Group dynamics in a spontaneously established group of newly weaned piglets

The weaning process poses considerable challenge in pig production. The standard procedure usually involves mixing multiple litters in a relatively dense group where the pigs do not have much space and opportunity to interact uninterruptedly and therefore cannot exhibit the full natural display of agonistic behaviour to establish a hierarchy. Understanding the newly-formed group dynamics in this way is hardly possible under standard weaning procedures. Therefore, we implemented a weaning approach that allowed piglets to mix spontaneously and willingly – each litter was weaned in its pen, separated with an empty pen, with all pens connected by narrow passages that were opened 24 h after weaning. Twelve litters (117 piglets) were included in the study. A total of 2792 fights and 1567 mounts were recorded during the 8-day study period. Fighting was the predominant interaction between litters, most frequent at the beginning of mixing. We found no territorial tendencies in the interactions, except for a lower probability of winning the fight in the piglet’s pen of origin. Heavier piglets were involved in fighting to a significantly greater extent immediately (first day) after mixing, while there were no differences in the frequency of fights regarding body weight later on. However, the probability of initiating a fight increased with body weight, and usually, the initiator was also the winner. On the other hand, mounting was the activity initiated and performed mainly by the piglets of medium body weight. In general, females were more involved in both fighting and mounting. These two interactions showed clear opposing temporal dynamics, with the frequency of fighting steadily decreasing over time, while mounting increased and surpassed fighting in relative occurrence. Thus, the formation of the new dominance hierarchy appears to be a two-phase process: an early phase of high aggression, followed by a phase of low aggression in which mounting replaces fighting. Mounting appeared to serve as a means of mutually determining strength in order to avoid fighting; thus, it plays an important role when the relative strengths of group members are known from previous confrontations. Therefore, in assessing the establishment of social structure, we should not neglect subtle agonistic behaviours (e.g., mounting) that serve as a means of establishing or reinforcing social positions within a group.