Deacon (2003) proposed a novel mechanism by which a biological property gains complexity. In his theory, the term "masking" is used to refer to an environmental change that masks a particular selection pressure. Similarly, the term "unmasking" is used to indicate a process by which a selection pressure becomes effective. Birdsong conveys two messages: species identification and individual vigor. To ensure the former, the signal should satisfy one or more species-specific features. To ensure the latter, the signal should reflect individual characteristics related to vigor. The Bengalese finch is a domesticated strain of the white-rumped munia, an endemic finch of East Asia. The process of domestication occurred about 250 years ago, comprising approximately 500 generations. Bengalese finches sing syntactically and phonologically complex songs, whereas white-rumped munias sing simpler songs in both domains (Okanoya, 2004). Female Bengalese finches engage in more breeding behaviors when stimulated by complex songs, suggesting that song complexity in Bengalese finches evolved by sexual selection (Okanoya, 2004). However, the model of Ritchie and Kirby (2005) demonstrated the possibility that domestication alone could account for the evolution of song complexity by relaxing selection pressures. In their study, domestication was used as a condition to disable selection pressure for species identification. If a system arises in which the need for species identification exhibits natural variation, that system can be used to directly test Deacon's theory
When several species of birds with similar plumage share the same environment (sympatric environment), birdsong should faithfully convey the species identification signal to avoid infertile hybridization. In Taiwan, white-rumped munias form mixed colonies with a closely related species, the spotted munia. We hypothesized that the rate of sympatry would affect song complexity. We conducted a field study at three locations in Taiwan: Huben (H), Mataian (M), and Taipei (T), where natural populations of white-rumped munias occur. During the summers of 2006-2008, we captured white-rumped munias using mist nets and recorded male songs. Totals of 30 (H), 23 (M), and 17 (T) male white-rumped munias were captured. The number of spotted munias at each location was also counted. Song linearity, an index of song simplicity, was calculated as (the number of song notes)/(the number of song note transition types). The value of this index equals one when the song sequence is completely linear and decreases when the song is less deterministic. The index is 1/N, where N is the number of song notes, when the song is completely random. The rate of sympatry was calculated as (the number of mixed flocks)/(the number of total flocks), where mixed flocks were those containing both white-rumped and spotted munias. We found that the rate of sympatry was lowest at Huben and higher at Taipei and Mataian (H < T, M; Fig. 1a). Song linearity was lowest (more complex) at Huben and greater at Taipei and Mataian (H < T, M; Fig. 1b). Therefore, the rate of sympatry corresponded with song linearity. These results were consistent with the prediction that lower pressure for species identification leads to higher complexity in birdsong.
Although other factors including sexual selection (Okanoya, 2004) are undoubtedly involved, the process of masking, as demonstrated here, may account for some proportion of signal evolution in Bengalese finches, as suggested by Deacon (2003). Similar mechanisms should be considered when examining the evolution of human language.
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