Behavioral Biology, Institute of Biology Leiden, Leiden University, Sylvius Laboratory, Sylviusweg 72, PO Box 9505, 2300 RA, Leiden, The Netherlands.
Proc Natl Acad Sci U S A. 2009 Dec 1;106(48):20538-43. doi: 10.1073/pnas.0908113106. Epub 2009 Nov 16.
According to a controversial hypothesis, a characteristic unique to human language is recursion. Contradicting this hypothesis, it has been claimed that the starling, one of the two animal species tested for this ability to date, is able to distinguish acoustic stimuli based on the presence or absence of a center-embedded recursive structure. In our experiment we show that another songbird species, the zebra finch, can also discriminate between artificial song stimuli with these structures. Zebra finches are able to generalize this discrimination to new songs constructed using novel elements belonging to the same categories, similar to starlings. However, to demonstrate that this is based on the ability to detect the putative recursive structure, it is critical to test whether the birds can also distinguish songs with the same structure consisting of elements belonging to unfamiliar categories. We performed this test and show that seven out of eight zebra finches failed it. This suggests that the acquired discrimination was based on phonetic rather than syntactic generalization. The eighth bird, however, must have used more abstract, structural cues. Nevertheless, further probe testing showed that the results of this bird, as well as those of others, could be explained by simpler rules than recursive ones. Although our study casts doubts on whether the rules used by starlings and zebra finches really provide evidence for the ability to detect recursion as present in "context-free" syntax, it also provides evidence for abstract learning of vocal structure in a songbird.
根据一个有争议的假说，递归是人类语言特有的一种特征。与该假说相反，有人声称，到目前为止，经过测试的两种具有这种能力的动物物种之一的八哥，能够根据中心嵌入的递归结构的存在与否来区分声音刺激。在我们的实验中，我们表明另一种鸣禽，即斑胸草雀，也可以区分具有这些结构的人工歌曲刺激。斑胸草雀能够将这种区分推广到使用属于同一类别的新元素构建的新歌曲中，这与八哥相似。然而，为了证明这是基于检测假设的递归结构的能力，关键是要测试鸟类是否也可以区分具有相同结构但属于不熟悉类别的元素的歌曲。我们进行了这项测试，结果表明，8 只斑胸草雀中有 7 只未能通过。这表明获得的区分是基于语音而不是句法的概括。然而，进一步的探测测试表明，这只鸟以及其他鸟的结果可以用比递归规则更简单的规则来解释。尽管我们的研究对八哥和斑胸草雀使用的规则是否真的提供了“无上下文”语法中检测递归能力的证据提出了质疑，但它也为鸣禽对声音结构的抽象学习提供了证据。
