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Cultural transmission of traditional songs in the Ryukyu Archipelago [1]

['Yuri Nishikawa', 'Department Of Biological Sciences', 'The University Of Tokyo', 'Bunkyoku', 'Tokyo', 'Yasuo Ihara']

Date: 2022-07

Abstract Geographic patterns of cultural variations are affected by how cultural traits are transmitted within and between populations. It has been argued that cultural traits are transmitted in different manners depending on their characteristics; for example, words for basic concepts are less liable to horizontal transmission between populations (i.e., borrowing) than other words. Here we examine the geographic variation of traditional songs in the Ryukyu Archipelago, southwestern islands of Japan, to explore cultural evolution of music with a focus on different social contexts in which songs are sung. Published scores of 1,342 traditional songs are coded using the CantoCore song classification scheme and distances between the songs are calculated from the codings. Neighbor-Net graphs of regions/islands are generated on the basis of the musical distances, and delta scores are obtained to examine the treelikeness of the networks. We also perform analysis of molecular variance (AMOVA) to evaluate the extent of musical diversification among regions/islands. Our results suggest that horizontal transmission between populations has played a greater role in the formation of musical diversity than that of linguistic diversity in the Ryukyu Archipelago and that the social context in which songs are sung has an effect on how they are transmitted within and between populations. In addition, we compare the observed patterns of song diversity among regions/islands with those of lexical and mitochondrial-DNA (mtDNA) diversity, showing that the variation of songs sung in the "work" context are associated with the linguistic variation, whereas no association is found between the musical and genetic variation.

Citation: Nishikawa Y, Ihara Y (2022) Cultural transmission of traditional songs in the Ryukyu Archipelago. PLoS ONE 17(6): e0270354. https://doi.org/10.1371/journal.pone.0270354 Editor: Søren Wichmann, Leiden University, GERMANY Received: October 8, 2021; Accepted: June 8, 2022; Published: June 24, 2022 Copyright: © 2022 Nishikawa, Ihara. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: Lexical data are available from Lee and Hasegawa (2011) (https://doi.org/10.1098/rspb.2011.0518). mtDNA HV-1 sequence data of Okinawa, Miyako, and Ishigaki islands are available from DDBJ/ENA/GenBank database (accession: AB493914-AB494137). mtDNA HV-1 sequence data of Kikai and Amami-Oshima islands are available from DDBJ/ENA/GenBank database (accession: LC653152- LC653253). The CantoCore song classification scheme is available in Savage et al. (2012) (https://iftawm.org/journal/oldsite/articles/2012a/Savage_Merritt_Rzeszutek_Brown_AAWM_Vol_2_1.htm). Funding: This study was supported by a grant from Yamaha Music Foundation (https://www.yamaha-mf.or.jp/) to YI and KAKENHI (https://www.jsps.go.jp/english/e-grants/index.html) JP17H06381 to YI. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction The study of cultural evolution, with the effective use of theories and concepts developed in evolutionary biology, has explored evolutionary dynamics in cultural change driven by transmission and innovation [1–3]. Like horizontal gene transfer in bacteria and archaea [4], the transmission of cultural traits occurs not only from parents to offspring (i.e., vertical transmission; [1]), but also through other pathways such as peer to peer (i.e., horizontal transmission) and from adults to unrelated young (i.e., oblique transmission). In cultural macroevolutionary or phylogenetic studies [5, 6], where populations rather than individuals are taken as a unit of analysis, vertical transmission implies population divergence, in which a daughter population inherits the cultural traits of the parent population, while horizontal transmission occurs between unrelated or related populations through migration and/or cultural exchange. The relative importance of vertical and horizontal transmission in observed patterns of population-level cultural diversity has long been a matter of debate, and various attempts have been made to disentangle them (e.g., [7–14]). In fact, the relative importance may vary across cultural traits. As a prime example, words for basic concepts, as those on Swadesh’s list of basic vocabulary [15], are considered more resistant to borrowing than other words are and predominantly vertically transmitted. For non-linguistic cultural traits, association with language is often considered as indicative of vertical transmission. Guglielmino et al. [7], for example, examined the associations of cultural traits with language, natural environment, and geography, each capturing the effect of vertical transmission, local adaptation, and horizontal transmission, respectively, in African societies documented in Ethnographic Atlas [16]. They suggested that of the six domains of cultural traits investigated ("family and kinship," "economy," "social stratification," "labor division by sex," "house," and "various others"), vertical transmission played a particularly important role in the "family and kinship" domain. In a more recent study to quantify the roles of shared ancestry and geography in the formation of cultural diversity documented in the Western North American Indian database (WNAI; [17]), Towner et al. [10] concluded that both vertical and horizontal transmission are important irrespective of cultural domains. Music, a supposed human universal [18], is also culturally transmitted both within and between populations. Cultural evolution of music is currently attracting renewed interests [19], beyond earlier attempts as represented by Alan Lomax’s Cantometrics Project [20], by incorporating methods of evolutionary biology. Most relevant to the present context, recent studies have applied methods for the analysis of population-level genetic diversity to analyze musical data across human populations [21–25]. By applying the methods of genetic studies, cultural data can be analyzed quantitatively, and human population history can be revealed from multiple perspectives using both genetic and cultural data. For example, Rzeszutek et al. [22] quantified pairwise distances among 421 traditional songs from 16 Austronesian-speaking populations in Taiwan and the Philippines using the CantoCore music classification scheme [26], and applied the analysis of molecular variance (AMOVA; [27]) to examine the within- and between-population diversity of the songs. Brown et al. [23] performed a similar analysis on 220 songs from nine indigenous populations of Taiwan; in addition, they compared patterns of population-level diversity in songs, genes, and languages, and found that the pattern of musical diversity resembles more genetic than linguistic diversity. Le Bomin et al. [25] argued that vertical transmission plays a major role in the formation of population-level musical diversity on the basis of their finding of a strong phylogenetic signal in a sample of 700 musical pieces collected in Gabon. In particular, they reported a delta score, a measure of deviation from treelike structure calculated from distance data [28, 29], of 0.29, which is comparable with those found in Indo-European languages (delta = 0.23, [9]) and Ainu languages (delta = 0.25, [30]), signaling a treelike structure inherent in the data. This result is in contrast to the aforementioned study on musical diversity in Austronesian-speaking populations, which suggested a greater departure from a treelike structure (delta = 0.46, [22]), indicative of horizontal transmission. Le Bomin et al. [25] further suggested that some character categories, such as rhythmic cells, metrics, and scales, exhibited a stronger phylogenetic signal than other categories. Some songs are sung in specific contexts: work songs, hymns, and lullabies to mention a few. Some social contexts are common to various human societies, and acoustic features of songs predict their contexts even across different societies [18]. In ethnomusicology, songs are often categorized based on the social context. Considering the different functional roles that different songs may have played in modern and ancient human societies, it is plausible that cultural transmission of songs may depend on the context in which they are sung. To evaluate this, we conduct a largely exploratory study of traditional songs associated with different social contexts in the Ryukyu Archipelago of Japan. The Ryukyu Archipelago is placed on the southwest part of Japan and composed of more than 150 islands, about 50 of which have residents. Since the islands have long been isolated by the sea, the Ryukyu Archipelago is known for its rich biodiversity and endemism [31–33]. The Ryukyuan population is genetically and morphologically differentiated from those in nearby regions, i.e., mainland Japan or Taiwan [34–42], and also genetically structured within the archipelago [43–45]. In terms of culture, Ryukyuan languages, which diverged from Japanese at least before the seventh century, can be divided into five subgroups [46]. They are spoken in distinct regions and differentiated from each other to the extent that they are often considered as independent languages rather than dialects [47]. The Ryukyu Archipelago is also known for its musical uniqueness. For example, a characteristic musical scale that is not seen in other part of Japan is widely shared within the archipelago [48, 49]. Common features of the music in the Ryukyu Archipelago include the presence of dance songs by members of community, the presence of songs by female religious leaders, and a high regard for the ability to sing and dance [49]. A stringed instrument called the sanshin is frequently used for accompaniment in every region of the archipelago, indicating the influence of the Ryukyu Kingdom, which was centered on Okinawa island from the 15th to 19th centuries. However, each region has unique cultural elements that are thought to have been originated before then especially in religious songs and songs for community events [50, 51]. In sum, these features seem to make the Ryukyu Archipelago an ideal field to study population-level musical diversity.

Discussion Both Neighbor-Net graphs and the delta scores suggested that musical networks of traditional songs in the Ryukyu Archipelago are less treelike than the linguistic network. A possible interpretation is that horizontal transmission between islands may have played a greater role in the formation of musical diversity than that of linguistic diversity. Deviation from treelike structure could also occur as a result of convergent evolution; however, evidence for association between song similarity and environmental or lifestyle similarity across islands, which would support this possibility, is thus far lacking. The network based on linguistic distances showed clear clusters of spatially close islands (S1B Fig), which may reflect vertical transmission of languages associated with the history of population divergence. Linguistic distance was also positively correlated with geographic distance between islands (Fig 4B). On the other hand, such correspondence with geography was not observed in the musical or genetic networks (S1A, S1C Fig, Fig 4A and 4C). This may be due to horizontal transmission between relatively distant islands. As a recent genomic analysis has suggested [44], there may have been undocumented small-scale migration events between islands within the Ryukyu Archipelago. Since the difficulty of an inter-island voyage depends not only on the geographic distance, but also on other factors such as tidal currents, these migration events may have attenuated the correlation between genetic and geographic distances. It is also plausible that songs culturally diffused through such migration events as well as during social interactions between the residents of different islands, the latter of which were likely contingent on economic and political factors. For example, the influence of Okinawa island, the largest island in the archipelago, may be the reason of the relatively small pairwise Φ ST values for music between Okinawa island and the other islands (Fig 3B, S2 Data). Conversely, those migrations and social interactions do not seem to have massively altered the basic vocabulary of each language in the Ryukyu Archipelago, perhaps because the conformity effect, by which children tend to acquire cultural variants used by the majority of people, played a greater role in the learning of basic vocabulary than that of songs. It is thus far difficult to connect the topologies of musical or genetic networks with known history of the area. For example, Amami-Oshima and Miyako islands, which are located close to each other in the genetic network (S1B Fig), are geographically separated and no historical records linking them have been recognized. For more detailed discussion, studies using whole genome in a large number of islands are awaited. While we were able to analyze large amount of musical data covering many islands, linguistic data were available only for a part of those islands, and scarcity is even more acute for genetic data. Furthermore, considering lexicon and phonology show quite different networks in Northeast Asia [62], the use of phonological data in addition to the lexical data may affect our results. Future research should take these issues into consideration. The extent of song diversification among populations in the Ryukyu Archipelago is smaller than that of Taiwan indigenous populations. However, the observed Φ ST values indicated that the extent of song diversification among regions/islands may vary depending on the social contexts associated with the songs; that is, the social context may have an effect on how songs are transmitted between and within populations. This is consistent with the previous argument that patterns of transmission vary across cultural domains [7, 8, 10, 63–66]. In particular, the "work" songs exhibited high degree of between-region diversity compared to songs with other social contexts. Association between the songs and mtDNA was not observed in the Ryukyu Archipelago, which is in contrast to the observed association between songs and mtDNA in Taiwan [23]. A possible reason for the discrepancy is that the association is weaker in the Ryukyuan people because they are genetically and culturally less diversified than Taiwan indigenous populations. While AMOVA for the five Ryukyuan islands obtained a statistically significant Φ ST value (Φ ST = 0.022, p < 0.001), the extent of between-population genetic differentiation is much lower than for the nine Taiwanese populations (Φ ST = 0.127, [23]). In fact, the Taiwanese indigenous populations are known to have striking genetic diversity between each other, presumably because of long-term social isolation and endogamy [67–69]. As for the Ryukyu Archipelago, on the other hand, where the present-day inhabitants are originated from migrations in the Holocene with little or no genetic contribution by the Pleistocene inhabitants [43, 70], the whole archipelago became a single cultural zone with exchanges and migrations between islands since the eleventh or twelfth century [44, 71]. In addition, the hypothesis that lullabies tend to be maternally transmitted was not supported by our mtDNA data. It is possible that the assumption that lullabies are vertically transmitted from mothers to children is false. In other words, lullabies are sometimes sung by baby-sitters other than mothers [49], and children may not necessarily learn the lullabies they are exposed to by their mothers. It should be noted, however, that we used mtDNA data from only five islands, and thus further analysis with extended data might obtain different results. The social bonding hypothesis for the evolution of musicality states that music has been adaptive because it enhances social bonding and cooperative behavior within group [72–77] as has been supported by several empirical studies [78–82]. There exists evidence suggesting that singing induces the elevation of oxytocin concentration [83, 84] and the release of endorphins [81, 82, 85]. Oxytocin and endorphins (or more generally, the endogenous opioid system, EOS) may be part of physiological mechanisms that enhance social bonding in humans (reviewed in Savage et al. [77]). It has also been suggested that EOS is associated with social bonding in non-human primates [86–90] and other animals [91–93]. Therefore, singing together in a group is thought to strengthen social bonding. It might be argued that our observation of large Φ ST value for "work" songs and statistically significant association between "work" songs and language are in line with the social bonding hypothesis, because singing during working may enhance social bonding of the collaborative team and increase work efficiency. This may be of particular adaptive value in food production like farming or fishing. To sing together, members of a team have to know the same song repertoires, and this may limit changes in songs either by innovation or horizontal transmission, leading to small within-group and large between-group divergence. While singing together may also promote social bonding in other contexts, the effect may be particularly important in joint works requiring precise coordination, and as a consequence, "work" songs may be functionally more restricted than other songs. In conclusion, horizontal transmission may have played a large role in song evolution in the Ryukyu Archipelago. The extent of song diversification varies depending on the social contexts in which songs are sung, and this indicates the importance of considering social context in studying cultural evolution of music. "Work" songs exhibited high degree of between-region diversity and association with languages, partially congruent with the social bonding hypothesis for the evolution of musicality.

Acknowledgments The authors would like to thank Patrick Savage for providing R source code and Sean Lee for providing lexical data.

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