Domestication and vocal behavior in finches
Bengalese finches (BFs) are a domesticated strain of wild white-rumped munias (WRMs) imported from China to Japan 250 years ago. BF songs are composed of multiple chunks. Each chunk is a combination of 2-4 song notes. Chunks are arranged in a syntactic rule. We studied how and why BFs sing such complex songs. We found the following facts. 1). WRMs sing simpler songs. 2). There is high learning specificity in WRMs but not in BFs. 3). BFs have larger brain structures related with song than WRMs. BFs also have higher level of plasticity-related gene expressions in the brain than WRMs. 4). Both BF and WRM females prefer complex songs as measured by the degree of reproductive effort they provide. Males with complex songs are physically fitted than the males with simpler songs. These results promoted sexual selection scenario of song complexity in BFs: Song complexity evolved because it served as a sexy signal for females.
We further examined factors related with domestication. We examined songs of WRMs in subpopulations of Taiwan. Where there is a sympatric species to WRMs, songs were simpler. This leads to a hypothesis that in the wild songs needed to be simple to secure species identification, but under domestication this constrains was set free. We also examined socio-emotional indexes including the corticosterone level, the degree of aggression, and the response to a novel stimulus. All indices suggested that WRMs have higher level of stress and social shyness, which should be adaptive under natural environment, but could be limiting opportunities for learning complex songs.
Thus, evolution of song complexity involves not only factors related with sexual selection and species identification, but also socio- emotional factors due to domestication. It is tempting to think about a scenario of language evolution in humans based on sexual selection and self-domestication.