经济学人下载:性选择与进化 美丽的雌性

Sexual selection and evolution
性选择与进化

My pretty maid
美丽的雌性

A glamorous raiment sometimes helps a female bird as much as it helps a male
华丽的外表有时给雌性的益处和雄性一样多

HOW the peacock got his tail is one of natural history's “Just So” stories that biologists like to think they have cracked. His tail is for showing off to the ladies just how fit he and his genes are. A less-than-perfect tail means no offspring. Genes for spectacular male tails are thus preserved and promoted over the generations in a process that is called sexual selection.
孔雀是如何得到它的尾巴，生物学家乐意认为这是他们已经破解的自然界奥秘故事之一。它的尾巴是向雌性炫耀它和它的基因是多么优秀。稍有瑕疵的尾巴意味着没有后代。受世代性选择的过程的影响，雄性华丽的尾巴的基因因此得到保留并进化。

There is, though, a problem with this story. Peahens, though not as showy as cocks, are by no means dowdy. Their heads have fetching crests, and their necks are a beautiful, iridescent blue. Such flummery is costly to grow and likely to attract predators. If you do not have to strut your stuff to get a mate, why do you need it?
然而，这个故事有一个疑问。雌孔雀虽不如雄性那样艳丽，但也绝不俗气。它们头上有迷人的羽冠，以及光泽艳丽的蓝色脖子。这种装饰物会威胁生存，可能招致天敌。如果你不是为了求偶炫耀，那你为什么需要它？

Even more confusingly, in many species both sexes are equally showy—the Gouldian finches overleaf, for example (the female is on the right). So, though no one thinks the theory is incorrect, as far as it goes, it clearly does not go far enough. To understand things better James Dale of Massey University, in Auckland, New Zealand, and his colleagues have therefore examined the plumage of both sexes of all 5,983 species of passerine bird (peafowl, not being passerines, are not among them), and compared them in exquisite detail.
更不解的是，许多物种雄性和雌性都绚丽——例如上图的彩虹鸟（右为雌性）。虽然没人认为这个理论是错的，但就现状而言，显然不够正确。为了了解更多，新西兰奥克兰区梅西大学的詹姆斯戴尔，和同事调查了总共5,983种雀形目鸟类雌雄的羽毛，并比较了他们的细微细节（孔雀不是雀形目，不在调查之中）。

First, the team had to devise a way to deal with the 11,966 types of plumage they had set out to examine. Using the “Handbook of the Birds of the World”, regarded by ornithologists as the definitive work in the field, they picked six points on a bird's body (nape, crown, forehead, throat, upper breast and lower breast) and performed a spectral analysis of each to measure how red, green and blue it was. The average let them plot each plumage as a dot on a graph with three axes—red, green and blue.
首先，团队设计了处理将要调查的11,966种羽毛的方法。用《全世界鸟类手册》——被鸟类学家视为该领域最为完整可靠的资料，他们在鸟身上挑了6处（颈部,羽冠,前额,喉咙,前胸和下胸），然后用光谱分析来测量红色，绿色和蓝色。在红绿蓝三维坐标系上用小点绘制了平均羽毛颜色的散点图。

To translate this into a “showiness” score, the researchers started from the fact that, despite the exceptions, showiness is still more a male than a female phenomenon. They therefore calculated, as a proxy for showiness, a “maleness” score for each dot, regardless of which sex it represented, by counting the sexes of its nearest 120 neighbours in the graph (ie, the nearest 1% of dots). They then ran these scores against characteristics, such as the size of a species, its habitat and its pattern of family life.
为了转化成“绚丽”分数，研究者从事实发现：除了个别例外，炫耀现象雄性多余雌性。因此他们计算出每一“雄性”点得分：无论雌性还是雄性有绚丽的装饰物，通过计算它附近120个两性点（将近1%的点）。然后将这些分数对比特征，比如物种的大小，栖息地以及生活习性。

Their first observation, as they report this week in Nature, was that in species where a few males monopolise all the females, with a consequent lack of male involvement in parental care, males were more colourful than females. This is what the theory of sexual selection would predict. What it would not predict in its simple form, though, was a second finding—that females in co-operatively breeding species (those in which, for lack of other opportunities, several females collaborate to raise the young of only one of them) are more ornamented than those in which all adult females have a chance of breeding. In this case it is females who are competing for the right to reproduce, thus putting themselves in a more male-like position.
他们第一次观察报告结果出版在本周《自然》上：在少量雄性配对所有雌性的物种，在亲代抚育中缺少雄性角色的缘故，雄性比雌性更艳丽。这是性选择理论做出的预测。然而，这个简单形式的理论不能预测的是第二个发现：在合作哺育的物种（缺少雄性情况下，几个雌性合作只能抚养其中一个后代），雌性比有机会和雄性一起繁殖后代的雌性更花哨。这个例子中正是雌性完成了繁殖的任务，因此他们扮演了雄性的角色。

Another widespread belief Dr Dale and his colleagues confirmed is that tropical species are more colourful than those from temperate climes. But again, there was a twist—the effect was much more marked in females than in males. Something about the tropics favours colourful females. It may be that tropical birds, which face more intense competition for food and nesting sites than temperate ones do because the tropics have more species, form more stable and collaborative pair-bonds than do temperate birds. In these circumstances males also need to be choosy, and females competitive. Selection for gaudy plumage therefore works in both directions.
戴尔博士和同事证实的另一个普遍的观点是，热带气候里的物种比温带的更鲜艳。但有一点不同——这种效果雌性比雄性表现得更明显。热带环境偏爱外表鲜艳的雌性。这可能是热带鸟类形成了更稳定更协作的一夫一妻制，因为热带有更多物种，热带鸟类比温带鸟类存在更激烈的食物竞争和巢穴竞争。这种环境下雄性挑选配偶，而雌性去竞争。因此绚丽羽毛在自然选择上往两个方向发展。

The final effect the researchers found was that big species are more colourful than small ones. That is true of both sexes, and probably reflects the fact that bigger birds are more difficult prey and thus have less need to hide. When released from the threat of predation, then, females tend to be gaudier. That suggests gaudiness is always good when it can be got away with (for even in a promiscuous species, pretty females are likely to be at an advantage to ugly ones)—and probably explains the decorated necks and heads of peahens, which are among the biggest of birds.
研究人员发现的最后一个结果是，体型大的物种比体型小的更鲜艳。雌性和雄性都符合这点，也反映了大型鸟不容易被捕杀故而不必隐藏的事实。一旦没有天敌的威胁，雌性就变得更花哨。这表明在没有天敌威胁下，华丽点总是好的（即使在一个滥交的物种，漂亮的雌性比丑的可能更有优势），也解释了为何雌孔雀拥有艳丽的脖子和头部，因为它是体型最大的鸟类。

Putting these results together, then, suggests that what is happening in the arena of sexual selection is as much to do with females as with males. Just as females are half the world, so the conventional explanation of the peacock's tail, though not wrong, is only half the story.
这些结果一起暗示了性选择过程中雌性和雄性变化一样多。就像雌性占世界的一半，孔雀尾巴的传言虽然不是错的，但只是故事的一半。