Heritabilities of agonistic behavioural traits in pigs and their relationships within and between different age groups

The aim of the present study was to estimate heritabilities for different agonistic behavioural traits after mixing female pigs of three age classes and to compare the agonistic behaviour between these age groups. The behavioural patterns were observed over a 48-h period, directly after weaning (PIG-28; n=884), 40 day later (growing pigs, PIG-68; n=351) and immediately after mixing gilts (GILTS; n=389) with an average age of about 242 day. Agonistic interactions were observed by noting the time of the start and finish of the interaction, the identity of the aggressor and receiver and the winner or loser of a fight. Starting from this, the frequency and the time involved were calculated for ten agonistic behavioural traits. Intermediate heritabilities were estimated for growing pigs, with the highest heritability for the number of won fights (h2=0.37). In comparison, the heritabilities for weaned piglets were at a lower level with the highest heritability for the duration spent in initiated fights (h2=0.20). Concerning gilts, moderate heritabilities were only found for the number of received aggressions (h2=0.42), the time spent in received aggressions (h2=0.15), the number of lost fights (h2=0.13) and the time spent in lost fights (h2=0.09). Within all three age groups, the correlations between the numbers of initiated and won fights were stronger than the correlations between the numbers of initiated and lost fights, indicating that pigs which initiated most of their fights also won the majority of their fights. Moderate correlations were found between the behavioural traits of growing pigs and gilts, but the number of relationships between the weaned piglets and older age groups was low. In conclusion, the results indicate that more aggressive growing pigs tend to also become more aggressive gilts. Additionally, the agonistic behaviour of weaned piglets cannot be compared with that of other age groups since the agonistic behaviour of weaned piglets is probably related to playfulness while in older pigs it is related to the establishment of dominance hierarchies.