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Description: **Supplementary Materials** What are the vocal experiences of children growing up on Malakula island, Vanuatu, where multilingualism is the norm? Long-form audio-recordings captured spontaneous speech behavior by, and around, 38 children (5-33 months, 23 girls) from 11 villages. Automated analyses revealed most children's vocal input came from female adults and other children’s voices, with small contributions from male adult voices. The greatest changes with age involved an increase in the input vocalizations from other children. Total input (collapsing across child-directed and overheard speech, and across languages) was ~11 minutes per hour, which was at least 5 minutes (31%) lower than that found in other populations studied using comparable methods in previous literature, as well as in archival American data analyzed with the same algorithm. In contrast, children's own vocalization counts were two to four times higher than previous reports for North-American English-learning monolingual infants at matched ages, and comparable to estimates from archival American data, consistent with a resilient language-learning cognitive system for this aspect of vocal development. The strongest association between input and output was with vocalizations by other children, rather than those by adults, which is consistent with research in anthropology but less so with current theoretical trends in developmental psychology. These results invite further research in populations that are under-represented in developmental science.