Koda

Cats grow fur in a variety of sizes and density throughout their coat, but their individual hairs maintain a mostly constant length through shedding and other natural processes. However, human head and facial hair will grow and grow, for many feet if not cut. So then, why does human hair grow long, while other animals such as cats tend to maintain a constant length?

This question has troubled me for a couple days. Here’s what Wikipedia has to say about Human Hair length:

Individual hairs alternate periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and armpit areas having the longest growth period). The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. The rate of growth is approximately 1.25 centimeters, or about 0.5 inches, per month. Anthropologists speculate that the functional significance of long head hair may be adornment, a byproduct of secondary natural selection once other somatic hair had been lost.

Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (with good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take ~80 months or just under 7 years to grow), and this would explain why long head hair (in both sexes) is viewed as “sexy” even now.

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Natural selection, eh? While I am not wholly satisfied by this explanation, it does provide some interesting details. The fact that socially we are so obsessed with the stuff would lend support to this conclusion. However, I can’t help but think that natural selection quality has been lost along the way of better living through chemistry. Though, perhaps that crazy hair stylin’ provides a social class cue?

I wonder if there is a certain length at which human facial/head hair will tend to stabilize, through the duration of hair growth reaching the rate of random hair loss. One more hairy tidbit:

From Human Anatomy and Physiology:

Although humans appear relatively hairless compared to that of other primates, with notable hair growth occurring chiefly on the top of the head, underarms and pubic area, the average human has more hair on its body than the average chimpanzee. The main distinction is that human hairs are shorter, finer, and less colored than the average chimpanzee’s, thus making them harder to see.

So I went off on hair today. There are far more interesting and pressing topics, and this is not one of them. And for that, dear reader, I do not apologize. Satisfying one’s curiousity is an important calling in life, and I might as well live by my values. And thus, I wrote about hair tonight.