Since the 1930s, the idea that creatures might advertise their " fitness " as mates through symmetry has been around. It rests on the notion that factors ranging from bad genes to coming off worse in too many fights will lead to asymmetries others can see. And over the past few years, experiments with animals and humans seemed to support the idea that symmetry in features such as plumage and facial characteristics influences mate selection.
But no one had actually checked that the animals in these experiments can actually see the subtle differences in symmetry, typically of between 1 and 2 percent. Now the first such experiment has been carried out. The results are worrying for advocates of symmetry detection as a powerful factor in mate selection.
John Swaddle, an ethnologist at the University of Bristol, performed the experiment using wild starlings, which have excellent eyesight. He trained them to hit keys marked with bar patterns with varying levels of asymmetry in order to receive food rewards. This showed the starlings could easily detect asymmetries of between 5 and 10 percent. But at between 1 and 2 percent, their performance plummeted to no better than random guessing.
" This suggests that the levels of asymmetry that birds encounter in nature will often be just too small to be detected, " says Swaddle. He says experiments that linked small asymmetries to fitness may have produced misleading correlations, and says the only direct evidence that birds use asymmetry as a cue to mate fitness involves levels of asymmetry of at least 10 percent. " I think signaling by asymmetry will probably only occur when species show such very large asymmetries—and this doesn’t occur that often. "
This raises questions about research suggesting that humans are influenced by visual asymmetry in their choice of partner. Michael Butt of the Perception Laboratory at the University of St. Andrews, who has carried out such experiments, concedes that no one has ever checked if humans have a threshold to asymmetry detection. He thinks a conclusive test of this would probably involve manipulating complex three-dimensional images: " It would be a very difficult experiment to do "
Burt adds that the abilities of animals to detect asymmetry in bar patterns may not reflect their talent for spotting asymmetries in body shape. " The visibility of an asymmetry may well depend on its type, " he says. Swaddle agrees, and is planning further experiments. " But I suspect that asymmetry is used as a visual cue less often than most people appear to presume, " he warns.
According to the passage, which of the following is TRUE（）
A. Swaddle thinks asymmetry and fitness are associated
B. Asymmetry is generally visible
C. Species use asymmetry as a cue to mate fitness
D. Nobody has checked human’s dependence on asymmetry
参考答案：D
解析：
是非判断题。第五段第二句提到“…no one has ever checked if humans have a threshold to asymmetry detection.”(没有人检验过人类是否能感知非对称性)，因此D项正确。第四段第二句提到“He says experiments that linked small asymmetries to fitness may have produced misleading correlations…”(他说那些将细微的非对称性与生物择偶时考虑的适合性联系起来的实验会让人们误以为两者具有相关性)，因此不能笼统地说非对称性与配偶适合性有关联，故排除A项；末段第二句提到“The visibility of an asymmetry may well depend on its type”(非对称性的可视度也许取决于它的类型)，也就是说不能脱离具体的非对称类型谈论其是否可视，例如文中的欧掠鸟对一定比例以下的非对称性便无法感知，故排除B项；第四段末句提到“I think signaling by asymmetry will probably only occur when species show such very large asymmetries—and this doesn’t occur that often.”(我认为只有当物种显示出非常大的非对称性时，非对称性才能被作为配偶选择时的信号，但是这种情况很少发生。)，也就是说非对称性如果没有达到足够大的程度，是无法被当作择偶信号的，故C项的说法过于绝对。