Allen et al - Proceedings B Supplementary Data - Raw Data from Cultural revolutions reduce complexity in the songs of humpback whales

Much evidence for non-human culture comes from vocally learned displays, such as the vocal dialects and song displays of birds and cetaceans. While many oscine birds use song complexity to assess male fitness, the role of complexity in humpback whale (<i>Megaptera novaeangliae</i>) song is uncertain due to population-wide conformity to one song pattern. Although songs change gradually each year, the eastern Australian population also completely replaces their song every few years in cultural ‘revolutions’. Revolutions involve learning large amounts of novel material introduced from the Western Australian population. We examined two measures of song structure, complexity and entropy, in the eastern Australian population over 13 consecutive years. These measures aimed to identify the role of complexity and information content in the vocal learning processes of humpback whales. Complexity was quantified at two hierarchical levels: the entire sequence of individual sound ‘units’ and the stereotyped arrangements of units which comprise a ‘theme’. Complexity increased as songs evolved over time but decreased when revolutions occurred. No correlation between complexity and entropy estimates suggests that changes to complexity may represent embellishment to the song which could allow males to stand out amidst population-wide conformity. The consistent reduction in complexity during song revolutions suggests a potential limit to the social learning capacity of novel material in humpback whales.

非人类文化的大量证据源自经声音学习形成的鸣唱与展示行为，例如鸟类与鲸类（cetaceans）的声音方言及鸣唱展示。尽管多数鸣禽（oscine birds）通过鸣唱复杂度评估雄性个体的适合度，但座头鲸（humpback whale，*Megaptera novaeangliae*）鸣唱的复杂度所扮演的角色尚不明确——原因在于其种群个体统一遵循单一的鸣唱模式。尽管座头鲸鸣唱每年都会发生渐进式变化，但东澳大利亚种群每隔数年便会通过文化“革命”彻底替换其鸣唱内容。此类文化革命涉及对西澳大利亚种群传入的大量全新鸣唱素材的学习。本研究针对东澳大利亚种群连续13年的鸣唱数据，分析了两项鸣唱结构指标：复杂度与信息熵（entropy）。这两项指标旨在探究复杂度与信息含量在座头鲸声音学习过程中的作用。复杂度的量化分为两个层级：其一为单个声音“单元”的完整序列，其二为构成“主题”的单元定型排布。随着鸣唱随时间渐进演化，复杂度逐步提升，但在文化革命发生时复杂度会出现下降。复杂度与熵的估算值之间不存在相关性，这表明复杂度的变化或许是对鸣唱的修饰，能够让雄性个体在种群统一的鸣唱模式中脱颖而出。鸣唱革命期间复杂度的持续性下降，暗示座头鲸在学习全新鸣唱素材的社会学习能力方面存在潜在上限。