 14,000 breeding proto-humans, our ancestors, the naked descendants
of a furry Eve.