Evolution of song complexity in Bengalese finches could mirror the emergence of human language

Abstract

Bengalese finches (BFs) are a domesticated strain of wild white-rumped munias imported from China to Japan 250 years ago. Bengalese songs are composed of multiple chunks of song notes, and each chunk is a combination of 2–5 song notes. Furthermore, chunks are arranged in a finite-state probabilistic automaton. We studied how and why BFs sing such complex songs. We found the following: (1) ancestral strains sing simpler songs; (2) there is high learning specificity in white-rumped munias but not in BFs; (3) BFs have larger song control nuclei and higher glutamate receptor gene expression levels than white-rumped munias; (4) both BFs and white-rumped munia females prefer complex songs as measured by the nest string assay, and males with complex songs are physically fitter than males with simpler songs. These results indicated the sexual selection scenario of song complexity in Bengalese finches. We further examined factors related to domestication. We examined songs of white-rumped munias in subpopulations of Taiwan. Where there is a sympatric species with white-rumped munias, songs were simpler. This led to a hypothesis that in the wild, songs needed to be simple to secure species identification, but under domestication this constraint was unnecessary. Based on these results, we extended our research into the socio-emotional domain. All indexes suggested that white-rumped munias have higher levels of stress and social shyness, which should be adaptive in natural environments, but could limit opportunities for learning complex songs. Evolution of song complexity involves factors related to the strength of sexual selection and relaxation of species identification as well as socio-emotional factors because of domestication. The results of the present study on BFs should be useful in discussing the possible biological origins of human speech relating to proximate and ultimate factors.