Why do Bedouins wear black robes when everyone knows that black is a strong heat absorber? A widely reported study has indicated that, overall, the black robes made desert dwellers no warmer than do white garments. However, that finding suggests an answer to the question that now turns out to be too simplistic.
If black robes are no disadvantages in the heat, then it would seem economically sensible for Bedouins to make clothes from the hair of their traditional black goats. This begs the original question, for why should goats bred to survive in a hot and arid land have black coats?
Razi Dmi'el, Avi Prevulotzky, and Amiram Shkolnik of Tel Aviv University point out that the question is asked the wrong way around. It is not the long hot season that is critical for the goats but the short, cold winter when food is scarce and bodily energy must be conserved. Black goats do indeed absorb more solar radiation and evaporate more water in summer than do white goats. But they survive the winter better.
In the original study published in Nature in late January, Shkolnik, his colleague Arien Borut, and C. Richard Taylor and Virginia Finch of Harvard University said that, while black Bedouin robes absorbed 2.5 times as much solar radiation as white robes, the wearer of black was no warmer than the wearer of white. They attributed this to enhanced ventilation -- "a bellows action" or "chimney effect" -- or some unknown process that compensates for the greater absorbtion of black.
At least one critic -- E. Mullinger of Windsor, England, writing in New Scientist -- has objected that the robes do not have large enough neck openings for a bellows effect. Also, he says, Bedouins wear white when crossing open desert, wearing black mainly in the shade.
In their study on goats, subsequently published in Nature, Shkolnik, Dmi'el, and Prevulotzky suggest that protection against winter cold may be as important for the Beduoins as for their animals.
They explain that heat and water are not critical factors for the goats, which are equipped physiologically to go long periods without drinking. But the animals are ill adapted for winter. Their loose and shaggy coats are poor insulators. In this case, the better absorbing power of black may be a crucial advantage.
Referring to the earlier study of black robes worn in summer heat, the three scientists say: "However, in the light of our measurements, it would be interesting to determine whether during the winter in the desert, the black robe has a role similar to that we assign to the black fur of the goat."
Long-established adaptations to local conditions are rarely simple. As the three researchers note, "One should take into consideration the entire climatic cycle." And, you could add, don't forget the complexity of an interdependent pastoral community in which both people and their animals have to do the best they can in that climate.