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 sexes 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 ______.
A. between male and female volunteers under all conditions
B. when one part of the skin was shaved while the other remained hairy
C. when some bugs were starved while the others were well-fed
D. when some volunteers were allowed to see while the others
weren’t