We investigated the hypothesis that infants search in an acoustic space for vocalisations that elicit adult utterances and vice versa, inspired by research on animal and human foraging. infant-worn recorders were used to collect day-long audio recordings, and infant speech-related and adult vocalisation onsets and offsets were automatically identified. We examined vocalisation-to-vocalisation steps, focusing on inter-vocalisation time intervals and distances in an acoustic space defined by mean pitch and mean amplitude, measured from the child's perspective. infant inter-vocalisation intervals were shorter immediately following a vocal response from an adult. Adult intervals were shorter following an infant response and adult inter-vocalisation pitch differences were smaller following the receipt of a vocal response from the infant. These findings are consistent with the hypothesis that infants and caregivers are foraging vocally for social input. increasing infant age was associated with changes in inter-vocalisation step sizes for both infants and adults, and we found associations between response likelihood and acoustic characteristics. Future work is needed to determine the impact of different labelling methods and of automatic labelling errors on the results. the study represents a novel application of foraging theory, demonstrating how infant behaviour and infant-caregiver interaction can be characterised as foraging processes. infant vocal development Human infants show massive growth in vocalising abilities during their first year 1-5. Cries and short, quiet sounds dominate early vocalisations. By about three months, infants demonstrate a much wider range of vocalisation types, varying pitch, amplitude, and other phonatory characteristics. During this time, they also start producing primitive consonant-vowel articulations. By 7 months, infants begin producing well-timed adult-like consonant-vowel alternations. This expansion in repertoire lays a foundation for later speech and other vocal communication production 2,6. For instance, there is continuity between the sounds produced during prelinguistic stages of vocal development and those sounds that make up infants' first words 7. Although anatomical changes and neuromaturation may account for some of the changes during the first year, even newborn infant vocal tracts are capable of producing a very wide range of sounds, and the dramatic changes in infants' vocalisations over the first year are believed to be primarily due to learning 8-10. Computational models of infant vocal learning have demonstrated that some combination of exploratory processes, social or intrinsic rewards, and imitation of adult speech input can result in the vocal learning we see in human infancy 11-17. Infant vocal learning may be viewed as a process that combines variation and selection, resulting in the evolution of a more adult-like repertoire of sound types. This perspective raises the question of how variability serves to explore the space of vocalisations in a way that select...