Abstract
Cultural transmission of behaviour is an important aspect of many animal communities ranging from humans to birds. Male humpback whales (Megaptera novaeangliae) sing a repetitive, stereotyped, socially learnt and culturally transmitted song display that slowly evolves each year. Most males within a population sing the same, slow-evolving song type; but in the South Pacific, song ‘revolutions’ have led to rapid and complete replacement of one song type by another introduced from a neighbouring population. Songs spread eastwards, from eastern Australia to French Polynesia, but the easterly extent of this transmission was unknown. Here, we investigated whether song revolutions continue to spread from the central (French Polynesia) into the eastern (Ecuador) South Pacific region. Similarity analyses using three consecutive years of song data (2016–2018) revealed that song themes recorded in 2016–2018 French Polynesian song matched song themes sung in 2018 Ecuadorian song, suggesting continued easterly transmission of song to Ecuador, and vocal connectivity across the entire South Pacific Ocean basin. This study demonstrates songs first identified in western populations can be transmitted across the entire South Pacific, supporting the potential for a circumpolar Southern Hemisphere cultural transmission of song and a vocal culture rivalled in its extent only by our own.
1. Introduction
Cultural traditions are significant to human society [1], but also shape non-human mammalian societies including primates, rats and cetaceans, and non-mammalian species such as fish and birds [2–10]. Vocally learnt displays play an important role in shaping culture in oscine birds and cetaceans, in particular [11–14]. Culture is defined here, following others, as the social learning of information or behaviours from conspecifics within a community [10,15–17]. Information can flow in a number of different directions. Horizontal transmission is within-generation spread of cultural traditions, while vertical transmission occurs from parent to offspring, and oblique transmission is spread from non-parental individuals belonging to the previous generation to the next generation [10,18,19].
Song is a striking example of non-human cultural transmission and evolution exhibited by oscine songbirds and possibly most baleen whales including humpback whales [12,20]. Bird song is a crucial part of courtship behaviour, and despite some basic song structure being innate, the complexity and detail are added through contact with conspecifics [21]. Further, some bird songs undergo changes from year to year by individuals dropping and adding syllables [22,23]. Corn buntings (Emberiza (Miliaria) calandra), for example, possess local dialects that are distinct from conspecifics beyond a geographical boundary [24,25]. The song of a certain dialect changes slowly each year, with all males adopting the novel version in unison [12,24,26]. This process of small changes through individuals performing their own rendition of the song is described as cultural evolution [27]. Song is also thought to play a role in breeding success of baleen whales [28–30]. The song produced by baleen whales ranges in complexity in terms of the number of sound types, structure and length. Blue whales (Balaenoptera musculus) and fin whales (Balaenoptera physalus) sing simple songs made up of only a few sound types [31,32]. By contrast, bowhead whales (Balaena mysticetus) and humpback whales sing complex songs that change over breeding seasons [30,33–35]. Unlike bowhead whale song where many song types appear to be present in one season [34,36], humpback whales within a population typically sing a single, shared song type that progressively evolves each year, much like that of corn buntings, but there are notable exceptions (see below) [29,30,37,38].
