Much ink has been spilled on the question of human hairlessness: why, as Desmond Morris put it in the title of a book published in i967, humankind is "The Naked Ape." Less attention has been paid, though, to the fact that humans are not really hairless at all. Per square centimeter human skin has as many hair follicles (毛囊) as that of other great apes. The difference is in the fineness of the hair. These fine human hairs do not seem to perform any of the functions of their counterparts in more hairy species (insulation and signaling). So what are they for
That is a question addressed by Isabelle Dean and Michael Siva-Jothy of Sheffield University, in Britain, who took tremendous persuasion and effort to recruit 29 volunteers--19 men and 10 women. Each had a patch of skin on one arm shaved, marked with a pen and surrounded by petroleum jelly, and a patch of the same size on the other marked and surrounded, but not shaved. The bugs had been fed a week and then starved, so they were eager to eat. Volunteers were asked to look away while a researcher put a bug on one of the skin patches. The volunteer was then supposed to record, using a press-button counter, the number of times he perceived the insect moving on his skin.
The difference was significant. When the bug was on a hairy patch it was detected, on average, every four seconds. When it was on a shaved patch, more than ten seconds elapsed between detections. Moreover, the bugs seemed to find it harder to locate a good spot to bite when they were surrounded by hair. Though no volunteer was actually bitten, because the vigilance of the watching researcher meant the insects were removed when they were ready for biting, bugs on hairy skin took about a fifth longer than those on shaved skin to attempt to bite their hosts.
In both cases men were better off than women when the bugs were released on unshaven patches of skin, though there was no significant difference between the es when they were shaved. The conclusion is that one reason human body hair has not disappeared completely is to warn and protect us from the attention of hostile insects. Significant difference was found ______.