A trait or a state - how consistent are tail biting phenotypes in pigs?

Abstract

The physiological, psychological and behavioural traits of tail biting/bitten pigs have been widely studied, with most research focusing on identifying traits to predict tail biting phenotypes (biters, victims, or uninvolved neutrals). However, it is not clear if these traits persist once pigs are no longer involved in performing or receiving tail bites. This study investigated whether there was a difference in behavioural responses to a novel object test (NOT) between pigs which were tail biting performers (BITER), recipients (VICTIM), or not involved in the biting events (NEUTRAL). We then investigated whether these differences in responses were still evident six weeks later, when tail biting was less prevalent. We hypothesised that biters would exhibit more behaviours indicative of boldness, but also stress, than victims or neutrals, and that these differences would still be present later. A total of 120 undocked pigs (BITER, n = 48; VICTIM, N = 48; NEUTRAL, n = 24; 60 male, 60 female) were selected for testing. At the time of the first test (one week after pigs were moved into the finisher house at 12 weeks of age; T1), the prevalence of tail biting peaked. The same test was repeated six weeks later (T2) when biting had eased. Each pig was tested separately in a novel arena. After a minute of habituation, a brush head was introduced by dropping it down from above, at which point the 5 min test began. A saliva sample was taken immediately before the habituation (baseline) and after each test to evaluate cortisol concentration. Direct continuous behaviour observation was conducted. Overall, salivary cortisol concentrations were higher after than before the NOT (P < 0.001), although VICTIM pigs had a reduced elevation in cortisol after the test (P = 0.02) compared to BITER and NEUTRAL pigs in T2. Between phenotypes, baseline saliva cortisol concentrations were similar. BITER pigs approached the object quicker than VICTIM pigs (P = 0.01), but also had more high-pitched vocalisations (P < 0.01), but these differences observed in T1 were no longer present in T2. The results suggested that the NOT was sensitive to detect different levels of behavioural response; however, differences in BITER pigs behavioural responses were transient and did not persist once biting behaviour ceased. The long-term consequence of chronic stress caused by being tail bitten was manifested in VICTIM pigs blunted cortisol elevation six weeks later, after severe tail biting events.