Fact #1: Hair grows on every inch of our bodies except for: (a) palms of our hands, (b) soles of our feet and (c) our lips. The fine, fuzzy hair that is hardly noticeable is called vellus hair. The longer, darker and noticeable hair is called terminal hair.
Fact #2: On average, we grow 590 miles of hair during our lifetimes.
Fact #3: All hair on our bodies has three phases: (1) growth, (2) intermediate/transition , and (3) resting . During the growth phase hair grows about 1/2 inch per month which is about six inches per year. In the transition phase hair stops growing and hair follicle begins to wither and die. During the resting phase the hair follicle renews itself and the strand of hair falls out. After a period of rest, growth phase starts up again. and a new hair strand is grown. So, hair growth runs in cycles, but in general, the human hair cycle is staggered so we don’t lose a bunch of hair all at once. In aggregate, hair on our heads is in the growth cycle 85% of the time. Other animals – like dogs and horses – are more seasonal in their hair cycles.
Fact #4: Hair grows long on our heads because it has a long growth phase: 2-6 years, largely determined by genetics, meaning that hair on our heads can grow 3 feet or so. Hair on other parts of our bodies has shorter growth phases than the hair on our heads – usually just a few weeks, but it varies by the location of the body hair.
Fact #5: Our hominid ancestors used to be much hairier than modern humans. Why did we shed our furry appearance? The most accepted evolutionary explanation is that as we began to walk upright and exited cool forests for the hot savanna having less hair was an advantage in a hotter climate – we could hunt and gather during the day when furry predators were napping in the shade. Our ability to sweat and quickly dissipate heat was a great survival advantage. We aren’t totally naked – we retained long hair on our heads because it shields the direct force of the sun from our heads as we walk upright. It also helps keep a cool layer of air between sun-heated hair and our scalps. Curly hair does this the best. Another leading theory as to why aren’t furry is to reduce the prevalence of external parasites that routinely infest fur.
Fact #6: Men are hairier than (most) women because men have more of androgenic hormones, such as testosterone, which cause vellus hair follicles to become terminal hair follicles (like that on our heads) at puberty. Women actually have the same amount of hair follicles as men on their bodies, but most of their body hair is of the vellus variety. Women given testosterone develop more terminal hair on their bodies and appear more hairy (absent shaving).
Fact #7: There may be an evolutionary reason for men being hairier. Reason one is that it signals the presence of testosterone and virility. Reason two – like a lion’s mane, a man with a beard is perceived by both men and women as being more aggressive and dominant. As such, men with beards may have success competing with beardless men for female attention by intimidating the competition. Note that studies of preferences show that women are all over the board as to whether they actually find beards attractive, leading some experts to think that beards are really just grown to impress other men.
Fact #8: Nearly half of men (and a quarter of women) will be affected by hair loss by the time they’re 50. Men go bald for a variety of factors including illness, medications, chemicals and toxins. The most common source of baldness is male (and female) pattern baldness which is largely genetic and is due to hair follicle sensitivity to a type of testosterone. It’s been hypothesized that baldness may have an evolutionary advantage. As a sign of age, baldness signals younger males that they’re less likely to be competition, and females that they’re less likely to be aggressive or prone to seeking multiple mates. Query – what about bald men with beards? Intimidating or not?
Fact #9: We lose about 100 hairs a day on average.
Fact #10: Why hair turns grey IFOD: http://www.theifod.com/does-stress-turn-hair-grey/