新托福TPO11阅读原文(三):Begging by Nestlings
TPO11-3：Begging by Nestlings
  Many signals that animals make seem to impose on the signalers costs that are overly damaging. A classic example is noisy begging by nestling songbirds when a parent returns to the nest with food. These loud cheeps and peeps might give the location of the nest away to a listening hawk or raccoon, resulting in the death of the defenseless nestlings. In fact, when tapes of begging tree swallows were played at an artificial swallow nest containing an egg, the egg in that “noisy”nest was taken or destroyed by predators before the egg in a nearby quiet nest in 29 of 37 trials.
  Further evidence for the costs of begging comes from a study of differences in the begging calls of warbler species that nest on the ground versus those that nest in the relative safety of trees. The young of ground-nesting warblers produce begging cheeps of higher frequencies than do their tree-nesting relatives. These higher-frequency sounds do not travel as far, and so may better conceal the individuals producing them, who are especially vulnera
ble to predators in their ground nests. David Haskell created artificial nests with clay eggs and placed them on the ground beside a tape recorder that played the begging calls of either tree-nesting or of ground-nesting warblers. The eggs “advertised”by the tree-nesters' begging calls were found bitten significantly more often than the eggs associated with the ground-nesters' calls.
  The hypothesis that begging calls have evolved properties that reduce their potential for attracting predators yields a prediction: baby birds of species that experience high rates of nest predation should produce softer begging signals of higher frequency than nestlings of other species less often victimized by nest predators. This prediction was supported by data collected in one survey of 24 species from an Arizona forest, more evidence that predator pressure favors the evolution of begging calls that are hard to detect and pinpoint.
  Given that predators can make it costly to beg for food, what benefit do begging nestlings derive from their communications? One possibility is that a noisy baby bird provides accurate signals of its real hunger and good health, making it worthwhile for the listening p
arent to give it food in a nest where several other offspring are usually available to be fed. If this hypothesis is true, then it follows that nestlings should adjust the intensity of their signals in relation to the signals produced by their nestmates, who are competing for parental attention. When experimentally deprived baby robins are placed in a nest with normally fed siblings, the hungry nestlings beg more loudly than usual—but so do their better-fed siblings, though not as loudly as the hungrier birds.
  If parent birds use begging intensity to direct food to healthy offspring capable of vigorous begging, then parents should make food delivery decisions on the basis of their offsprings’calls. Indeed, if you take baby tree swallows out of a nest for an hour feeding half the set and starving the other half, when the birds are replaced in the nest, the starved youngsters beg more loudly than the fed birds, and the parent birds feed the active beggars more than those who beg less vigorously.
  As these experiments show, begging apparently provides a signal of need that parents use to make judgments about which offspring can benefit most from a feeding. But the que
stion arises, why don't nestlings beg loudly when they aren't all that hungry? By doing so, they could possibly secure more food, which should result in more rapid growth or larger size, either of which is advantageous. The answer lies apparently not in the increased energy costs of exaggerated begging—such energy costs are small relative to the potential gain in calories—but rather in the damage that any successful cheater would do to its siblings, which share genes with one another. An individual's success in propagating his or her genes can be affected by more than just his or her own personal reproductive success. Because close relatives have many of the same genes, animals that harm their close relatives may in effect be destroying some of their own genes. Therefore, a begging nestling that secures food at the expense of its siblings might actually leave behind fewer copies of its genes overall than it might otherwise.
TPO11-3译文：雏鸟的乞食行为
  有些动物发出的信号可能会给他们自身带来极大危害。一个典型的例子就是歌鸟的雏鸟在它们的父母带着食物归巢时吵闹的乞食行为。这些喧闹的叫声可能会让巢外的老鹰和浣熊听
offspring怎么读到并获取到它们的位置信息，从而致使毫无抵抗能力的雏鸟丧命。事实上，如果在一个盛有鸟蛋的人工燕窝旁播放树燕讨食的录音，这个试验做了37次，有29次的结果都是，这个“嘈杂”鸟巢里的树燕蛋比周围安静的鸟巢里的树燕蛋更早被捕食者掠走或破坏。
  一项关于地面筑巢的黄莺与住在相对安全的树上的黄莺对比的研究进一步为乞食行为的代价提供了证据。地面筑巢的黄莺雏鸟发出乞食叫声的频率要高于树上筑巢的黄莺。这种高频的声音不会传播的很远，可以更好地隐藏在地面鸟巢里单独发出这种声音而容易成为捕食者攻击的雏鸟。David Haskell 制做了一些装有泥制鸟蛋的‘假巢”并放在录音机旁的地面上，播放着地面筑巢或树上筑巢的黄莺的乞食声音。置于树上筑巢的声音旁边的“被注意的”鸟蛋被发现的几率显然要比地面筑巢的黄莺的鸟蛋高得多。
  一个关于乞食行为的假说认为，乞食声已经进化出一种避免引起捕食者注意并及时作出预警的特性：比起那些较少受到捕食者捕食的雏鸟相比，被捕食率高的鸟类的雏鸟需要发出更轻柔频率更好的叫声。对亚利桑那森林里的24个物种的调查所收集的数据证实了这一假说，更多的证据也表明捕食者的存在迫使乞食声变得难以察觉和难以定位。
  既然捕食者可以让雏鸟为食物付出巨大代价，那么乞食的雏鸟们到底可以从这种交流方式
中得到什么益处?可能原因之一是吸引注意力的雏鸟可以准确传达它们很饿而且很健康的信号，它们这么做是为了让父母在同一鸟巢的众多雏鸟中将食物喂给自己。如果这一假说成立，那么我们可以断定雏鸟会根据其他争相引起父母注意的同伴所发出的信号来调整它们信号的强度。人们做了一个实验，将饥饿的知更鸟雏鸟放进那些正常喂养的同类的巢中，饥饿的雏鸟会发出比平时更响亮的乞食声，而其他喂养的很好的雏鸟们也是如此，尽管没有饥饿的雏鸟们叫的响。
  如果鸟父母是根据乞食声音的响亮程度来给那些健康且更积极乞食的幼鸟喂食，那么鸟父母应该是根据幼崽乞食声来分配食物的。所以，如果你将树燕雏鸟带离鸟巢一个小时，并将一半雏鸟喂饱同时不让另外一半吃东西，当把雏鸟们放回巢时，饥饿的雏鸟们会比已经吃饱的雏鸟们叫得更响，而鸟父母会给积极乞食的雏鸟们比不积极的雏鸟喂更多的食物。
  这些实验表明，乞食行为很明显为鸟父母提供了一个判断谁能吃的最多的需求信号。但是问题又出现了，为什么雏鸟不在它们不饿的时候大声乞食呢?如果它们这么做，就可以保证更多的食物，也就能更快的成长或者拥有更壮的身体，怎么说都是有利的。这个问题的答案显然不是因为过分乞食会消耗更多的能量——损耗的能量相比于其潜在能得到的热量来说只
是很小部分——而是因为任何这么做成功骗取食物的雏鸟会带来跟它们拥有相同基因的同伴们造成危害。
一个物种成功延续它的基因所产生的影响要比它自身繁殖所带来的影响大的多。因为近亲中有很多相似基因，动物伤害它们的近亲的同时很可能会摧毁一些它们特有的基因。因此，一个乞食的雏鸟如果以牺牲它的同类为代价来获取食物，事实上可能它能保存下来的基因要远远少于相反的做法。
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