Persistence of sociality in group dynamics of dairy cattle

•Location Big data of 158 cows during four months to study their individual and group social dynamics.•We found that sociality is an individual trait, unrelated to the group and relatively persistent once established.•Findings have the potential to help understanding the social dynamic when mixing dairy cows. In many species, animals live in highly structured groups. In these groups, individual differences in the number and identity of social contacts (alters) of each ego define the social network structure of the animal group. The composition of groups can be disturbed by grouping animals according to age or production stage, which can in turn induce stress. We investigated whether the preference of two animals to stay together depends on the sociality of each individual (defined by the time it spends close to other animals) or on the composition of the group. We observed 158 dairy cows distributed in 6 pens during 17 weeks. The precise positions of the cows were monitored with positional loggers 24/7 in two groups with fixed independent populations and during the formation of new groups with varying population. In fixed groups, the sociality of a cow was maintained over the entire observation period, but this trait depended on the area contacts occurred (inside vs. outside the resting area). When introducing foreign individuals into social groups, the sociality of individual cows was maintained independently of the group; this sociality was therefore not necessarily influenced by the time spent in the group but by the social characteristics of individual cows. During the formation of new groups, newly introduced cows dynamically interacted with resident ones, forming a few strong short-lasting contacts between newcomers and resident cows. However, a few long-lasting interactions occurred between resident and newcomers and the whole network is weakened. Our study suggests that each cow has its own sociality independent of the group and tend to establish relations with specific partners when the population is fixed. In addition, when introducing cows into a group, the social network of the group is weakened, with absence of strong links between newcomers and resident cows and a decrease in the links between resident cows for at least 2 weeks.