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

Huddling, a tactile, thermoregulatory and affiliative social interaction, is a predominant and conserved behaviour for mammalian and bird species. Huddling is particularly important in early life, when thermoregulation, touch and bonding are influential for survival, brain and behavioural development. Our study aims to understand how tactile, vocal and kinship dynamics develop in rat pups. We designed a huddling apparatus where we synchronise huddle formation with ultrasonic vocalisations (USVs). 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; in all ages, join transitions were preferred to breaking. The spectral characteristics of rat pup USVs change in development, with higher peak frequency in P18-20 pups. In all age-groups, we observe vocal quieting as aggregons form. We hypothesised that kinship is a determinant of huddling dynamics but findings reveal otherwise. When comparing kin versus non-kin groups, we found no differences in aggregon durations or switches. In the youngest age (P6-8), USVs were reduced in kin versus non-kin groups, though not in older pups. To address the role of social touch in quieting, we integrated dividers in the huddle arena. Without skin/fur contact, USVs increased in P6-8, P11-14, but not in P18-20 kin groups. We suggest that rat pups have a strong internal drive towards huddling regardless of whether partners are related by kinship. USV analysis suggests that huddling has a calming effect, where related sibling young pups show less USVs overall; absence of social touch is associated with increased vocalisation in young huddle groups. Thus, huddling is a natural social behaviour shared between both related siblings and unrelated conspecifics. Huddling has calming effects-indicated by USVs-that depend on kinship and tactile social contact during the early stages of development.