机翻,未校对
动物发出的许多信号似乎都对信号器造成了过度破坏的成本。一个典型的例子是,当父母带着食物回到巢穴时,雏鸟发出嘈杂的乞讨。这些响亮的哔哔声和窥视声可能会将巢穴的位置告诉鹰或浣熊,导致手无寸铁的雏鸟死亡。事实上,当在一个包含鸡蛋的人工燕窝中播放乞求树燕的录音带时,在 37 次试验中的 29 次中,那个“嘈杂”的巢中的蛋在附近安静巢穴中的蛋之前被掠食者夺走或摧毁。
乞讨成本的进一步证据来自一项研究,研究了在地面筑巢的莺与在相对安全的树木中筑巢的莺的乞讨叫声的差异。地面筑巢莺的幼鸟发出的乞求哔哔声比它们在树上筑巢的亲戚发出的乞讨声更高。这些较高频率的声音不会传播得那么远,因此可能会更好地隐藏产生它们的个体,这些个体特别容易受到地巢中的捕食者的攻击。大卫·哈斯克尔用粘土蛋创造了人工巢穴,并将它们放在地上的录音机旁边,录音机播放着树巢或地巢莺的乞求叫声。被树巢者乞讨的叫声“宣传”的鸡蛋被咬的次数明显多于与地面巢穴有关的鸡蛋。
乞求呼叫已进化出降低其吸引捕食者潜力的特性的假设产生了一个预测:与巢穴捕食者较少受害的其他物种的雏鸟相比,经历高巢捕食率的物种的幼鸟应该产生更高频率的更柔和的乞讨信号。这一预测得到了对亚利桑那州森林 24 个物种的一项调查中收集的数据的支持,更多的证据表明捕食者的压力有利于难以检测和确定的乞讨呼叫的演变。
鉴于掠食者可以使乞讨食物变得昂贵,乞讨雏鸟从它们的交流中获得什么好处?一种可能性是,一只吵闹的幼鸟提供了它真正饥饿和健康状况的准确信号,这使得倾听的父母在通常可以喂食其他几个后代的巢穴中给它食物是值得的。如果这个假设是正确的,那么雏鸟应该根据它们的同伴产生的信号来调整它们的信号强度,这些同伴正在争夺父母的注意力。当实验被剥夺的小知更鸟与正常喂养的兄弟姐妹一起放在巢中时,饥饿的雏鸟比平时更大声地乞求——但他们吃得更好的兄弟姐妹也是如此,尽管没有饥饿的鸟那么大声。
如果亲鸟使用乞讨强度将食物引导给能够进行剧烈乞讨的健康后代,那么父母应该根据后代的呼唤做出送餐决定。的确,如果你把小树燕从巢里拿出一个小时,喂了一半,另一半饿死,当鸟儿被重新安置在巢里时,饥饿的幼鸟比喂食的鸟更大声地乞求,而父母鸟主动乞讨的人比乞讨不那么激烈的乞讨者多喂食。
正如这些实验表明的那样,乞讨显然提供了一种需求信号,父母可以利用这种信号来判断哪些后代可以从喂养中受益最多。但是问题来了,为什么雏鸟不那么饿的时候不大声乞求呢?通过这样做,他们可能会获得更多的食物,这应该会导致更快的生长或更大的体型,这两者都是有利的。答案显然不在于夸大乞讨所增加的能量成本——这样的能量成本相对于卡路里的潜在收益来说是很小的——而是任何成功的作弊者都会对其兄弟姐妹造成的伤害,因为兄弟姐妹彼此共享基因。一个人在传播他或她的基因方面的成功可能不仅仅受到他或她自己的个人繁殖成功的影响。因为近亲有许多相同的基因,伤害近亲的动物实际上可能正在破坏它们自己的一些基因。因此,以牺牲兄弟姐妹为代价来获取食物的乞讨雏鸟实际上可能会留下比其他情况更少的基因拷贝。
原文
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 vulnerable 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 parent 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 offspring’s 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 question 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.
来源 TOEFL TPO 11 R 03