Ontogeny of tactile, vocal and kinship dynamics in rat pup huddling

Huddling, a tactile, thermoregulatory and filial social interaction, is a predominant and conserved social interaction mammalian and bird species. The act of huddling is particularly important in early life, when thermoregulation, social touch and bonding are influential for survival and healthy brain and behavioural development. We ask how tactile, vocal and kinship dynamics of rat pup interactions develop. We designed a huddling apparatus where we record and synchronise huddle formation with ultrasonic vocalisation analysis. With development, we see that groups (P6–8 vs older pups) stay longer in triad aggregon (pup huddle) configurations in the huddle trial period. Older pups (P18–20) switch huddle states more often. The spectral characteristics of rat pup vocalisation during huddling task change in development, with a higher peak frequency in P18–20 pups. In all age-groups we observe vocal quieting as aggregons form. We hypothesized that kinship should be a strong determinant of huddling interactions but our findings reveal otherwise. When comparing kin vs non-kin groups we found no differences in aggregon durations or switches. In the youngest age group (P6–8) the amount of USVs were reduced in kin vs non-kin groups, though not in older age groups. To address the role of tactile contact in quieting we integrated touch dividers in the huddle arena. Without touch, we saw that vocalisations significantly increased in P6–8 and P11 – 14, but not in P18–20 kin sibling groups. We suggest that rat pups have a strong internal drive towards huddling behaviour regardless of whether huddle partners are related by kinship. USV analysis suggests that huddling has a calming effect, where related sibling young pups show less distress overall; absence of touch is associated with increased distress in P6–8 and P11 – 14 kin huddle groups. Thus, huddling is a natural social behaviour that is intrinsically rewarding and shared between both related siblings and unrelated conspecifics; it has calming effects - as indicated by USVs - that depend on kinship and tactile contact during the earliest stages of development.