because of mothers' greater needs for vitamin D._
To turn to another curious feature of human hair, when did you last see a chimpanzee getting a haircut? Human head hair differs from that of apes in that it never stops growing. If the hair follicles on the human head behaved like those of chimpanzees, they would follow an orderly cycle in which each would grow a hair for several weeks; the hair, after reaching a certain length, would then be shed, and the follicle would grow another hair. With people, this cycle has been lengthened from weeks to years. The reason that uncontrolled hair growth was favored by natural selection may have been that it offered a means of signaling copious amounts of social information. In every society in the world, people spend an inordinate amount of time in cutting, shaping, braiding, plaiting, curling, straightening, decorating and otherwise gussying up the appearance of their hair. Much the same is true of men's beards and mustaches. To let one's hair grow unkempt is a sign of the outcast, or that one is in deep mourning. Trimmed hair sends a variety of important signals about the wearer's health, wealth and social status. But for all this social signaling activity to occur, humans had first to abandon the self-maintaining
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