Why Humans are Relatively Hairless

(This is derived from “Androgens in Human Evolution,” Rivista di Biologia / Biology Forum 2001; 94: 345-362.)

Testosterone reduces hair growth. Estrogen reduces hair growth. DHEA increases hair growth. Human males and females produce more testosterone than chimpanzee males and females respectively. Chimpanzees produce much more DHEA than humans. Hence, we are relatively hairless.

In the following abstract, note that testosterone and estrogen both reduce hair formation. Therefore, women are less hairy than men. When “fetal calf serum” was added to the growth culture medium, hair growth increased. The “fetal calf serum” contains DHEA.

It is testosterone that reduces hair growth in humans.

Arch Dermatol Res. 1990;282(7):442-5.

Organ culture of human scalp hair follicles: effect of testosterone and oestrogen on hair growth.

Kondo S, Hozumi Y, Aso K.

Department of Dermatology, Yamagata University School of Medicine, Yamagata City, Japan.

Whole human scalp hair follicles were cultured. The follicles were dissected from skin pieces of normal scalp and put into 1.5 ml of incubation medium in a closed 5 ml glass tube under an atmosphere of 95% O2 and 5% CO2. The tube was rolled at 15 rpm at 36 degrees C. Remarkable hair growth was noticed for 7 to 8 days. Hair root sheaths also grew with the hair shafts. The structure of the hair bulbs was well maintained for at least 6 days, and then the hair matrix cells started to degenerate. Fetal calf serum was not essential for hair growth in vitro, but increased the growth rate slowly. Testosterone and oestrogen inhibited hair growth in vitro to a similar extent. The minimum effective doses of both hormones to suppress hair growth were around 5 ng/ml, which corresponds well to the normal plasma level of testosterone in adult males in vivo, suggesting that scalp hair growth may be critically controlled by testosterone in adult males.