Sexual communication and domestication may give rise to the signal complexity necessary for the emergence of language: An indication from songbird studies.

For language to be a vehicle of thought, protolanguage must develop a degree of complexity that allows for the syntactic manipulation of symbol sequences. Thus, before language emerged, a process in which signals became complex must have occurred. Here, we submit a scenario describing the process in which courtship songs gained in complexity during the course of domestication of Bengalese finches. By comparing domesticated Bengalese finches with their wild strain, white-rumped munias, we found that female preferences for complex songs were coupled with domesticated relaxed environments to give rise to song complexity. Furthermore, we found that the outcomes of domestication in Bengalese finches include decreased psychological and physiological stress reactions, decreased aggressiveness, and changed plumage colors. These characteristics are consistent with "domestication syndrome," originally proposed for mammals but now possibly applicable to birds. Our finding that domestication was a cause of signal complexity might be helpful in considering the process of signal evolution in human language.