Tail biting is an adverse behaviour characterised by manipulation of a pig’s tail by another pig resulting in tail damage and a possible tail biting outbreak. Tail biting is a common problem in the pig husbandry causing economic losses and reduced animal welfare worldwide. To prevent tail biting, the majority of newborn piglets are tail docked, a procedure which is not only painful but generates more and more public concern. This emphasizes the need to prevent the occurrences of tail biting without having to dock a pig’s tail. So far, research focused mainly on the risk factors that can induce tail biting. However, the way a tail biting outbreak evolves in a group of pigs (the ‘aetiology’) is still poorly understood. For that reason, the main aim of this thesis was to gain more insight in the aetiology of a tail biting outbreak. This will not only enhance our understanding of the current preventive and curative treatments of tail biting, but can also generate more effective measures to prevent, predict and counteract a tail biting outbreak. Therefore, the development of tail biting behaviour and tail damage was studied in relation to preventive and curative measures, group composition and indicators for an upcoming tail biting outbreak. The results showed that the provision of twice daily a handful of long straw strongly reduced tail biting. Furthermore, this measure was also effective in counteracting an ongoing tail biting outbreak (an outbreak was defined as the first day with a minimum of one piglet with a tail wound or two piglets with bite marks in a pen), although this outbreak could not be totally eliminated. In pens without straw almost all pigs performed and received tail biting behaviour at low levels prior to a tail biting outbreak. However, considerable variation in tail biting behaviour between pigs was found. In most pens one or a few pigs could be identified as pronounced biters prior to the tail biting outbreak. Although less clear, often one or a few pigs could similarly be identified as pronounced victims. In mixed-sex pens male pigs developed tail damage most rapidly, while in single-sex pens the quickest tail damage development was found in all-female groups. These results indicate that female pigs are more likely to become biters and male pigs are more likely to become victims. More detailed study of pronounced biters and victims showed that prior to a tail biting outbreak, biters not only directed more of their biting behaviour to their penmates’ tail, but also to the enrichment device. Victims were the heavier pigs in the pen and tended to be more often male and more restless preceding an outbreak. Victims also performed more aggressive behaviour, while biters tended to receive more aggressive behaviour. Furthermore, it was found that tail posture is a predictor for tail damage. Pigs with their tail between the legs had a higher chance of tail damage 2-3 days later.
Based on the results of this research an aetiology model of a tail biting outbreak was developed. Subsequently practical suggestions were given to prevent (e.g. providing effective environmental enrichment), predict (e.g. observing the pigs’ tail posture) and counteract (e.g. removing the biter) a tail biting outbreak. This provides opportunities to omit tail docking without the negative consequence of tail biting.