School of Biological Sciences, University of Bristol, Woodland Road, Bristol, BS8 1UG, United Kingdom.
Evolution. 2011 Mar;65(3):900-6. doi: 10.1111/j.1558-5646.2010.01180.x. Epub 2010 Nov 18.
Parasitism generally imposes costs on victims, yet many victims appear to tolerate their parasites. We suggest that in some cases this may be because parasites provide victims with mitigating benefits, paradoxically giving rise to selection for advertisement rather than concealment by parasites. We investigate this possibility using the interaction between an avian kleptoparasite, the fork-tailed drongo (Dicrurus adsimilis), and one of its victims, the pied babbler (Turdoides bicolor). Combining field observations and a playback experiment, we demonstrate that a conspicuous vocal signal broadcast by drongos perched waiting to steal food from foraging babblers allows the latter to improve their own foraging efficiency, although not to the same extent as that experienced in response to conspecific sentinel calling. We argue that "sentinel" calling by drongos may originally have arisen as a means of manipulating babblers: because babblers find more food items and venture into the open more in response to these vocalizations, drongos are presented with more kleptoparasitism opportunities. However, the resulting benefit to babblers could be sufficient to reduce selection for the evolution of defenses against drongos, and the current situation may represent a rare example of an interspecific relationship in transition from a parasitism to a mutualism.
寄生虫通常会给宿主带来成本，但许多宿主似乎能够容忍它们的寄生虫。我们认为，在某些情况下，这可能是因为寄生虫为宿主提供了缓解益处，从而导致寄生虫的广告宣传而不是隐藏选择。我们使用一种鸟类食腐寄生虫——叉尾卷尾（Dicrurus adsimilis）与其一种受害者——斑胸短脚鹎（Turdoides bicolor）之间的相互作用来研究这种可能性。通过野外观察和播放实验，我们证明了停在枝头等待从觅食的短脚鹎那里偷食的卷尾鸟发出的一种显眼的叫声，使后者能够提高自己的觅食效率，尽管不如对同种哨兵鸣叫的反应程度高。我们认为，卷尾鸟的“哨兵”鸣叫最初可能是一种操纵短脚鹎的手段：因为短脚鹎会在听到这些声音后找到更多的食物，并且更愿意冒险进入开阔地，所以卷尾鸟就有了更多的食腐机会。然而，这对短脚鹎来说是有益的，足以减少针对卷尾鸟进化防御的选择，而当前的情况可能代表了一种从寄生虫到共生关系的转变的罕见的种间关系的例子。
