Early life experiences involving multisensory interactions with social conspecifics influence brain development and social behaviour in rat pups. Rat pups, reared with their kin, interact through contact, olfaction, and ultrasonic vocalisations, with the predominant multisensory social interaction pre-weaning being huddling behaviour. We investigated how rat pups develop huddling behaviour through these interactions, hypothesising that kin status and developmental stage influence huddle organisation and formation.
To test this, we used an open arena while recording video and ultrasonic vocalisations, both synchronised to observe huddle formation in groups consisting of three animals at three different group ages. We adhered to the regulations outlined by the University of Edinburgh Animal Welfare Committee, and all experimental tests were conducted under a United Kingdom Home Office project licence. Observing development from P6-8 (n=22 groups: kin=14 and non-kin=8), P11-14 (n=22 groups; kin=13; non-kin=9), and P18-20 (n=23 groups: kin=14 and non-kin=9), we found that groups formed more triad aggregons as they aged (H(3)=22.5, p<0.0001, non-parametric Kruskal-Wallis test), and these groups remained intact once formed.
Contact quieting, where vocalisations decrease with huddle formation, was noted especially during P6-8, but also in the P11-14 age groups. In the youngest age group (P6-8), non-kin groups exhibited more vocal behaviour compared to kin groups (number of vocalisations per 20-minute session: kin groups = 86.9 ± 6.2, non-kin groups = 138.0 ± 6.2; mean ± SEM; U = 40.0, p < 0.001, non-parametric Mann-Whitney U test). Exposure of kin groups to foreign bedding increased vocalisations, indicating that olfaction and contact interact to produce contact quieting, and that kin detection through olfaction leads to quieter vocal states in kin/familiar settings.
Overall, we observed that huddling behaviour is highly regulated by development, relies on the integration of multisensory cues, and may depend on the kinship composition of the huddling group.