The features human hair shares with other mammal’s fur follicles
Humans have hair. We’ve got hair on our heads, faces, backs, armpits, shins, etc. And yet with all those follicles, we don’t consider ourselves especially fuzzy, much less furry. While some mammals are obviously shaggier than others, how different is the hair on a dog’s head from the coif on ours? After all, each thin strand is made up of the same keratin, but somehow the styling on sheep looks pretty different from what’s on us. There’s not actually a clear binary difference in hair versus fur, as hair types are actually spread along a bit of a continuum, each with different roles, some of which humans have given up.
The most distinct hair type is vibrissal. These hairs, found on many mammals, are more commonly referred to as whiskers. Unlike when we refer facial hair as “whiskers,” these hairs actually have movement-sensitive nerves inside. Each touch to a whisker then triggers the nerve inside the follicle, providing feedback about an animal’s surroundings. It’s a significant shift from what’s thought to be the original point of hair, which was to keep creatures warm.
Retaining body heat helps endothermic animals like mammals keep their temperature stable. The hairs most specialized for this job are the undercoat, or downy hair. These hairs are usually curly and thin, coming together to form a layer of insulation next to the skin. They trap warm air in, which is probably why you don’t find them sprouting around the base of the hair follicles on humans, since our hair seems to have undergone an overhaul to help us lose heat a little over a million year ago. For other animals, like otters or musk oxen, these lightweight hairs allow them to stay warm in cold conditions.
On top of the undercoat are guard hairs, which is what more or less what humans are most familiar with. On most mammals, these hairs form an outer layer of protection, helping keep the undercoat dry and intact. It’s also where most stripes and spots turn up, although on humans we usually settle for one color of hair, guarding… our scalp? Sure, it’s nice to keep your skull warm, but at this point it’s theorized that the long locks on our heads have evolved to emphasize our health for possible mates and rivals. In the end, the hair isn’t that different, it’s just evolved to have longer anagen cycles, growing for longer periods of time than most other follicles and allowing us to make use of braiding and ponytails without an actual tail.
Soft to scratchy sub-types
It might seem odd that out of the three major types of hair, humans basically have one, but clearly have different kinds of hair growing on different parts of our bodies. The hair on your head isn’t a perfect match for what’s in your armpit, which brings up some of the finder details in how we categorize our fuzz, including so-called “peach fuzz” itself.
Vellus hair is the thin, wispy hairs that are easy to overlook from a distance. It covers almost the entire body, and like our missing undercoat, actually helps with temperature regulation. Rather than trap warm air against our skin though, vellus hair usually helps wick warm sweat out to speed up cooling. Structurally, it’s very similar to the hair on your head, just shorter due to shorter anagen cycles.
The real contrast lies in the androgenic hair that turns up in patches across our bodies after reaching puberty. This hair grows thanks to “male” hormones in the body, and is often slightly thicker than what grows on your head. The benefits of these tufts of post-pubescent hair aren’t completely understood, but they help with displaying sexual maturity as well as sharing smells originating from our apocrine sweat glands. It’s not quite as handy as a good whisker, but our pseudo-fur is clearly designed for something other than just staying warm.
Source: Fur, Wool, Hair: What's the Difference? by Mindy Weisberger, Live Science