HIV is common in sub-Saharan Africa. Sexually transmitted bacterial and fungal infections increase the chance of HIV infection. Melanin can prevent the penetration of skin and mucus membranes by microorganisms, and soluble melanin can inhibit HIV replication. We suggest that melanin may reduce the incidence of HIV infection through venereally acquired skin lesions, thus reducing the risk of sero-conversion and slow the progress to AIDS. Indigenous sub-Saharan peoples are highly melanized, but there is pigment variation between populations. We show that skin reflectance, a negative correlate of melanin, is positively associated with adult rate of HIV in sub-Saharan countries. There is no such relationship in populations outside sub-Saharan Africa. We suggest that melanin concentration in black people may correlate with resistance to HIV infection.
Since HIV is a relatively new disease it seems unlikely that it is the reason behind the high levels of eumelanin in Africa, but it may play a role in protecting against a wide range of sexually transmitted diseases. If people with high levels of dark melanin are better protected against STDs then they may also have higher levels of testosterone since this androgen affects sex drive. Here is another abstract showing a link between pigmentation and testosterone:
IT is well known that there is an increased pigmentation of the skin in certain areas of the body in pregnancy. It has also been established that the occurrence of a malignant change in benign pigmented tumours or moles is rare before puberty in both sexes. Furthermore it is accepted that the growth of malignant melanomas and the formation of metastases is accelerated during pregnancy1. All these facts suggest that the activity of the melanocyte is under hormonal influence. Edwards, Hamilton, Duntley and Hubert2 reported that the human male castrate has a reduced amount of melanin in the skin. Treatment of these men with testosterone usually increased the melanin content in all areas, although the response was of small degree. Kupperman3, working on the male golden hamster, showed that testosterone increases the pigmentation of the skin, whereas castration causes a reduction in pigmentation. Wheeler et al. 4 found by macroscopical and microscopical observations that topical application of testosterone to the nipples and areolæ of immature castrated male guinea pigs produced no increased pigmentation. The present investigation was designed to investigate histochemically the effect of testosterone and orchidectomy on the activity of the melanocyte.
If there is a connection between melanin and testosterone then it may explain why people associate spring with mating since there is significantly more sunlight - and therefore opportunities for tanning - than there is in winter.