Effects of familiarity and relatedness on agonistic pair relationships in newly mixed domestic pigs

The effects of familiarity and relatedness on agonistic pair relationships (dyads) in different pen regions (pen area or trough area) were studied in 16 groups of newly mixed domestic pigs of similar weight (9 pigs per group) at an age of 12 weeks. The agonistic interactions (AI) within 124 familiar (related) and 452 unfamiliar (related and unrelated) dyads were continuously recorded for 3 days (10 h daily) after mixing. Whereas pigs, both in dyads familiar and unfamiliar to each other, showed the same frequency of AI in the trough area, unfamiliar dyads exhibited significantly more AI in the pen area than familiar dyads. The relatedness of unfamiliar dyads had apparently no influence on AI. It is discussed that, besides establishing a dominance hierarchy, pigs react aggressively on strange subjects. Furthermore, the results are briefly discussed with reference to dominance and resource usage in pigs.